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The Solcurity Standard

Opinionated security and code quality standard for Solidity smart contracts.

Based off work by BoringCrypto, Mudit Gupta, Runtime Verification, and ConsenSys Diligence.

General Review Approach:

• Read the project's docs, specs, and whitepaper to understand what the smart contracts are meant to do.
• Construct a mental model of what you expect the contracts to look like before checking out the code.
• Glance over the contracts to get a sense of the project's architecture. Tools like Surya can come in handy.
• Compare the architecture to your mental model. Look into areas that are surprising.
• Create a threat model and make a list of theoretical high level attack vectors.
• Look at areas that can do value exchange. Especially functions like transfer, transferFrom, send, call, delegatecall, and selfdestruct. Walk backward from them to ensure they are secured properly.
• Look at areas that interface with external contracts and ensure all assumptions about them are valid like share price only increases, etc.
• Do a generic line-by-line review of the contracts.
• Do another review from the perspective of every actor in the threat model.
• Glance over the project's tests + code coverage and look deeper at areas lacking coverage.
• Run tools like Slither/Solhint and review their output.
• Look at related projects and their audits to check for any similar issues or oversights.

Variables

• V1 - Can it be internal?
• V2 - Can it be constant?
• V3 - Can it be immutable?
• V4 - Is its visibility set? (SWC-108)
• V5 - Is the purpose of the variable and other important information documented using natspec?
• V6 - Can it be packed with an adjacent storage variable?
• V7 - Can it be packed in a struct with more than 1 other variable?
• V8 - Use full 256 bit types unless packing with other variables.
• V9 - If it's a public array, is a separate function provided to return the full array?
• V10 - Only use private to intentionally prevent child contracts from accessing the variable, prefer internal for flexibility.

Structs

• S1 - Is a struct necessary? Can the variable be packed raw in storage?
• S2 - Are its fields packed together (if possible)?
• S3 - Is the purpose of the struct and all fields documented using natspec?

Functions

• F1 - Can it be external?
• F2 - Should it be internal?
• F3 - Should it be payable?
• F4 - Can it be combined with another similar function?
• F5 - Validate all parameters are within safe bounds, even if the function can only be called by a trusted users.
• F6 - Is the checks before effects pattern followed? (SWC-107)
• F7 - Check for front-running possibilities, such as the approve function. (SWC-114)
• F8 - Is insufficient gas griefing possible? (SWC-126)
• F9 - Are the correct modifiers applied, such as onlyOwner/requiresAuth?
• F10 - Are return values always assigned?
• F11 - Write down and test invariants about state before a function can run correctly.
• F12 - Write down and test invariants about the return or any changes to state after a function has run.
• F13 - Take care when naming functions, because people will assume behavior based on the name.
• F14 - If a function is intentionally unsafe (to save gas, etc), use an unwieldy name to draw attention to its risk.
• F15 - Are all arguments, return values, side effects and other information documented using natspec?
• F16 - If the function allows operating on another user in the system, do not assume msg.sender is the user being operated on.
• F17 - If the function requires the contract be in an uninitialized state, check an explicit initialized variable. Do not use owner == address(0) or other similar checks as substitutes.
• F18 - Only use private to intentionally prevent child contracts from calling the function, prefer internal for flexibility.
• F19 - Use virtual if there are legitimate (and safe) instances where a child contract may wish to override the function's behavior.

Modifiers

• M1 - Are no storage updates made (except in a reentrancy lock)?
• M2 - Are external calls avoided?
• M3 - Is the purpose of the modifier and other important information documented using natspec?

Code

• C1 - Using SafeMath or 0.8 checked math? (SWC-101)
• C2 - Are any storage slots read multiple times?
• C3 - Are any unbounded loops/arrays used that can cause DoS? (SWC-128)
• C4 - Use block.timestamp only for long intervals. (SWC-116)
• C5 - Don't use block.number for elapsed time. (SWC-116)
• C7 - Avoid delegatecall wherever possible, especially to external (even if trusted) contracts. (SWC-112)
• C8 - Do not update the length of an array while iterating over it.
• C9 - Don't use blockhash(), etc for randomness. (SWC-120)
• C10 - Are signatures protected against replay with a nonce and block.chainid (SWC-121)
• C11 - Ensure all signatures use EIP-712. (SWC-117 SWC-122)
• C12 - Output of abi.encodePacked() shouldn't be hashed if using >2 dynamic types. Prefer using abi.encode() in general. (SWC-133)
• C13 - Careful with assembly, don't use any arbitrary data. (SWC-127)
• C14 - Don't assume a specific ETH balance. (SWC-132)
• C15 - Avoid insufficient gas griefing. (SWC-126)
• C16 - Private data isn't private. (SWC-136)
• C17 - Updating a struct/array in memory won't modify it in storage.
• C18 - Never shadow state variables. (SWC-119)
• C19 - Do not mutate function parameters.
• C20 - Is calculating a value on the fly cheaper than storing it?
• C21 - Are all state variables read from the correct contract (master vs. clone)?
• C22 - Are comparison operators used correctly (>, <, >=, <=), especially to prevent off-by-one errors?
• C23 - Are logical operators used correctly (==, !=, &&, ||, !), especially to prevent off-by-one errors?
• C24 - Always multiply before dividing, unless the multiplication could overflow.
• C25 - Are magic numbers replaced by a constant with an intuitive name?
• C26 - If the recipient of ETH had a fallback function that reverted, could it cause DoS? (SWC-113)
• C27 - Use SafeERC20 or check return values safely.
• C28 - Don't use msg.value in a loop.
• C29 - Don't use msg.value if recursive delegatecalls are possible (like if the contract inherits Multicall/Batchable).
• C30 - Don't assume msg.sender is always a relevant user.
• C31 - Don't use assert() unless for fuzzing or formal verification. (SWC-110)
• C32 - Don't use tx.origin for authorization. (SWC-115)
• C33 - Don't use address.transfer() or address.send(). Use .call.value(...)("") instead. (SWC-134)
• C34 - When using low-level calls, ensure the contract exists before calling.
• C35 - When calling a function with many parameters, use the named argument syntax.
• C36 - Do not use assembly for create2. Prefer the modern salted contract creation syntax.
• C37 - Do not use assembly to access chainid or contract code/size/hash. Prefer the modern Solidity syntax.
• C38 - Use the delete keyword when setting a variable to a zero value (0, false, "", etc).
• C39 - Comment the "why" as much as possible.
• C40 - Comment the "what" if using obscure syntax or writing unconventional code.
• C41 - Comment explanations + example inputs/outputs next to complex and fixed point math.
• C42 - Comment explanations wherever optimizations are done, along with an estimate of much gas they save.
• C43 - Comment explanations wherever certain optimizations are purposely avoided, along with an estimate of much gas they would/wouldn't save if implemented.
• C44 - Use unchecked blocks where overflow/underflow is impossible, or where an overflow/underflow is unrealistic on human timescales (counters, etc). Comment explanations wherever unchecked is used, along with an estimate of how much gas it saves (if relevant).
• C45 - Do not depend on Solidity's arithmetic operator precedence rules. In addition to the use of parentheses to override default operator precedence, parentheses should also be used to emphasise it.
• C46 - Expressions passed to logical/comparison operators (&&/||/>=/==/etc) should not have side-effects.
• C47 - Wherever arithmetic operations are performed that could result in precision loss, ensure it benefits the right actors in the system, and document it with comments.
• C48 - Document the reason why a reentrancy lock is necessary whenever it's used with an inline or @dev natspec comment.
• C49 - When fuzzing functions that only operate on specific numerical ranges use modulo to tighten the fuzzer's inputs (such as x = x % 10000 + 1 to restrict from 1 to 10,000).
• C50 - Use ternary expressions to simplify branching logic wherever possible.
• C51 - When operating on more than one address, ask yourself what happens if they're the same.

External Calls

• X1 - Is an external contract call actually needed?
• X2 - If there is an error, could it cause DoS? Like balanceOf() reverting. (SWC-113)
• X3 - Would it be harmful if the call reentered into the current function?
• X4 - Would it be harmful if the call reentered into another function?
• X5 - Is the result checked and errors dealt with? (SWC-104)
• X6 - What if it uses all the gas provided?
• X7 - Could it cause an out-of-gas in the calling contract if it returns a massive amount of data?
• X8 - If you are calling a particular function, do not assume that success implies that the function exists (phantom functions).

Static Calls

• S1 - Is an external contract call actually needed?
• S2 - Is it actually marked as view in the interface?
• S3 - If there is an error, could it cause DoS? Like balanceOf() reverting. (SWC-113)
• S4 - If the call entered an infinite loop, could it cause DoS?

Events

• E1 - Should any fields be indexed?
• E2 - Is the creator of the relevant action included as an indexed field?
• E3 - Do not index dynamic types like strings or bytes.
• E4 - Is when the event emitted and all fields documented using natspec?
• E5 - Are all users/ids that are operated on in functions that emit the event stored as indexed fields?
• E6 - Avoid function calls and evaluation of expressions within event arguments. Their order of evaluation is unpredictable.

Contract

• T1 - Use an SPDX license identifier.
• T2 - Are events emitted for every storage mutating function?
• T3 - Check for correct inheritance, keep it simple and linear. (SWC-125)
• T4 - Use a receive() external payable function if the contract should accept transferred ETH.
• T5 - Write down and test invariants about relationships between stored state.
• T6 - Is the purpose of the contract and how it interacts with others documented using natspec?
• T7 - The contract should be marked abstract if another contract must inherit it to unlock its full functionality.
• T8 - Emit an appropriate event for any non-immutable variable set in the constructor that emits an event when mutated elsewhere.
• T9 - Avoid over-inheritance as it masks complexity and encourages over-abstraction.
• T10 - Always use the named import syntax to explicitly declare which contracts are being imported from another file.
• T11 - Group imports by their folder/package. Separate groups with an empty line. Groups of external dependencies should come first, then mock/testing contracts (if relevant), and finally local imports.
• T12 - Summarize the purpose and functionality of the contract with a @notice natspec comment. Document how the contract interacts with other contracts inside/outside the project in a @dev natspec comment.

Project

• P1 - Use the right license (you must use GPL if you depend on GPL code, etc).
• P2 - Unit test everything.
• P3 - Fuzz test as much as possible.
• P4 - Use symbolic execution where possible.
• P5 - Run Slither/Solhint and review all findings.

DeFi

• D1 - Check your assumptions about what other contracts do and return.
• D2 - Don't mix internal accounting with actual balances.
• D3 - Don't use spot price from an AMM as an oracle.
• D4 - Do not trade on AMMs without receiving a price target off-chain or via an oracle.
• D5 - Use sanity checks to prevent oracle/price manipulation.
• D6 - Watch out for rebasing tokens. If they are unsupported, ensure that property is documented.
• D7 - Watch out for ERC-777 tokens. Even a token you trust could preform reentrancy if it's an ERC-777.
• D8 - Watch out for fee-on-transfer tokens. If they are unsupported, ensure that property is documented.
• D9 - Watch out for tokens that use too many or too few decimals. Ensure the max and min supported values are documented.
• D10 - Be careful of relying on the raw token balance of a contract to determine earnings. Contracts which provide a way to recover assets sent directly to them can mess up share price functions that rely on the raw Ether or token balances of an address.
• D11 - If your contract is a target for token approvals, do not make arbitrary calls from user input.