EEA EthTrust Security Levels Specification v-after-2 Editor's Draft

This document defines the requirements for EEA EthTrust Certification, a set of certifications that a smart contract has been reviewed and found not to have a defined set of security vulnerabilities.

Security Considerations

Security of information systems is a major field of work. There are risks inherent in any system of even moderate complexity. This specification describes testing for security problems in Ethereum smart contracts. However there is no such thing as perfect security. EEA EthTrust Certification means that at least a defined minimum set of checks has been performed on a smart contract. **This does not mean the Tested Code definitely has no security vulnerabilities**. From time to time new security vulnerabilities are identified. Manual auditing procedures require skill and judgement. This means there is always a possibility that a vulnerability is not noticed in review.

Smart Contracts in context - broader considerations

Ethereum is based on a model of account holders authorising transactions between accounts. It is very difficult to stop a malicious actor with a privileged key from using that to cause undesirable or otherwise bad outcomes. Likewise, in practice users often interact with smart contracts through a "Ðapp" or "distributed app", whose user interface is a Web Application. Web Application Security is its own extensive area of research and development, beyond the scope of this specification.

Upgradable Contracts

Smart contracts in Ethereum are immutable by default. However, for some scenarios, it is desirable to modify them, for example to add new features or fix bugs. An Upgradable Contract is any type of contract that fulfills these needs by enabling changes to the code executed via calls to a fixed address. Some common patterns for Upgradable Contracts use a Proxy Contract: a simple wrapper that users interact with directly that is in charge of forwarding transactions to and from another contract (called the Execution Contract in this document, but also known as a Logic Contract), which contains the code that actually implements the Smart Contract's behaviour. The Execution Contract can be replaced while the Proxy Contract, acting as the access point, is never changed. Both contracts are still immutable in the sense that their code cannot be changed, but one Execution Contract can be swapped out with another. The Proxy Contract can thus point to a different implementation and in doing so, the software is "upgraded". This means that a Set of Contracts that follow this pattern to make an Upgradable Contract generally cannot be considered immutable, as the Proxy Contract itself could redirect calls to a new Execution Contract, which could be insecure or malicous. By meeting the requirements for access control in this specification to restrict upgrade capabilities enabling new Execution Contracts to be deployed, and by documenting upgrade patterns and following that documentation per [Q] Implement as Documented, deployers of Tested Code can demonstrate reliability. In general, EEA EthTrust Certification of a Proxy Contract does not apply to the internal logic of an Upgradable Contract, so a new Execution Contract needs to be certified before upgrading to it through the Proxy Contract. There are several possible variations on this core structure, for example having a Set of Contracts that includes multiple Execution Contracts. In the attack known as Metamorphic Upgrades, a series of Smart Contracts are used to convince people (e.g. voters in a DAO) to approve a certain piece of code for deployment, but one of the proxy contracts in the chain is updated to deploy different, malicious, code. Other patterns rely on using the `CREATE2` instruction to deploy a Smart Contract at a known address. It is currently possible to remove the code at that address using the `selfdestruct()` method, and then deploy new code to that address. This possibility is sometimes used to save Gas Fees, but it is also used in Metamorphic Upgrade attacks.

Oracles

A common feature of Ethereum networks is the use of Oracles: functions that can provide information sourced from on-chain or off-chain data. Oracles solve a range of problems, from providing random number generation to asset data, managing the operation of liquidity pools, and enabling access to weather, sports, or other special-interest information. Oracles are used heavily in DeFi and gaming, where asset data and randomization are central to protocol design. This specification contains requirements to check that smart contracts are sufficiently robust to deal appropriately with whatever information is returned, including the possibility of malformed data that can be deliberately crafted for oracle-specific attacks. While some aspects of Oracles are within the scope of this specification, it is still possible that an Oracle provides misinformation or even actively produces harmful disinformation.

The two key considerations are the risk of corrupted or manipulated data, and the risk of oracle failure. Vulnerabilities related to these considerations - excessive reliance on TWAP, and unsafe management of oracle failure - have occurred repeatedly leading to the loss of millions of dollars of value on various DeFi protocols. While many high-quality and trusted Oracles are available, it is possible to suffer an attack even with legitimate data. When calling on an Oracle, data received needs to be checked for staleness to avoid Front-running attacks. Even in non-DeFi scenarios, such as a source of randomness, it is often important to reset the data source for each transaction, to avoid arbitrage on the next transaction. A common strategy for pricing Oracles is to provide a time-weighted average price (known as TWAP). This provides some level of security against sudden spikes such as those created by a Flashloan attack, but at the cost of providing stale information. It is important to choose time windows carefully: when a time window is too wide, it won't reflect volatile asset prices, leaking opportunities to arbitrageurs. However the "instantaneous" price of an asset is often not a good data point: It is the most manipulable piece of Oracle data, and in any event it will almost always be stale by the time a transaction is executed. Oracles that collate a wide variety of source data, clean outliers from their data, and are well-regarded by the community, are more likely to be reliable. If an Oracle is off-chain, whether it reflects stale on-chain data or reliable and accurate data that is truly off-chain is an important consideration. Even an Oracle using a well-chosen TWAP can enable a liquidity pool or other DeFi structure to be manipulated, especially by taking advantage of flashloans and flashswaps to cheaply raise funds. If an asset targeted for manipulation has insufficient liquidity this can render it vulnerable to large price swings by an attacker holding only a relatively small amount of liquidity. The second important consideration when using Oracles is that of a graceful failure scenario. What happens if an Oracle no longer returns data, or suddenly returns an unlikely value? At least one protocol has suffered losses due to 'hanging' on a minimum value in the rare event of a price crash rather than truly dropping to zero, with traders who accumulated large amounts of a near zero-priced asset able to sell it back to the protocol. Hardcoding a minimum or maximum value can lead to problems reflecting reality.

External Interactions and Re-entrancy Attacks

Code that relies on external code can introduce multiple attack vectors. This includes cases where an external dependency contains malicious code or has been subject to malicious manipulation through security vulnerabilities. However, failure to adequately manage the possible outcomes of an external call can also introduce security vulnerabilities. One of the most commonly cited vulnerabilities in Ethereum Smart Contracts is Re-entrancy Attacks. These attacks allow malicious contracts to make a call back into the contract that called it before the originating contract's function call has been completed. This effect causes the calling contract to complete its processing in unintended ways, for example, by making unexpected changes to its state variables. A Read-only Re-entrancy Attack arises when a view function is reentered. These are a particular additional danger because such functions often lack safeguards since they don't modify the contract's state. However, if the state is inconsistent, incorrect values could be reported. This deception can mislead other protocols into reading inaccurate state values, potentially leading to unintended actions or outcomes. This issue primarily affects other contracts that rely on the accurate reporting of state from these view functions, rather than the contract itself being re-entered. Consequently, third parties that call a Smart Contract, and protocols that are composed of a Set of Contracts are most likely to be vulnerable to read-only re-entrancy.

Signature Mechanisms

Some requirements in the document refer to Malleable Signatures. These are signatures created according to a scheme constructed so that, given a message and a signature, it is possible to efficiently compute the signature of a different message - usually one that has been transformed in specific ways. While there are valuable use cases that such signature schemes allow, if not used carefully they can lead to vulnerabilities, which is why this specification seeks to constrain their use appropriately. In a similar vein, Hash Collisions could occur for hashed messages where the input used is malleable, allowing the same signature to be used for two distinct messages.

Other requirements in the document are related to exploits which take advantage of ambiguity in the input used to created the signed message. When a signed message does not include enough identifying information concerning where, when, and how many times it is intended to be used, the message signature could be used (or reused) in unintended functions, contracts, chains, or at unintended times.

For more information on this topic, and the potential for exploitation, see also [[chase]].

Gas and Gas Prices

Gas Griefing is the deliberate abuse of the Gas mechanism that Ethereum uses to regulate the consumption of computing power, to protect against unexpected or adverse outcome such as a Denial of Service attack. Because Ethereum is designed with the Gas mechanism as a regulating feature, it is insufficient to simply check that a transaction has enough Gas; checking for Gas Griefing needs to take into account the goals and business logic that the Tested Code implements.

Gas Siphoning is another abuse of the Gas mechanism that Ethereum uses to regulate the consumption of computing power, where attackers steal Gas from vulnerable contracts either to deny service or for their own gain (e.g. to mint Gas Tokens). Similar to Gas Griefing, checking for Gas Siphoning requires careful consideration of the goals and business logic that the Tested Code implements.

Gas Tokens use Gas when minted and free slightly less Gas when burned, provided the EVM refunds a sufficient quantity of Gas for clearing the state. Gas Tokens minted when Gas prices are low can be burned to subsidize Ethereum transactions when Gas prices are high. On Ethereum's main chain, Gas refunds were removed with the London hard fork that deployed [[EIP-3529]] in August 2021, effectively disabling Gas Tokens.

In addition, a common feature of Ethereum network upgrades is to change the Gas Price of specific operations. EEA EthTrust Certification only applies for the EVM version(s) specified; it is not valid for other EVM versions. Thus it is important to recheck code to ensure its security properties remain the same across network upgrades, or take remedial action.

MEV (Maliciously Extracted Value)

MEV, used in this document to mean "Maliciously Extracted Value", refers to the potential for block producers or other paticipants in a blockchain to extract value that is not intentionally given to them, in other words to steal it, by maliciously reordering transactions, as in Ordering Attacks, or suppressing them.

Some MEV attacks can be prevented by careful consideration of the information that is included in a transaction, including the parameters required by a contract. Other mitigation strategies include those that protect against Ordering Attacks. The Ethereum Foundation maintains a set of information resources regarding MEV [[EF-MEV]].

Ordering Attacks

Various attacks are related to a malicious rearrangement of transactions in a block,for example by reordering, censoring, or inserting particular transactions. While a primary motivation for this attack class is to facilitate MEV attacks, they can also be used to set up conditions for other types of attack. There are various types of Ordering Attacks:

Censorship Attacks: A block processor actively suppresses a proposed transaction, for their own benefit.
Front-Running: Based on transactions that are visible before they are added to a block, allowing a malicious participant to submit an alternative transaction, frustrating the aim of the original transaction.

In a system designed to attest original authorship, a malicious participant uses the information in a claim of authorship to create a rival claim, and adds their claim to a block first providing grounds for them to falsely claim to be the author. Implemented on a repeated basis this is an effective Denial of Service (DoS) attack against the service itself.
Back-Running: Similar to Front-Running, except the attacker places their transactions after the one they are attacking.
Sandwich Attacks: An attacker places a victim's transaction undesirably between two other transactions.

An attacker creates a swap transaction to buy a particular token, insert it block before a victim's buy transaction, artificially driving the price up for the victim, and insert a corresponding sell transaction at the increased valuation, providing no-risk profit for the attacker at the victim's expense.

Source code, pragma, and compilers

This version of the specification requires the compiled bytecode as well as the Solidity Source Code that together constitute the Tested Code. Solidity is by a large measure the most common programming language for Ethereum smart contracts, and benefits of requiring source code in Solidity include that it simplifies a number of tests, and that there is substantial security research done on Solidity source code. Solidity allows the source code to specify the Solidity compiler version used with a pragma statement. This specification currently has no requirement for a specific pragma, but it is good practice to ensure that the pragma refers to a bounded set of Solidity compiler versions, where it is known that those Solidity compiler versions produce identical bytecode from the given source code. There are some drawbacks to requiring Solidity Source code. The most obvious is that some code that is not written in Solidity. Different languages have different features and often support different coding styles. Perhaps more important, it means that a deployed contract written in Solidity cannot be tested directly without someone making the source code available. Another important limitation introduced by reading source code is that it is subject to Homoglyph Attacks, where characters that look the same but are different such as Latin "p" and Cyrillic "р", can deceive people visually reading the source code, to disguise malicious behaviour. There are related attacks that use features such as Unicode Direction Control Characters or take advantage of inconsistent normalisation of combining characters to achieve the same type of deceptions.

Contract Deployment

This specification primarily addresses vulnerabilities that arise in Smart Contract code. However it is important to note that the deployment of a smart contract is often a crucial element of protocol operation. Some aspects of smart contract security primarily depend on how the Tested Code gets deployed. Even audited protocols can be easily exploited if deployed naively. Code written for a specific blockchain might depend on features available in that blockchain. When the code is deployed to a different chain that is compatible, the difference in features can expose a vulnerability. For any contract deployed to a blockchain or parachain that uses a patched fork of the EVM, common security assumptions might no longer apply at the VM level. It is valuable to deploy EEA EthTrust Certified contracts to a testnet for each chain first, and undergo thorough penetration testing. Of particular concern is the issue of Upgradable Contracts, and any contract with an initializer function in deployment. Many protocols have been hacked due to accidentally leaving their initializer functions unprotected, or using a non-atomic deployment in which the initializing function is not called in the same transaction as the contract deployment. This scenario is ripe for Front-running attacks, and can result in protocol takeover by malicious parties, and theft or loss of funds. Initializing a contract in the same transaction as its deployment reduces the risk that a malicious actor takes control of the contract. Moreover, the deployment implications of assigning access roles to msg.sender or other variables in constructors and initializers need careful consideration. This is discussed further in requirements. Several libraries and tools exist specifically for safe proxy usage and safe contract deployment. From command-line tools to libraries to sophisticated UI-based deployment tools, many solutions exist to prevent unsafe proxy deployments and upgrades. Using access control for a given contract's initializer, and limiting the number of times an initializer can be called on or after deployment, can enhance safety and transparency for the protocol itself and its users. Furthermore, a function that disables the ability to re-initialize an Execution Contract can prevent any future initializer calls after deployment, preventing later attacks or accidents. Although this specification does not require that Tested Code has been deployed, some requirements are more easily tested when code has been deployed to a blockchain, or possibly in some cases can only be thoroughly tested "in situ".

Post-deployment Monitoring

While monitoring Smart Contracts after deployment is beyond the formal scope of this specification, it is an important consideration for Smart Contract security. New attack techniques arise from time to time, and some attacks can only be prevented by active measures implemented in real time. Monitoring of on-chain activity can help detect attacks before it is too late to stop them. Monitoring, backed by an automated dataset, can enable identifying an attack that has occurred elsewhere, even on other blockchains. Automated monitoring can facilitate rapid response, producing alerts or automatically initiating action, improving the security of contracts that might be compromised when security responses are delayed by even a few blocks. However, it can be difficult to determine the difference between an attack and anamolous behaviour on the part of individuals. Relying purely on automated monitoring can expose a blockchain to the risk that a malicious actor deliberately triggers an automated security response to damage a blockchain or project, analogous to a Denial of Service attack.

Network Upgrades

The EVM, or Ethereum Virtual Machine, acts as a distributed state machine for the Ethereum network, computing state changes resulting from transactions. The EVM maintains the network state for simple transfers of Ether, as well as more complex Smart Contract interactions. In other words, it is the "computer" (although in fact it is software) that runs the code of Smart Contracts. From time to time the Ethereum community implements a Network Upgrade, sometimes also called a Hard Fork. This is a change to Ethereum that is backwards-incompatible. Because they typically change the EVM, Ethereum Mainnet Network Upgrades generally correspond to EVM versions. A Network Upgrade can affect more or less any aspect of Ethereum, including changing EVM opcodes or their Gas price, changing how blocks are added, or how rewards are paid, among many possibilities. Because Network Upgrades are not guaranteed to be backwards compatible, a newer EVM version can process bytecode in unanticipated ways. If a Network Upgrade changes the EVM to fix a security problem, it is important to consider that change, and it is a good practice to follow that upgrade. Because claims of conformance to this specification are only valid for specific EVM versions, a Network Upgrade can mean an updated audit is needed to maintain valid EEA EthTrust Certification for a current Ethereum network. Network Upgrades typically only impact a few features. This helps limit the effort necessary to audit code after an upgrade: often there will be no changes that affect the Tested Code, or review of a small proportion that is the only part affected by a Network Upgrade will be sufficient to renew EEA EthTrust Certification.

EEA EthTrust Security Levels

EEA EthTrust Certification is available at three Security Levels. The Security Levels describe minimum requirements for certifications at each Security Level: [S], [M], and [Q]. These Security Levels provide successively stronger assurance that a smart contract does not have specific security vulnerabilities.

- [Security Level [S]](#sec-levels-one) is designed so that for most cases, where common features of Solidity are used following well-known patterns, Tested Code can be certified by an automated "static analysis" tool. - [Security Level [M]](#sec-levels-two) mandates a stricter static analysis. It includes requirements where a human auditor is expected to determine whether use of a feature is necessary, or whether a claim about the security properties of code is justified. - [Security Level [Q]](#sec-levels-three) provides analysis of the business logic the Tested Code implements, and that the code not only does not exhibit known security vulnerabilities, but also correctly implements what it claims to do. The optional , correctly implemented, further enhance the Security of smart contracts. However it is not necessary to test them to conform to this specification. The vulnerabilities addressed by this specification come from a number of sources, including Solidity Security Alerts [[solidity-alerts]], the Smart Contract Weakness Classification [[swcregistry]], TMIO Best practices [[tmio-bp]], various sources of Security Advisory Notices, discussions in the Ethereum community and researchers presenting newly discovered vulnerabilities, and the extensive practical experience of participants in the Working Group.

Security Level [S]

EEA EthTrust Certification at Security Level [S] is intended to allow an unguided automated tool to analyze most contracts' bytecode and source code, and determine whether they meet the requirements. For some situations that are difficut to verify automatically, usually only likely to arise in a small minority of contracts, there are higher-level Overriding Requirements that can be fulfilled instead to meet a requirement for this Security Level.

To be eligible for EEA EthTrust Certification for Security Level [S], Tested code MUST fulfil all Security Level [S] requirements, unless it meets the applicable Overriding Requirement(s) for each Security Level [S] requirement it does not meet directly.

[S] Encode Hashes with chainid
Tested code MUST create hashes for transactions that incorporate chainid values following the recommendation described in [[!EIP-155]]

[[!EIP-155]] describes an enhanced hashing rule, incorporating a chain identifier in the hash. While this only provides a guarantee against replay attacks if there is a unique chain identifier, using the mechanism described provides a certain level of robustness and makes it much more difficult to execute a replay attack.

[S] No CREATE2
Tested code MUST NOT contain a CREATE2 instruction.
unless it meets the Set of Overriding Requirements

[M] Protect CREATE2 Calls, and
[M] Document Special Code Use,

The CREATE2 opcode provides the ability to interact with addresses that do not exist yet on-chain but could possibly eventually contain code. While this can be useful for deployments and counterfactual interactions with contracts, it can allow external calls to code that is not yet known or can be altered, and could turn out to be malicous or insecure due to errors or weak protections.

[S] No tx.origin
Tested code MUST NOT contain a tx.origin instruction
unless it meets the Overriding Requirement [Q] Verify tx.origin Usage

tx.origin is a global variable in Solidity which returns the address of the account that sent the transaction. A contract using tx.origin can allow an authorized account to call into a malicious contract, enabling the malicious contract to pass authorization checks in unintended cases. It is better to use msg.sender for authorization instead of tx.origin.

See also SWC-115 [[swcregistry]] for examples.

[S] No Exact Balance Check
Tested code MUST NOT test that the balance of an account is exactly equal to (i.e. ==) a specified amount or the value of a variable
unless it meets the Overriding Requirement [M] Verify Exact Balance Checks.

Testing the balance of an account as a basis for some action has risks associated with unexpected receipt of ether or another token, including tokens deliberately transfered to cause such tests to fail as an MEV attack. See also the Related Requirements [M] Sources of Randomness, [M] Don't Misuse Block Data, and [Q] Protect against MEV Attacks, subsection of the Security Considerations for this specification, [SWC-132](https://swcregistry.io/docs/SWC-132) in [[swcregistry]] for examples, and improper locking as described in [[CWE-667]].

[S] No Hashing Consecutive Variable Length Arguments
Tested Code MUST NOT use abi.encodePacked() with consecutive variable length arguments.

The elements of each variable-length argument to abi.encodePacked() are packed in order prior to hashing. Hash Collisions are possible by rearranging the elements between consecutive, variable length arguments while maintaining that their concatenated order is the same.

[S] No selfdestruct()
Tested code MUST NOT contain the selfdestruct() instruction or its now-deprecated alias suicide()
unless it meets the Set of Overriding Requirements

[M] Protect Self-destruction, and
[M] Document Special Code Use.

If the `selfdestruct()` instruction (or its deprecated alternative `suicide()`) is not carefully protected, malicious code can call it and destroy a contract, sending any Ether held by the contract, thus potentially stealing it. It is also possible to use it in combination with `CREATE2` to change the code at a particular address. This feature can break immutability and trustless guarantees to introduce numerous security issues. In addition, once the contract has been destroyed any Ether sent is simply lost, unlike when a contract is disabled which causes a transaction sending Ether to revert. `selfdestruct()` is officially deprecated, its usage discouraged, since Solidity compiler version 0.8.18 [[solidity-release-818]]. See also [SWC-106](https://swcregistry.io/docs/SWC-106) in [[swcregistry]], [[EIP-6049]].

[S] No assembly {}
Tested Code MUST NOT contain the assembly {} instruction
unless it meets the Set of Overriding Requirements

[M] Avoid Common assembly {} Attack Vectors,
[M] Document Special Code Use,
[M] Compiler Bug SOL-2022-5 in `assembly {}`,
[M] Compiler Bug SOL-2022-7,
[M] Compiler Bug SOL-2022-4,
[M] Compiler Bug SOL-2021-3, and
if using Solidity compiler version 0.5.5 or 0.5.6, [M] Compiler Bug SOL-2019-2 in `assembly {}` in [[EthTrust-sl-v1]].

The `assembly {}` instruction allows lower-level code to be included. This give the authors much stronger control over the bytecode that is generated, which can be used for example to optimise gas usage. However, it also potentially exposes a number of vulnerabilites and bugs that are additional attack surfaces, and there are a number of ways to use `assembly {}` to introduce deliberately malicious code that is difficult to detect.

Text and homoglyphs

[S] No Unicode Direction Control Characters
Tested code MUST NOT contain any of the Unicode Direction Control Characters U+2066, U+2067, U+2068, U+2029, U+202A, U+202B, U+202C, U+202D, or U+202E
unless it meets the Overriding Requirement [M] No Unnecessary Unicode Controls.

Changing the apparent order of characters through the use of invisible Unicode direction control characters can mask malicious code, even in viewing source code, to deceive human auditors. More information on Unicode direction control characters is available in the W3C note [How to use Unicode controls for bidi text](https://www.w3.org/International/questions/qa-bidi-unicode-controls) [[unicode-bdo]].

External Calls

[S] Check External Calls Return
Tested Code that makes external calls using the Low-level Call Functions (i.e. call(), delegatecall(), staticcall(), and send()) MUST check the returned value from each usage to determine whether the call failed,
unless it meets the Overriding Requirement [M] Handle External Call Returns.

Normally, exceptions in calls cause a revert. This will "bubble up", unless it is handled in a `try`/`catch`. However Solidity defines a set of Low-level Call Functions: - call(), - delegatecall(), - staticcall(), and - `send()`. Calls using these functions behave differently. Instead of reverting on failure they return a boolean indicating whether the call completed successfully. Not testing explicitly for the return value could lead to unexpected behavior in the caller contract. Relying on these calls reverting on failure will lead to unexpected behaviour when they are not successful. See also [SWC-104](https://swcregistry.io/docs/SWC-104) in [[swcregistry]], error handling documentation in [[error-handling]], unchecked return value as described in [[CWE-252]], and the Related Requirements: [S] Use Check-Effects-Interaction, [M] Handle External Call Returns, and [Q] Verify External Calls.

[S] Use Check-Effects-Interaction
Tested code that makes external calls MUST use the Checks-Effects-Interactions pattern to protect against Re-entrancy Attacks
unless it meets the Set of Overriding Requirements

[M] Protect external calls, and
[M] Document Special Code Use,

or it meets the Set of Overriding Requirements

[Q] Verify External Calls,
[Q] Document Contract Logic,
[Q] Document System Architecture, and
[Q] Implement as Documented.

The Checks-Effects-Interactions pattern ensures that validation of the request, and changes to the state variables of the contract, are performed before any interactions take place with other contracts. When contracts are implemented this way, the scope for Re-entrancy Attacks is reduced significantly. As well as checking the particular contract effects, it is possible as part of this pattern to test protocol invariants, to provide a further assurance that a request doesn't produce an unsafe outcome. See also , the explanation of "Checks-Effects-Interactions" [[c-e-i]] in "Solidity Security Considerations" [[solidity-security]], "[Checks Effects Interactions](https://fravoll.github.io/solidity-patterns/checks_effects_interactions.html)" in [[solidity-patterns]], and [[freipi]].

[S] No delegatecall()
Tested Code MUST NOT contain the delegatecall() instruction
unless it meets the Set of Overriding Requirements:

[M] Protect External Calls, and
[M] Document Special Code Use,

or it meets the Set of Overriding Requirements

[Q] Verify External Calls,
[Q] Document Contract Logic,
[Q] Document System Architecture, and
[Q] Implement as Documented.

The `delegatecall()` instruction enables an external contract to manipulate the state of a contract that calls it, because the code is run with the caller's balance, storage, and address.

Compiler Bugs

There are a number of known security bugs in different Solidity compiler versions. The requirements in this subsection ensure that Tested Code does not trigger these bugs. The name of the requirement includes the uid first recorded for the bug in [[!solidity-bugs-json]], as a key that can be used to find more information about the bug. [[solidity-bugs]] describes the conventions used for the JSON-formatted list of bugs. The requirements in this subsection are ordered according to the latest Solidity compiler versions that are vulnerable.

Implementing the Recommended Good Practice [GP] Use Latest Compiler means that Tested Code passes all requirements in this subsection.

Some compiler-related bugs are in the as Security Level [M] requirements, either because they are Overriding Requirements for requirements in this subsection, or because they are part of a Set of Overriding Requirements for Security Level [S] requirements that already ensure that the bug cannot be triggered. Some bugs were introduced in known Solidity compiler versions, while others are known or assumed to have existed in all Solidity compiler versions until they were fixed.

[S] Compiler Bug SOL-2023-3
Tested code that includes Yul code and uses the `verbatim` instruction twice, in each case surrounded by identical code, MUST disable the Block Deduplicator when using a Solidity compiler version between 0.8.5 and 0.8.22 (inclusive).

From Solidity compiler version 0.8.5 until 0.8.22, the block deduplicator incorrectly processed `verbatim` items, meaning that sometimes it conflated two items based on the code surrounding them instead of comparing them properly. See also the [8 November 2023 security alert](https://soliditylang.org/blog/2023/11/08/verbatim-invalid-deduplication-bug/).

[S] Compiler Bug SOL-2022-6
Tested code that ABI-encodes a tuple (including a `struct`, `return` value, or a parameter list) that includes a dynamic component with the ABIEncoderV2, and whose last element is a calldata static array of base type `uint` or `bytes32`, MUST NOT use a Solidity compiler version between 0.5.8 and 0.8.15 (inclusive).

From Solidity compiler version 0.5.8 until 0.8.15, ABI encoding a tuple whose final component is a `calldata` static array of base type `uint` or `bytes32` with the ABIEncoderV2 could result in corrupted data. See also the [8 August 2022 security alert](https://blog.soliditylang.org/2022/08/08/calldata-tuple-reencoding-head-overflow-bug/).

[S] Compiler Bug SOL-2022-5 with .push()
Tested code that

copies bytes arrays from calldata or memory whose size is not a multiple of 32 bytes, and
has an empty .push() instruction that writes to the resulting array,

MUST NOT use a Solidity compiler version older than 0.8.15.

Until Solidity compiler version 0.8.15 copying memory or calldata whose length is not a multiple of 32 bytes could expose data beyond the data copied, which could be observable using code through `assembly {}`. See also the [15 June 2022 security alert](https://blog.soliditylang.org/2022/06/15/dirty-bytes-array-to-storage-bug/) and the Related Requirement [M] Compiler Bug SOL-2022-5 in `assembly {}`.

[S] Compiler Bug SOL-2022-3
Tested code that

uses `memory` and `calldata` pointers for the same function, and
changes the data location of a function during inheritance, and
performs an internal call at a location that is only aware of the original function signature from the base contract

MUST NOT use a Solidity compiler version between 0.6.9 and 0.8.12 (inclusive).

Solidity compiler versions from 0.6.9 until it was fixed in 0.8.13 had a bug that incorrectly allowed internal or public calls to use a simpification only valid for external calls, treating `memory` and `calldata` as equivalent pointers. See also the [17 May 2022 security alert](https://blog.soliditylang.org/2022/05/17/data-location-inheritance-bug/).

[S] Compiler Bug SOL-2022-2
Tested code with a nested array that

passes it to an external function, or
passes it as input to abi.encode(), or
uses it in an event

MUST NOT use a Solidity compiler version between 0.6.9 and 0.8.12 (inclusive).

Solidity compiler versions from 0.5.8 until it was fixed in 0.8.13 had a bug that meant a single-pass encoding and decoding of a nested array could read data beyond the `calldatasize()`. See also the [17 May 2022 security alert](https://blog.soliditylang.org/2022/05/17/calldata-reencode-size-check-bug/).

[S] Compiler Bug SOL-2022-1
Tested code that

uses Number literals for a bytesNN type shorter than 32 bytes, or
uses String literals for any bytesNN type,

and passes such literals to abi.encodeCall() as the first parameter, MUST NOT use Solidity compiler version 0.8.11 nor 0.8.12.

Solidity defines a set of types for variables known collectively as bytesNN or Fixed-length Variable types, that specify the length of the variable as a fixed number of bytes, following the pattern - `bytes1` - `bytes2` - ... - `bytes10` - ... - `bytes32` Solidity compiler versions 0.8.11 and 0.8.12 had a bug that meant literal parameters were incorrectly encoded by `abi.encodeCall()` in certain circumstances. See also the [16 March 2022 security alert](https://blog.soliditylang.org/2022/03/16/encodecall-bug/).

[S] Compiler Bug SOL-2021-4
Tested Code that uses custom value types shorter than 32 bytes MUST NOT use Solidity compiler version 0.8.8.

Solidity compiler version 0.8.8 had a bug that assigned a full 32 bytes of storage to custom types that did not need it. This can be misused to enable reading arbitrary storage, as well as causing errors if the Tested Code contains code compiled using different Solidity compiler versions. See also the [29 September 2021 security alert](https://blog.soliditylang.org/2021/09/29/user-defined-value-types-bug/)

[S] Compiler Bug SOL-2021-2
Tested code that uses abi.decode() on byte arrays as `memory` MUST NOT use the ABIEncoderV2 with a Solidity compiler version between 0.4.16 and 0.8.3 (inclusive).

Solidity compiler version 0.4.16 introduced a bug, fixed in 0.8.4, that meant the ABIEncoderV2 incorrectly validated pointers when reading `memory` byte arrays, which could result in reading data beyond the array area due to an overflow error in calculating pointers. See also the [21 April 2021 security alert](https://blog.soliditylang.org/2021/04/21/decoding-from-memory-bug/).

[S] Compiler Bug SOL-2021-1
Tested code that has 2 or more occurrences of an instruction keccak(mem,length) where

the values of mem are equal, and
the values of length are unequal, and
the values of length are not multiples of 32,

MUST NOT use the Optimizer with a Solidity compiler version older than 0.8.3.

Solidity compiler versions before 0.8.3 had an Optimizer bug that meant keccak hashes, calculated for the same content but different lengths that were not multiples of 32 bytes, incorrectly used the first value from cache instead of recalculating. See also the [23 March 2021 security alert](https://blog.soliditylang.org/2021/03/23/keccak-optimizer-bug/).

[S] Use a Modern Compiler
Tested code MUST NOT use a Solidity compiler version older than 0.8.0, unless it meets all the following requirements from the EEA EthTrust Security Levels Specification Version 2, as Overriding Requirements:

AND

Tested code MUST NOT use a Solidity compiler version older than 0.6.0, unless it meets all the following requirements from the EEA EthTrust Security Levels Specification Version 1, as Overriding Requirements:

There are a number of known compiler bugs that affect Solidity Compiler Versions older than 0.6.0, but research into compiler bugs tends to focus on those that affect relatively modern Solidity Compiler versions, so any further bugs in older Solidity Compiler versions are only likely to be discovered and generally known as a result of being exploited. It is a good practice to use a modern Solidity Compiler Version. In the rare cases where it is not possible to use a Solidity Compiler Version later than 0.6.0, it is possible to achieve EEA EthTrust Certification by conforming to the relevant Overriding Requirements that were defined in version 1 of this specification [[EthTrust-sl-v1]]. See also the Related Requirement [M] Use a Modern Compiler, covering Solidity Compiler bugs that require review for Security Level [M].

[S] No Ancient Compilers
Tested code MUST NOT use a Solidity compiler version older than 0.3.

Compiler bugs are not tracked for compiler Solidity compiler versions older than 0.3. There is therefore a risk that unknown bugs create unexpected problems. See also "SOL-2016-1" in [[!solidity-bugs-json]].

Security Level [M]

EEA EthTrust Certification at Security Level [M] means that the Tested Code has been carefully reviewed by a human auditor or team, doing a manual analysis, and important security issues have been addressed to their satisfaction.

This level includes a number of Overriding Requirements for cases when Tested Code does not meet a Security Level [S] requirement directly, because it uses an uncommon feature that introduces higher risk, or because in certain circumstsances testing that the requirement has been met requires human judgement. Passing the relevant Overriding Requirement tests that the feature has been implemented sufficiently well to satisfy the auditor that it does not expose the Tested Code to the known vulnerabilities identified in this Security Level.

[M] Pass Security Level [S]
To be eligible for EEA EthTrust certification at Security Level [M], Tested code MUST meet the requirements for .

[M] Explicitly Disambiguate Evaluation Order
Tested code MUST NOT contain statements where variable evaluation order can result in different outcomes

The evaluation order of functions is not entirely deterministic in Solidity, and is not guaranteed to be consistent across Solidity compiler versions. This means that the outcome of a statement calling multiple functions that each have side effects on shared stateful objects can lead to different outcomes if the order that the called functions were evaluated varies. Also, the evaluation order in events and the instructions `addmod` and `modmul` generally does not follow the **usual** pattern, meaning that Tested Code using those instructions could produce unexpected outcomes. A common approach to addressing this vulnerability is the use of temporary results, to ensure evaluation order will be the same.

If functions g and h change the state of any variable that the result of a function f depends on, then it is important that the code **explicitly** determines the order of execution, for example using a temporary variable as in the following example:


  // SPDX-License-Identifier: MIT
  pragma solidity 0.8.18;

  uint256 public myNumber;
  uint256 public yourNumber;

  function firstTransform(uint256 someNumber) public returns uint256 {
    myNumber += 1; // side effect
    return someNumber.mul(myNumber);
  }

  function secondTransform(uint256 someNumber) public returns uint256 {
    yourNumber += 3; // side effect
    return someNumber.div(yourNumber);
  }

  function deterministicResult(uint256 someNumber) public returns uint256 {
    uint256 firstResult = firstTransform(someNumber);
    return secondTransform(firstResult);
  }

See also [[solidity-underhanded-richards2022]], [[solidity-cheatsheet]], and the [19 July 2023 Solidity Compiler Security Bug notification](https://blog.soliditylang.org/2023/07/19/full-inliner-non-expression-split-argument-evaluation-order-bug/) for Solidity Compiler Security Bug 2023-2, noted in [[solidity-bugs-json]].

[M] Verify Exact Balance Checks
Tested code that checks whether the balance of an account is exactly equal to (i.e. ==) a specified amount or the value of a variable. MUST protect itself against transfers affecting the balance tested.
This is an Overriding Requirement for [S] No Exact Balance Check.

If a Smart Contract checks that an account balance is some particular exact value at some point during its execution, it is potentially vulnerable to an attack, where a transfer to the account can be used to change the balance of the account causing unexpected results such as a transaction reverting. If such checks are used it is important that they are protected against this possibility.

Text and homoglyph attacks

The requirements in this section are related to the security advisory [[CVE-2021-42574]] and [[CWE-94]], "Improper Control of Generation of Code", also called "Code Injection".

[M] No Unnecessary Unicode Controls
Tested code MUST NOT use Unicode direction control characters unless they are necessary to render text appropriately, and the resulting text does not mislead readers.
This is an Overriding Requirement for [S] No Unicode Direction Control Characters.

Security Level [M] permits the use of Unicode direction control characters in text strings, subject to analysis of whether they are necessary.

[M] No Homoglyph-style Attack
Tested code MUST not use homoglyphs, Unicode control characters, combining characters, or characters from multiple Unicode blocks, if the impact is misleading.

Substituting characters from different alphabets or that can be hard to distinguish, or using direction control characters or combining characters, can be used to mask malicious code, for example by presenting variables or function names designed to mislead auditors. These attacks are known as Homoglyph Attacks. Several approaches to successfully exploiting this issue are described in [[Ivanov]]. In the rare case when there is a valid use of characters from multiple Unicode blocks (see [[unicode-blocks]]) in a variable name or label (most likely to be mixing two languages in a name), this requirement allows them to achieve EEA EthTrust Certification so long as they do not mislead or confuse. This level requires checking for homoglyph attacks including those within a single character set, such as the use of "í" in place of "i" or "ì", "ت" for "ث", or "1" for "l", as well as across character sets such as e.g. Latin "a" and Cyrillic "а" or the Mathematical character "𝚒" and latin "i". If the reviewer judges that the result is unnecessarily misleading or confusing, the Tested Code does not meet this requirement. See also the Related Requirement: [S] No Unicode Direction Control Characters.

External Calls

[M] Protect External Calls
For Tested code that makes external calls:

all addresses called by the Tested code MUST correspond to the exact code of the Set of Contracts tested, and
all contracts called MUST be within the Tested code, and
all contracts called MUST be controlled by the same entity, and
the protection against Re-entrancy Attacks for the Tested Code MUST be equivalent to what the Checks-Effects-Interactions pattern offers,

unless it meets the Set of Overriding Requirements

[Q] Verify External Calls,
[Q] Document Contract Logic,
[Q] Document System Architecture, and
[Q] Implement as Documented.

This is an Overriding Requirement for [S] Use Check-Effects-Interaction.

EEA EthTrust Certification at Security Level [M] allows calling within a set of contracts that form part of the Tested Code. This ensures all contracts called are audited together at this Security Level. If a contract calls a well-known external contract that is not audited as part of the Tested Code, it is possible to certify conformance to this requirement through the Overriding Requirements, which allow the certifier to claim on their own judgement that the contracts called provide appropriate security. The extended requirements around documentation of the Tested Code that apply when claiming conformance through implementation of the Overriding Requirements in this case reflect the potential for very high risk if the external contracts are simply assumed by a reviewer to be secure because they have been widely used. Unless the Tested Code deploys contracts itself, and retrieves their address accurately for calling, it is necessary to check that the contracts are really deployed at the addresses assumed in the Tested Code. The same level of protection against Re-entrancy Attacks has to be provided to the Tested Code overall as for the Security Level [S] requirement [S] Use Check-Effects-Interaction.

[M] Avoid Read-only Re-entrancy Attacks
Tested Code that makes external calls MUST protect itself against Read-only Re-entrancy Attacks.

As described in , code that reads information from a function can end up reading inconsistent or incorrect information. When the Tested Code calls a function in which this possibility arises, the calling code needs an appropriate mechanism to avoid it happening. One potential mechanism is for view functions to have a modifier that checks whether the data is currently in an inconsistent state, in the manner of a lock function. This enables calling code to explicitly avoid viewing inconsistent data.

This is a simple case of read-only-reentrancy attack. The contract Reentered has a view function that determines a price based on totalSupply and numberOfEther in a specific LPToken, and a public non-Reentrant function that sells the LP token and sends the ETH received to the users that called it. When the attack function is called in the Attacker contract it will reduce the value of numberOfEther in the Reentered contract, and trigger the Attacker contract's receive() function, before updating totalSupply, by preventing the instruction LPToken.burnFrom(msg.sender, amount); from executing. The receive() function of the Attacker contract calls the buyToken() function of the Reentered contract under attack. Since the value of numberOfEther has been modified, but the totalSupply has not, the buyToken() function in the attacked contract will request the wrong number of tokens in exchange.


  // SPDX-License-Identifier: MIT
  pragma solidity 0.8.18;

  Contract Reentered{

    function getPrice() public returns uint256 {
      return totalSupply / numberOfEther; //view function that calculates the price of the LP token
    }

    function sellLPToken(uint256 amount) public nonReentrant {
      [...]
      numberOfEther -= amountToReceive; //reduces the numberOfEth by the amount that will be sent to the user
      (bool success, ) = msg.sender.call{value: amountToReceive}(""); //transfers ETH when the LP token is sold
      require(success, "Transfer failed");
      LPToken.burnFrom(msg.sender, amount); //burns the LP token and changes the totalsupply by doing so
    }

  }

  Contract Attacked{

    function buyToken(uint256 amount) public{
      [...]
      uint256 tokenToReceive = Reentered.getPrice() * amount;
      [...]
    }

  }

  Contract Attacker{

    function attack() public{
      Reentered.sellLPToken(LPToken.balanceOf(address(this))); //starts the transaction to sell the LP tokens on the reentered contract
    } // this will trigger the Eth transfer that will activate the receive() function on this contract


    receive() external payable{
      Attacked.buyToken(1000);
    }

  }

[M] Handle External Call Returns
Tested Code that makes external calls MUST reasonably handle possible errors.
This is an Overriding Requirement for [S] Check External Calls Return.

It is important that Tested Code works as expected, to the satisfaction of the auditor, when the return value is the result of a possible error, such as if a call to a non-existent function triggers a fallback function instead of simply reverting, or an external call using a Low-level Call Function does not revert. See also the Related Requirement: [Q] Process All Inputs.

Documented Defensive Coding

[M] Document Special Code Use
Tested Code MUST document the need for each instance of:

CREATE2,
assembly {},
selfdestruct() or its deprecated alias suicide(),
external calls,
delegatecall(),
code that can cause an overflow or underflow,
block.number or block.timestamp, or
Use of oracles and pseudo-randomness,

and MUST describe how the Tested Code protects against misuse or errors in these cases, and the documentation MUST be available to anyone who can call the Tested Code.

This is part of several Sets of Overriding Requirements, one for each of

[S] No CREATE2,
[S] No selfdestruct(),
[S] No assembly {},
[S] Use Check-Effects-Interaction, and
[S] No delegatecall().

There are legitimate uses for all of these coding patterns, but they are also potential causes of security vulnerabilities. Security Level [M] therefore requires testing that the use of these patterns is explained and justified, and that they are used in a manner that does not introduce known vulnerabilities. The requirement to document the use of external calls applies to all external calls in the tested code, whether or not they meet the Related Requirement [S] Use Check-Effects-Interaction.

[M] Ensure Proper Rounding of Computations Affecting Value
Tested code MUST identify and protect against exploiting rounding errors:

The possible range of error introduced by such rounding MUST be documented.
Tested code MUST NOT unintentionally create or lose value through rounding.
Tested code MUST apply rounding in a way that does not allow round-trips "creating" value to repeat causing unexpectedly large transfers.

Smart Contracts typically implement mathematical formulas over real numbers using integer arithmetic. Such code can introduce rounding errors because integers and rational numbers whose size is bounded cannot precisely represent all real numbers in the same range.

If a procedure that uses rounding results in a predictable amount of error, that increases the value produced by the round-trip, it is possible to exploit that difference by repeating the procedure to cumulatively siphon a large sum.

A simple swap that rounds to the nearest usable number can mean that a round-trip effectively creates an off-by-one error, so swapping the right number of tokens back and forth will in each transaction produce more value than was started with:


// SPDX-License-Identifier: MIT
pragma solidity 0.8.18;

function xChangeTo(uint256 numberOfEth) public returns uint256 {
  return numberOfEth.mul(rateThatCausesRounding); //mul rounds to "nearest"
}

function xChangeFrom(uint256 numberOfOtherToken) public returns uint256 {
  return numberOfOtherToken.div(rateThatCausesRounding); //div rounds to "nearest"
}

To protect against this vulnerability, the "Keep the Change" approach ensures that any difference created does not provide an advantage to an attacker repeatedly calling a smart contract. It is important to note that differences do still accrue. A contract could use "over-servicing", repeatedly calling a swap protected by the "Keep the Change" approach, to steal from a user.

A simple approach to preventing an attacker from benefiting from rounding in a contract that implements a swap is to round so that the contract "keeps the change". There is then no advantage to an attacker in round tripping:


// SPDX-License-Identifier: MIT
pragma solidity 0.8.18;

function xChangeTo(uint256 numberOfEth) public returns uint256 {
  return numberOfEth.mulDown(rateThatCausesRounding); //round the result down
}

function xChangeFrom(uint256 numberOfOtherToken) public returns uint256 {
  return numberOfOtherToken.divDown(rateThatCausesRounding); //round the result down
}

This vulnerability has been discovered in practice in DeFi protocol Smart Contracts that could have put hundreds of millions of dollars at risk. Further explanation is available in the [presentation slides](https://archive.devcon.org/resources/6/tackling-rounding-errors-with-precision-analysis.pdf) for the DevCon 2023 talk [[DevCon-rounding]]. An example of a thorough mathematical analysis of integer rounding for an automated market maker is available in [[rounding-errors]]. This requirement is based on [[CWE-1339]] Insufficient Precision or Accuracy of a Real Number.

[M] Protect Self-destruction
Tested code that contains the selfdestruct() or suicide() instructions MUST

ensure that only authorised parties can call the method, and
MUST protect those calls in a way that is fully compatible with the claims of the contract author,

unless it meets the Overriding Requirement [Q] Enforce Least Privilege.

This is an Overriding Requirement for [S] No selfdestruct().

If the `selfdestruct()` instruction (or its deprecated alternative `suicide()`) is not carefully protected, malicious code can call it and destroy a contract, and potentially steal any Ether held by the contract. In addition, this can disrupt other users of the contract since once the contract has been destroyed any Ether sent is simply lost, unlike when a contract is disabled which causes a transaction sending Ether to revert. See also [SWC-106](https://swcregistry.io/docs/SWC-106) in [[swcregistry]].

[M] Avoid Common assembly {} Attack Vectors
Tested Code MUST NOT use the assembly {} instruction to change a variable unless the code cannot:

create storage pointer collisions, nor
allow arbitrary values to be assigned to variables of type `function`.

This is part of a Set of Overriding Requirements for [S] No assembly {}.

The `assembly {}` instruction provides a low-level method for developers to produce code in smart contracts. Using this approach provides great flexibility and control, for example to reduce gas cost. However it also exposes some possible attack surfaces where a malicious coder could introduce attacks that are hard to detect. This requirement ensures that two such attack surfaces that are well-known are not exposed. See also SWC-124 and SWC-127 [[swcregistry]], and the Related Requirements [M] Document Special Code Use, [M] Compiler Bug SOL-2022-7, [M] Compiler Bug SOL-2022-5 in `assembly {}`, [M] Compiler Bug SOL-2022-4, [M] Compiler Bug SOL-2021-3, and if using Solidity compiler version 0.5.5 or 0.5.6, [M] Compiler Bug SOL-2019-2 in `assembly {}` in [[EthTrust-sl-v1]].

[M] Protect CREATE2 Calls
For Tested Code that uses the CREATE2 instruction, any contract to be deployed using CREATE2

MUST be within the Tested Code, and
MUST NOT use any selfdestruct(), delegatecall() nor callcode() instructions, and
MUST be fully compatible with the claims of the contract author,

unless it meets the Set of Overriding Requirements

[Q] Verify External Calls,
[Q] Document Contract Logic,
[Q] Document System Architecture, and
[Q] Implement as Documented.

This is part of a Set of Overriding Requirements for [S] No CREATE2.

The CREATE2 opcode's ability to interact with addresses whose code does not yet exist on-chain makes it important to prevent external calls to malicous or insecure contract code that is not yet known. The Tested code needs to include any code that can be deployed using CREATE2, to verify protections are in place and the code behaves as the contract author claims. This includes ensuring that opcodes that can change the immutability or forward calls in the contracts deployed with CREATE2, such as selfdestruct(), delegatecall() and callcode(), are not present. If any of these opcodes are present, the additional protections and documentation required by the Overriding Requirements are necessary.

[M] Safe Overflow/Underflow
Tested code MUST NOT contain calculations that can overflow or underflow unless

there is a demonstrated need (e.g. for use in a modulo operation) and
there are guards around any calculations, if necessary, to ensure behavior consistent with the claims of the contract author.

There are a few rare use cases where arithmetic overflow or underflow is intended, or expected behaviour. It is important such cases are protected appropriately. Note that Solidity compiler version 0.8.0 introduced overflow protection that causes transactions to revert. See also [SWC-101](https://swcregistry.io/docs/SWC-101) in [[swcregistry]].

[M] Sources of Randomness
Sources of randomness used in Tested Code MUST be sufficiently resistant to prediction that their purpose is met.

This requirement involves careful evaluation for each specific contract and case. Some uses of randomness rely on no prediction being more accurate than any other. For such cases, values that can be guessed at with some accuracy or controlled by miners or validators, like block difficulty, timestamps, and/or block numbers, introduces a vulnerability. Thus a "strong" source of randomness like an oracle service is necessary. Other uses are resistant to "good guesses" because using something that is close but wrong provides no more likelihood of gaining an advantage than any other guess. See also the Related Requirements [S] No Exact Balance Check, [M] Don't Misuse Block Data, and [Q] Protect against MEV Attacks.

[M] Don't Misuse Block Data
Block numbers and timestamps used in Tested Code MUST NOT introduce vulnerabilities to MEV or similar attacks.

Block numbers are vulnerable to approximate prediction, although they are generally not reliably precise indicators of elapsed time. `block.timestamp` is subject to manipulation by malicious actors. It is therefore important that these data are not trusted by Tested Code to function as if they were highly reliable or random information. The description of [SWC-116](https://swcregistry.io/docs/SWC-116) in [[swcregistry]] includes some code examples for techniques to avoid, for example using `block.number / 14` as a proxy for elapsed seconds, or relying on `block.timestamp` to indicate a precise time has passed. For probabilitsitic low precision use, such as "about 1/2 an hour has passed", an expression like `(block.number / 14 > 1800)` can be sufficiently robust on main net, or a blockchain with a similar regular block period of around 14 seconds. But using this approach to determine that e.g. "exactly 36 seconds" have elapsed fails the requirement. Likewise, because block.timestamp depends on settings that can be manipulated by a malicious node operator, in cases likes Ethereum mainnet it is suitable for use as a coarse-grained approximation (on a scale of minutes) but the same code on a different blockchain can be vulnerable to MEV attacks. Note that this is related to the use of Oracles, which can also provide inaccurate information. See also the Related Requirements [S] No Exact Balance Check, [M] Sources of Randomness, and [Q] Protect against MEV Attacks.

Signature Management

[M] Proper Signature Verification
Tested Code MUST properly verify signatures to ensure authenticity of messages that were signed off-chain.

Some smart contracts process messages that were signed off-chain to increase flexibility, while maintaining authenticity. Smart contracts performing their own signature verification need to verify such messages' authenticity.

Using `ecrecover()` for signature verification, it is important to validate the address returned against the expected outcome. In particular, a return value of `address(0)` represents a failure to provide a valid signature. See also SWC-122 [[swcregistry]]. For code that does use `ecrecover()` and a Solidity compiler version older than 0.4.14, see the Related Requirement [M] Use a Modern Compiler, specifically [M] Validate `ecrecover()` Input in [[EthTrust-sl-v1]]

[M] No Improper Usage of Signatures for Replay Attack Protection
Tested Code using signatures to prevent replay attacks MUST ensure that signatures cannot be reused:

In the same function to verify the same message, nor
In more than one function to verify the same message within the Tested Code, nor
In more than one contract address to verify the same message, in which the same account(s) may be signing messages, nor
In the same contract address across multiple chains,

unless it meets the Overriding Requirement [Q] Intended Replay. Additionally, Tested Code MUST verify that multiple signatures cannot be created for the same message, as is the case with Malleable Signatures.

In Replay Attacks, an attacker replays correctly signed messages to exploit a system. The signed message needs to include enough identifying information so that its intended setting is well-defined. Malleable Signatures allow an attacker to create a new signature for the same message. Smart contracts that check against hashes of signatures to ensure that a message has only been processed once could be vulnerable to replay attacks if malleable signatures are used.

Security Level [M] Compiler Bugs and Overriding Requirements

Some solidity compiler bugs described in have Overriding Requirements at Security Level [M], and some have trigger conditions that are not readily detectable in software.

Implementing the Recommended Good Practice [GP] Use Latest Compiler means that Tested Code passes all requirements in this subsection.

[M] Solidity Compiler Bug 2023-1
Tested code that contains a compound expression with side effects that uses `.selector` MUST use the viaIR option with Solidity compiler versions between 0.6.2 and 0.8.20 inclusive.

A bug introduced in Solidity compiler version 0.6.2 and fixed in Solidity compiler version 0.8.21 meant that when compound expressions accessed the `.selector` member, the expression would not be evaluated, unless the viaIR pipeline was used. Thus any side effects caused by the expression would not occur. See also the [19 July 2023 security alert](https://blog.soliditylang.org/2023/07/19/missing-side-effects-on-selector-access-bug/).

[M] Compiler Bug SOL-2022-7
Tested code that has storage writes followed by conditional early terminations from inline assembly functions containing return() or stop() instructions MUST NOT use a Solidity compiler version between 0.8.13 and 0.8.16 inclusive.
This is part of the Set of Overriding Requirements for [S] No assembly {}.

A bug fixed in Solidity compiler version 0.8.17 meant that storage writes followed by conditional early terminations from inline assembly functions would sometimes be erroneously dropped during optimization. See also the [5 September 2022 security alert](https://blog.soliditylang.org/2022/09/08/storage-write-removal-before-conditional-termination/).

[M] Compiler Bug SOL-2022-5 in `assembly {}`
Tested code that

copies bytes arrays from calldata or memory whose size is not a multiple of 32 bytes, and
has an `assembly {}` instruction that reads that data without explicitly matching the length that was copied,

MUST NOT use a Solidity compiler version older than 0.8.15.
This is part of the Set of Overriding Requirements for [S] No assembly {}.

Until Solidity compiler version 0.8.15 copying `memory` or `calldata` whose length is not a multiple of 32 bytes could expose data beyond the data copied, which could be observable using `assembly {}`. See also the [15 June 2022 security alert](https://blog.soliditylang.org/2022/06/15/dirty-bytes-array-to-storage-bug/) and related requirements [S] Compiler Bug SOL-2022-5 with `.push()`, [M] Avoid Common assembly {} Attack Vectors, [M] Document Special Code Use, [M] Compiler Bug SOL-2022-4, and [M] Compiler Bug SOL-2021-3.

[M] Compiler Bug SOL-2022-4
Tested code that has at least two `assembly {}` instructions, such that

one writes to memory e.g. by storing a value in a variable, but does not access that memory again, and
code in a another `assembly {}` instruction refers to that memory,

MUST NOT use the yulOptimizer with Solidity compiler versions 0.8.13 or 0.8.14.
This is part of the Set of Overriding Requirements for [S] No assembly {}.

Solidity compiler version 0.8.13 introduced a yulOptimizer bug, fixed in Solidity compiler version 0.8.15, where memory created in an `assembly {}` instruction but only read in a different `assembly {}` instruction was discarded. See also the [17 June 2022 security alert](https://blog.soliditylang.org/2022/06/15/inline-assembly-memory-side-effects-bug/) and related requirements [M] Avoid Common assembly {} Attack Vectors, [M] Document Special Code Use, [M] Compiler Bug SOL-2022-7, [M] Compiler Bug SOL-2022-5 in `assembly {}`, and [M] Compiler Bug SOL-2021-3.

[M] Compiler Bug SOL-2021-3
Tested code that reads an `immutable` signed integer of a `type` shorter than 256 bits within an `assembly {}` instruction MUST NOT use a Solidity compiler version between 0.6.5 and 0.8.8 (inclusive).
This is part of the Set of Overriding Requirements for [S] No assembly {}.

Solidity compiler version 0.6.8 introduced a bug, fixed in Solidity compiler version 0.8.9, that meant immutable signed integer types shorter than 256 bits could be read incorrectly in inline `assembly {}` instructions. See also the [29 September 2021 security alert](https://blog.soliditylang.org/2021/09/29/signed-immutables-bug/), and the related requirements [M] Safe Use of assembly {}, [M] Document Special Code Use, [M] Compiler Bug SOL-2022-5 in `assembly {}`, and [M] Compiler Bug SOL-2022-4.

[M] Use a Modern Compiler
Tested code MUST NOT use a Solidity compiler version older than 0.8.0, unless it meets the requirement [M] Compiler Bug Check Constructor Payment from the EEA EthTrust Security Levels Specification Version 2, as an Overriding Requirement,

AND

See also the Related Requirement [S] Use a Modern Compiler, covering Solidity Compiler bugs that require review for Security Level [S].

Security Level [Q]

In addition to automatable static testing verification (Security Level [S]), and a manual audit (Security Level [M]), EEA EthTrust Certification at Security Level [Q] means checking that the intended functionality of Tested code is sufficiently well documented that its functional correctness can be verified, that the code and documentation has been thoroughly reviewed by a human auditor or audit team to ensure that they are both internally coherent and consistent with each other, carefully enough to identify complex security vulnerabilities. This level of review is especially relevant for tokens using ERC20 [[ERC20]], ERC721 [[ERC721]], and others; [[token-standards]] identifies a number of other standards that can define tokens. At this Security Level there are also checks to ensure the code does not contain errors that do not directly impact security, but do impact code quality. Code is often copied, so Security Level [Q] requires code to be as well-written as possible. The risk being addressed is that it is easy, and not uncommon, to introduce weaknesses after copying existing code as a starting point.

[Q] Pass Security Level [M]
To be eligible for EEA EthTrust Certification at Security Level [Q], Tested code MUST meet the requirements for .

[Q] Code Linting
Tested code

MUST NOT create unnecessary variables, and
MUST NOT use the same name for functions, variables or other tokens that can occur within the same scope, and
MUST NOT include assert() statements that fail in normal operation, and
MUST NOT include code that cannot be reached in execution
except for code explicitly intended to manage unexpected errors, such as assert() statements, and
MUST NOT contain a function that has the same name as the smart contract unless it is explicitly declared as a constructor using the `constructor` keyword, and
MUST explicitly declare the visibility of all functions and variables.

Code is often copied from "good examples" as a starting point for development. Code that has achieved Security Level [Q] EEA EthTrust Certification is meant to be high quality, so it is important to ensure that copying it does not encourage bad habits. It is also easier to review Tested Code that does not contain pointless code. Code that has the same names for functions and variables is generally harder to read. If those items can overlap in scope, compilation will disambiguate them, but they will often require significant work on the part of reviewers to mentally separate. Using i,j as counters in multiple non-overlapping loops is fine, but having a variable in an "outer scope" whose name is replicated in an "inner scope" by some other variable or function increases the effort required to follow the execution patterns. Code designed to trap unexpected errors, such as `assert()` instructions, is explicitly allowed, because it would be very unfortunate if defensively written code that successfully eliminates the possibility of triggering a particular error could not achieve EEA EthTrust Certification. `assert()` statements are meant for invariants, not as a generic error-handling mechanism. If an `assert()` statement fails in routine operation because it is being used as a mechanism to catch errors, it is better to replace it with a `require()` statement or similar mechanism explicitly designed for the use case. If it fails due to a coding bug, that needs to be fixed. The requirement on `assert()` statements is based on [[CWE-670]] Always-Incorrect Control Flow Implementation.

[Q] Manage Gas Use Increases
Sufficient Gas MUST be available to work with data structures in the Tested Code that grow over time, in accordance with descriptions provided for [Q] Document Contract Logic.

Some structures such as arrays can grow, and the value of variables is (by design) variable. Iterating over a structure whose size is not clear in advance, whether an array that grows, a bound that changes, or something determined by an external value, can result in significant increases in gas usage. What is reasonable growth to expect needs to be considered in the context of the business logic intended, and how the Tested Code protects against Gas Griefing attacks, where malicious actors or errors result in values occurring beyond the expected reasonable range(s). See also SWC-126, SWC-128 [[swcregistry]] and the Related Requirements in .

[Q] Protect Gas Usage
Tested Code MUST protect against malicious actors stealing or wasting gas.

Smart contracts allowing "gasless" transactions enable users to submit transactions without having to supply their own gas. They need to be carefully implemented to prevent Denial of Service from Gas Griefing and Gas Siphoning attacks. See also The Gas Siphon Attack: How it Happened and How to Protect Yourself from the DevCon 2019 talk [[DevCon-siphoning]].

[Q] Protect against Oracle Failure
Tested Code MUST protect itself against malfunctions in Oracles it relies on.

Some Oracles are known to be vulnerable to manipulation, for example because they derive the information they provide from information vulnerable to Read-only Re-entrancy Attacks, or manipulation of prices through the use of flashloans to enable an MEV attack, among other well-known attacks.

In addition, as networked software Oracles can potentially suffer problems ranging from latency issues to outright failure, or being discontinued. It is important to check the mechanism used by an Oracle to generate the information it provides, and the potential exposure of Tested Code that relies on that Oracle to the effects of it failing, or of malicious actors manipulating its inputs or code to enable attacks. See also the Related Requirements [Q] Protect against Ordering Attacks, and [Q] Protect against MEV Attacks.

[Q] Protect against Ordering Attacks
Tested Code MUST manage information in such a way that it protects against Ordering Attacks.

In Ordering Attacks, an attacker places their transaction in a beneficial position compared to that of a victim's. This can be done by a malicious block producer or by an attacker monitoring the mempool, and preempting susceptible transactions by broadcasting their own transactions with higher transaction fees. Removing incentives generally means applying mitigations such as hash commitment schemes [[hash-commit]] or batch execution. See also the Related Requirement [Q] Protect against MEV Attacks.

[Q] Protect against MEV Attacks
Tested Code that is susceptible to MEV attacks MUST follow appropriate design patterns to mitigate this risk.

MEV refers to the potential that a block producer can maliciously reorder or suppress transactions, or another participant in a blockchain can propose a transaction or take other action to gain a benefit that was not intended to be available to them.

This requirement entails a careful judgement by the auditor, of how the Tested Code is vulnerable to MEV attacks, and what mitigation strategies are appropriate. Some approaches are discussed further in . Many attack types need to be considered, including Ordering Attacks. See also the Related Requirements [S] No Exact Balance Check, [M] Sources of Randomness, [M] Don't Misuse Block Data, and [Q] Protect against Oracle Failure, and [Q] Protect against Ordering Attacks.

[Q] Protect Against Governance Takeovers
Tested Code which includes a governance system MUST protect against one external entity taking control via exploit of the governance design.

Governance attacks are specific to the system that is exploited. Depending on the governance proposal system, some areas of vulnerability may include:

The issued governance token;
The method of distribution for the governance token;
The design of the acceptance and execution of governance proposals.

For example, if a staking contract is used to distribute governance tokens as a reward, it is important that the staking contract is not vulnerable to a Flash Loan Attack, where a large amount of tokens are borrowed in a very short-term flash loan, then staked atomically to gain a temporary majority of governance tokens, that are then used to make a governance decision, such as draining all the funds held to an attacker's wallet. See also the Related Requirement [Q] Protect against Ordering Attacks.

[Q] Process All Inputs
Tested Code MUST validate inputs, and function correctly whether the input is as designed or malformed.

Code that fails to validate inputs runs the risk of being subverted through maliciously crafted input that can trigger a bug, or behaviour the authors did not anticipate. See also SWC-123 [[swcregistry]] which notes that it is important to consider whether input requirements are too strict, as well as too lax, [[CWE-573]] Improper Following of Specification by Caller, and note that there are several Related Requirements that are specific to particular Solidity compiler versions in .

[Q] State Changes Trigger Events
Tested code MUST emit a contract event for all transactions that cause state changes.

Events are convenience interfaces that give an abstraction on top of the EVM's logging functionality. Applications can subscribe and listen to these events through the RPC interface of an Ethereum client. See more at [[solidity-events]]. Events are generally expected to be used for logging all state changes as they are not just useful for off-chain applications but also security monitoring and debugging. Logging all state changes in a contract ensures that any developers interacting with the contract are made aware of every state change as part of the ABI and can understand expected behavior through event annotations, as per [Q] Annotate Code with NatSpec.

[Q] No Private Data
Tested code MUST NOT store Private Data on the blockchain.

This is a Security Level [Q] requirement primarily because the question of what is private data often requires careful and thoughtful assessment and a reasoned understanding of context. In general, this is likely to include an assessment of how the data is gathered, and what the providers of data are told about the usage of the information.

Private Data is used in this specification to refer to information that is not intended to be generally available to the public. For example, an individual's home telephone number is generally private data, while a business' customer enquiries telephone number is generally not private data. Similarly, information identifying a person's account is normally private data, but there are circumstances where it is public data. In such cases, that public data can be recorded on-chain in conformance with this requirement.

PLEASE NOTE: In some cases regulation such as the [[GDPR]] imposes formal legal requirements on some private data. However, performing a test for this requirement results in an expert technical opinion on whether data that the auditor considers private is exposed. A statement about whether Tested Code meets this requirement does not represent any form of legal advice or opinion, attorney representation, or the like.

[Q] Intended Replay
If a signature within the Tested Code can be reused, the replay instance MUST be intended, documented, and safe for re-use.

This is an Overriding Requirement for [M] No Improper Usage of Signatures for Replay Attack Protection.

In some rare instances, it may be the intention of the Tested Code to allow signatures to be replayed. For example, a signature may be used as permission to participate in a whitelist for a given period of time. In these exceptional cases, the replay must be included in documentation as a known allowance. Further, it must be verified that the reuse cannot be maliciously exploited.

Documentation requirements

Security Level [Q] conformance requires a detailed description of how the Tested Code is intended to behave. Alongside detailed testing requirements to check that it does behave as described wth regard to specific known vulnerabililies, it is important that the claims made for it are accurate. This requirement helps ensure that the Tested Code fulfils claims made for it outside audit-specific documentation.

The combination of these requirements helps ensure there is no malicious code, such as malicious "back doors" or "time bombs" hidden in the Tested Code. Since there are legitimate use cases for code that behaves as a "time bomb", "phones home", or the like, this combination helps ensure that testing focuses on real problems. The requirements in this section extend the coverage required to meet the Security Level [M] requirement [**[M] Document Special Code Use**](#req-2-documented). As with that requirement, there are multiple requirements at this level that require the documentation mandated in this subsection.

[Q] Document Contract Logic
A specification of the business logic that the Tested code functionality is intended to implement MUST be available to anyone who can call the Tested Code.

Contract Logic documented in a human-readable format and with enough detail that functional correctness and safety assumptions for special code use can be validated by auditors helps them assess complex code more efficiently and with higher confidence. It is important to document how the logic protects against potential attacks such as Flash Loan Attacks (especially on governance or price manipulation), MEV, and other complex attacks that take advantage of ecosystem features or tokenomics.

[Q] Document System Architecture
Documentation of the system architecture for the Tested code MUST be provided that conveys the overrall system design, privileged roles, security assumptions and intended usage.

System documentation provides auditor(s) information to understand security assumptions and ensure functional correctness. It is helpful if system documentation is included or referenced in a README file of the code repository, alongside documentation for how the source code can be tested, built and deployed. The management of variables and more complex data structures over time is an important part of this documentation. That aspect of this requirement is likely to be met through work to meet the Related Requirement [Q] Manage Gas Use Increases. See also the Related Requirement [Q] Annotate Code with NatSpec.

[Q] Annotate Code with NatSpec
All Public Interfaces contained in the Tested code MUST be annotated with inline comments according to the [[NatSpec]] format that explain the intent behind each function, parameter, event, and return variable, along with developer notes for safe usage.

Inline comments are important to ensure that developers and auditors understand the intent behind each function and other code components. Public Interfaces means anything that would be contained in the ABI of the compiled Tested code. It is also recommended to use inline comments for private or internal functions that implement sensitive and/or complex logic. Following the [[NatSpec]] format allows these inline comments to be understood by the Solidity compiler for extracting them into a machine-readable format that could be used by other third-party tools for security assessments and automatic documentation, including documentation shown to users by wallets that integrate with source code verification tools like Sourcify. This could also be used to generate specifications that fully or partially satisfy the Requirement to [Q] Document Contract Logic.

[Q] Implement as Documented
The Tested code MUST behave as described in the documentation provided for [Q] Document Contract Logic, and [Q] Document System Architecture.

The requirements at Security Level [Q] to provide documentation are important. However, it is also crucial that the Tested Code actually behaves as documented. If it does not, it is possible that this reflects insufficient care and that the code is also vulnerable due to bugs that were missed in implementation. It is also possible that the difference is an attempt to hide malicious code in the Tested Code.

Access Control

[Q] Enforce Least Privilege
Tested code that enables privileged access MUST implement appropriate access control mechanisms that provide the least privilege necessary for those interactions, based on the documentation provided for [Q] Document Contract Logic.
This is an Overriding Requirement for [M] Protect Self-destruction.

There are several common methods to implement access control, such as Role-Based Access Control [[RBAC]] and [[Ownable]], and bespoke access control is often implemented for a given use case. Using industry-standard methods can help simplify the process of auditing, but is not sufficient to determine that there are no risks arising either from errors in implementation or due to a maliciously-crafted contract. It is important to consider access control at both the protocol operation and deployment levels. If a protocol is deployed in a deterministic manner, for example allowing a multi-chain deployment to have the same address across all chains, it is important to explicitly set an owner rather than defaulting to msg.sender, as that may leave a simple factory deployment contract as the insufficent new admininstrator of your protocol. It is particularly important that appropriate access control applies to payments, as noted in [SWC-105](https://swcregistry.io/docs/SWC-105), but other actions such as overwriting data as described in [SWC-124](https://swcregistry.io/docs/SWC-126), or changing specific access controls, also need to be appropriately protected [[swcregistry]]. This requirement matches [[CWE-284]] Improper Access Control. See also "[Access Restriction](https://fravoll.github.io/solidity-patterns/access_restriction.html)" in [[solidity-patterns]].

[Q] Use Revocable and Transferable Access Control Permissions
If the Tested code makes uses of Access Control for privileged actions, it MUST implement a mechanism to revoke and transfer those permissions.

Privileged Accounts can perform administrative tasks on the Set of Contracts. If those accounts are compromised or responsibility to perform those tasks is assigned to different people, it is important to have a mechanism to revoke and transfer those permissions.

[Q] No Single Admin EOA for Privileged Actions
If the Tested code makes uses of Access Control for privileged actions, it MUST ensure that all critical administrative tasks require multiple signatures to be executed, unless there is a multisg admin that has greater privileges and can revoke permissions in case of a compromised or rogue EOA and reverse any adverse action the EOA has taken.

Privileged accounts can perform administrative tasks on the Set of Contracts. If a single EOA can perform these actions, and that permission cannot be revoked, the risks to a Smart Contract posed by a compromised or lost private key can be existential.

[Q] Verify External Calls
Tested Code that contains external calls

MUST document the need for them, and
MUST protect them in a way that is fully compatible with the claims of the contract author.

This is part of a Set of Overriding Requirements for [S] Use Check-Effects-Interaction, and for [M] Protect External Calls.

At Security Level [Q] auditors have a lot of flexibility to offer EEA EthTrust Certification for different uses of External Calls. This requirement effectively allows a reviewer to declare that the destination of an external call is not a security risk. It is important to note that any such declaration reflects very closely on the reputation of a reviewer. It is inappropriate to assume that a smart contract is secure just because it is widely used, and it is unacceptable to assume that a smart contract provided by a user in the future will be secure - this is a known vector that has been used for many serious security breaches. It is also important to consider how any code referenced and declared safe by the reviewer could be vulnerable to attacks based on its use of external calls. To take a common example, swap contracts that allow a user to provide any pair of token contracts are potentially at risk if one of those contracts is malicious, or simply vulnerable, in a way the swap contract does not anticipate and protect against. See also the related requirements [Q] Document Contract Logic, [Q] Document System Architecture, and [Q] Implement as Documented.

[Q] Verify tx.origin Usage
For Tested Code that uses tx.origin, each instance

MUST be consistent with the stated security and functionality objectives of the Tested Code, and
MUST NOT allow assertions about contract functionality made for [Q] Document Contract Logic or [Q] Document System Architecture to be violated by another contract, even where that contract is called by a user authorized to interact directly with the Tested Code.

This is an Overriding Requirement for [S] No tx.origin.

`tx.origin` can be used to enable phishing attacks, tricking a user into interacting with a contract that gains access to all the funds in their account. It is generally the wrong choice for authorization of a caller for which `msg.sender` is the safer choice. See also Related Requirements [**[Q] Document Contract Logic**](#req-3-documented), [Q] Enforce Least Privilege, the [section "`tx.origin`"](https://docs.soliditylang.org/en/latest/security-considerations.html?highlight=tx.origin) in Solidity Security Considerations [[solidity-security]], and CWE 284: Improper Access Control [[CWE-284]].

Recommended Good Practices

This section describes good practices that require substantial human judgement to evaluate, where there isn't a clear way to determine if it has been done or not, or where a poor implementation can reduce rather than increase the security of the Tested Code. Testing for, and meeting these requirements does not directly affect conformance to this document. Note however that meeting the Recommended Good Practice [**[GP] Meet as Many Requirements as Possible**](#req-R-meet-all-possible) will in practice mean that Tested Code meets all the Requirements based on Compiler Bugs, including the majority of Requirements for Security Level [S].

[GP] Check For and Address New Security Bugs
Check [[!solidity-bugs-json]] and other sources for bugs announced after 1 November 2023 and address them.

This version of the specification was finalized late in 2023. New vulnerabilities are discovered from time to time, on an unpredictable schedule. The latest solidity compiler bug accounted for in this version is SOL-2023-3. Checking for security alerts published too late to be incorporated into the current version of this document is an important technique for maintaining the highest possible security. There are other sources of information on new security vulnerabilities, from [[CWE]] to following the blogs of many security-oriented organizations such as those that contributed to this specification.

[GP] Meet as Many Requirements as Possible
The Tested Code SHOULD meet as many requirements of this specification as possible at Security Levels above the Security Level for which it is certified.

While meeting some requirements for a higher EEA EthTrust Certification Security Level makes no change to the formal conformance level of the Tested Code, each requirement is specified because meeting it provides protection against specific known attacks. If it is possible to meet a particular requirement, even if it is not necessary for conformance at the Security Level being tested, meeting that requirement will improve the security of the Tested Code and is therefore worth doing.

[GP] Use Latest Compiler
The Tested Code SHOULD use the latest available stable Solidity compiler version.

The Solidity compiler is regularly updated to improve performance but also specifically to fix security vulnerabilities that are discovered. There are many requirements in that are related to vulnerabilities known at the time this specification was written, as well as enhancements made to provide better security by default. In general, newer Solidity compiler versions improve security. Unless there is a specific known reason not to do so, using the latest Solidity compiler version available will result in better security.

[GP] Write Clear, Legible Solidity Code
The Tested Code SHOULD be written for easy understanding.

There are no strict rules defining how to write clear code. It is important to use sufficiently descriptive names, comment code appropriately, and use structures that are easy to understand without causing the code to become excessively large, because that also makes it difficult to read and understand. Excessive nesting, unstructured comments, complex looping structures, and the use of very terse names for variables and functions are examples of coding styles that can also make code harder to understand. It is important to note that in some cases, developers can sacrifice easy reading for other benefits such as reducing gas costs - this can be mitigated somewhat by well-documented code. Likewise, for complex code involving multiple individual smart contracts, the way source is organised into files can help clarify or obscure what's happening. In particular, naming source code files to match the names of smart contracts they define is a common pattern that eases understanding. This Good Practice extends somewhat the Related Requirement [Q] Code Linting, but judgements about how to meet it are necessarily more subjective than in the specifics that requirement establishes. Those looking for additional guidance on code styling can refer to the [[Solidity-Style-Guide]].

[GP] Follow Accepted ERC Standards
The Tested Code SHOULD conform to finalized [[ERC]] standards when it is reasonably capable of doing so for its use-case.

An ERC is a category of [[EIP]] (Ethereum Improvement Proposal) that defines application-level standards and conventions, including smart contract standards such as token standards [[ERC20]] and name registries [[ERC137]]. While following ERC standards will not inherently make Solidity code secure, they do enable developers to integrate with common interfaces and follow known conventions for expected behavior. If the Tested Code does claim to follow a given ERC, its functional correctness in conforming to that standard can be verified by auditors.

[GP] Define a Software License
The Tested Code SHOULD define a software license

A software license provides legal guidance on how contributors and users can interact with the code, including auditors and whitehats. Because bytecode deployed to public networks can be read by anyone, it is common practice to use an Open-Source license for the Solidity code used to generate it. It is important to choose a [[software-license]] that best addresses the needs of the project, and clearly link to it throughout the Tested Code and documentation, e.g. using a prominent LICENSE file in the code repository and referencing it from each source file.

[GP] Disclose New Vulnerabilities Responsibly
Security vulnerabilities that are not addressed by this specification SHOULD be brought to the attention of the Working Group and others through responsible disclosure as described in .

New security vulnerabilities are discovered from time to time. It helps the efforts to revise this specification to ensure the Working Group is aware of new vulnerabilities, or new knowledge regarding existing known vulnerabilities. The EEA has agreed to manage a specific email address for such notifications - and if that changes, to update this specification accordingly.

[GP] Use Fuzzing
Fuzzing SHOULD be used to probe Tested Code for errors.

Fuzzing is an automated software testing method that repeatedly activates a contract, using a variety of invalid, malformed, or unexpected inputs, to reveal defects and potential security vulnerabilities.

Effective Fuzzing can take days or even weeks: it is better to be patient than to stop it prematurely. Fuzzing relies on a Corpus - A set of inputs for a fuzzing target. It is important to maintain the Corpus to maximise code coverage, and helpful to prune unnecessary or duplicate inputs for efficiency. Many tools and input mutation methods can help to build the Corpus for Fuzzing. Good practice is to build on and leverage community resources where possible, always checking licensing restrictions. Another important part of Fuzzing is the set of specification rules that is checked throughout the processes. While Corpus is the set of inputs for Fuzzing targets, the specification rules are business logic checks created specifically and evaluated for each Fuzzing input. This additional set of rules important as it gets triggered if Fuzzing finds an edge case. That way the process doesn't just rely on the checks and reverts already within the contracts and the compiler.

A simple example of a property written using Scribble, a specification language that turns annotations in Solidity into concrete assertions. The annotation is the comment after "///". The property derived from that annotation that is being checked ensures that if the call to the contract `Foo` and function `add` succeeds, then the state variable `x` has to be equal to its value before the call - `old(x)` - added to the function parameter `y`. Essentially, fuzzing with this property would check whether or not the state's storage gets updated correctly.


      contract Foo {
        int x;
        /// #if_succeeds {:msg "test"} x == old(x) + y;
        function add(int y) public {
            require(y > 0);
            x += y;
        }
    }

Fuzzing rules and properties can be complex and depend on specific contracts, functions, variables, their values before and/or after execution, and potentially many other things. If any vulnerabilities are discovered in the Solidity compiler version by Fuzzing please disclose them responsibly.

[GP] Use Formal Verification
The Tested Code SHOULD undergo formal verification.

Formal verification is a family of techniques that can mathematically prove functional correctness of smart contracts. It has been used in other applications such as embedded systems. There are many uses for formal verification in smart contracts, such as testing liveness, protocol invariants for safety at a high level, or proving narrower, more specific properties of a program's execution. In formal verification, a formal (symbolic or mathematical) specification of the expected or desired outcome of a smart contract is created, enabling a formal mathematical proof of a protocol's correctness. The smart contract itself is often translated into a formal language for this purpose. Several languages and programs exist for creating fromal verification proofs, some with the explicit aim of making formal verification more accessible to casual users and non-mathematicians. Please see [[EF-SL]] for some examples. When implemented correctly by a practitioner with experience and skill, formal verification can make guarantees that fuzzing and testing cannot provide. However, that is often difficult to achieve in practice. Formal verification requires substantial manual labor and expertise. A comprehensive formal verification most likely has a much a higher cost and complexity than unit or integration testing, fuzzing, or other methods. The immutable nature of many smart contracts, and the complexity of upgrading contracts when it is possible, makes formal verification appealing to administrators and stakeholders of protocols.

[GP] Select an Appropriate Threshold for Multisig Wallets
Multisignature requirements for privileged actions SHOULD have a sufficient number of signers, and NOT require "1 of N" nor all signatures.

Requiring multiple signatures for administrative actions has become the standard for many teams. When not managed carefully, they can become a source of attack even if the smart contract code is secure. The problem with "1 of N" setups, that enable a single account to execute transactions, is that it is relatively easy to exploit. "N of N" setups meanwhile mean that if even one signer loses access to their account or will not approve an action, there is no possibility for approval. This can affect necessary operations such as the replacement of one signer with another, for example to ensure operational continuity, which can have a very serious impact. Choosing a lower number of signatures to meet the requirement allows for quicker response, while a higher value requires stronger majority support. Consider using an "M of N" multisignature where M = (N/2) + 1, in other words, the smallest possible majority of signatures are necessary for approval, as a starting point. However it is important to consider how many potential signers there are, and the specific situations where signatures are needed, to determine a reasonably good value for M in a given case.

[GP] Use TimeLock Delays for Sensitive Operations
Sensitive operations that affect all or a majority of users SHOULD use [[TimeLock]] delays.

Sensitive operations, such as upgrades and [[RBAC]] changes impact all or a majority of users in the protocol. A [[TimeLock]] delay allows users to exit the system if they disagree with the proposed change, and allows developers to react if they detect a suspicious change.

Introduction

How to read this specification

Overview of this Document

Typographic Conventions

How to Read a Requirement

Overriding Requirements

Related Requirements

Why Certify Contracts?

Developing Secure Smart Contracts

Feedback and new vulnerabilities

Conformance Claims

Who can offer EEA EthTrust Certification?

Identifying what is certified

Security Considerations

Smart Contracts in context - broader considerations

Upgradable Contracts

Oracles

External Interactions and Re-entrancy Attacks

Signature Mechanisms

Gas and Gas Prices

MEV (Maliciously Extracted Value)

Ordering Attacks

Source code, pragma, and compilers

Contract Deployment

Post-deployment Monitoring

Network Upgrades

EEA EthTrust Security Levels

Security Level [S]

Text and homoglyphs

External Calls

Compiler Bugs

Security Level [M]

Text and homoglyph attacks

External Calls

Documented Defensive Coding

Signature Management

Security Level [M] Compiler Bugs and Overriding Requirements

Security Level [Q]

Documentation requirements

Access Control

Recommended Good Practices

Additional Information

Defined Terms

Summary of Requirements

Acknowledgments

Changes

New Requirements

Updated Requirements

Requirements removed

Previously removed requirements