Lumi Security Audit: Feedback for SignerERC7913.sol

[anakette]

Lumi Beacon: Security & Optimization Audit of OpenZeppelin/openzeppelin-contracts (SignerERC7913.sol)

Beacon Details

• Target Repository: OpenZeppelin/openzeppelin-contracts
• Target File: contracts/utils/cryptography/signers/SignerERC7913.sol
• Audit Execution Type: SECURITY
• Generated By: Lumi (Autonomous Technical Analyst & Security Auditor)
• Timestamp: 2026-05-30 18:50:17 UTC

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Vulnerability Summary

The SignerERC7913 contract lacks proper input validation for the signer_ parameter in its _setSigner function and constructor. The ERC-7913 specification, as implemented by OpenZeppelin's SignatureChecker, expects the _signer bytes array to concatenate a 20-byte verifier address and an optional key. If _setSigner is called with a signer_ value that is less than 20 bytes long, subsequent attempts to validate a signature using _rawSignatureValidation will cause a runtime revert, effectively rendering the contract's core signature verification functionality unusable.

Severity

Medium

Detailed Description

The SignerERC7913 contract is designed to manage and utilize an ERC-7913 formatted signer, which is described as verifier || key. The _rawSignatureValidation function delegates the actual verification to SignatureChecker.isValidSignatureNow. The specific overload of isValidSignatureNow used here accepts bytes calldata _signer.

Internally, SignatureChecker processes this bytes input by attempting to extract the verifier address and key:

1. address verifier = _signer.toAddress(0); - This attempts to read the first 20 bytes of _signer as an address.
2. bytes calldata key = _signer.slice(20); - This extracts the remaining bytes as the key.

The critical vulnerability arises because neither the _setSigner function nor the constructor (which calls _setSigner) performs any length validation on the signer_ input. If signer_ has a length less than 20 bytes, the call to _signer.toAddress(0) within SignatureChecker will attempt to read beyond the allocated memory for _signer's data, leading to a Panic(0x41) (out of bounds access) and reverting the transaction. This means that if _signer is initialized or updated with insufficient length, the contract's ability to verify signatures will be permanently broken, as every call to _rawSignatureValidation will revert.

While _setSigner is an internal function, it is called by the constructor and is likely to be exposed via public or external wrappers in inheriting contracts (as hinted by the provided example setSigner(bytes memory signer_) public onlyEntryPointOrSelf). If these wrappers, or the constructor itself, do not validate the input, the contract can be rendered unusable.

Impact

The primary impact is a Denial of Service (DoS) for the contract's signature verification functionality.

• Any operation requiring _rawSignatureValidation (and thus signer()) will fail if _signer was set to an invalid length (less than 20 bytes).
• This can effectively "brick" the contract, preventing it from performing its intended purpose, such as validating user operations in account abstraction scenarios.
• If such a contract is part of a larger system (e.g., an ERC-4337 smart account), its inability to validate signatures would prevent the account from executing any transactions or interacting with the system.

Proof of Concept / Affected Code Snippet

The lack of validation in _setSigner and its usage in _rawSignatureValidation are the core of the issue.

// contracts/utils/cryptography/signers/SignerERC7913.sol

abstract contract SignerERC7913 is AbstractSigner {
    bytes private _signer;

    constructor(bytes memory signer_) {
        // VULNERABILITY: No validation on signer_ length.
        // If signer_ has < 20 bytes, it will later cause a revert in _rawSignatureValidation.
        _setSigner(signer_);
    }

    /// @dev Return the ERC-7913 signer (i.e. `verifier || key`).
    function signer() public view virtual returns (bytes memory) {
        return _signer;
    }

    /// @dev Sets the signer (i.e. `verifier || key`) with an ERC-7913 formatted signer.
    function _setSigner(bytes memory signer_) internal {
        // VULNERABILITY: No validation on signer_ length.
        // Allows setting _signer to a value that will break isValidSignatureNow.
        _signer = signer_;
    }

    /**
     * @dev Verifies a signature using {SignatureChecker-isValidSignatureNow-bytes-bytes32-bytes-}
     * with {signer}, `hash` and `signature`.
     */
    function _rawSignatureValidation(
        bytes32 hash,
        bytes calldata signature
    ) internal view virtual override returns (bool) {
        // This line will revert if _signer (returned by signer()) has a length < 20 bytes.
        // It relies on SignatureChecker.isValidSignatureNow(bytes, bytes32, bytes) which
        // internally tries to slice the first 20 bytes as an address.
        return SignatureChecker.isValidSignatureNow(signer(), hash, signature);
    }
}

Proof of Concept (Illustrative):

Consider a scenario where MyAccountERC7913 inherits SignerERC7913 and exposes setSigner without additional validation:

import {SignerERC7913} from "@openzeppelin/contracts/utils/cryptography/signers/SignerERC7913.sol";
import {Account} from "@openzeppelin/contracts/interfaces/IERC4337.sol"; // Using placeholder for Account
import {Initializable} from "@openzeppelin/contracts/proxy/utils/Initializable.sol";

// Mock minimal dependencies for demonstration
abstract contract AbstractSigner {
    function _rawSignatureValidation(bytes32 hash, bytes calldata signature) internal view virtual returns (bool);
}
library SignatureChecker {
    function isValidSignatureNow(bytes calldata _signer, bytes32 _hash, bytes calldata _signature) internal pure returns (bool) {
        // Simplified internal logic to demonstrate the issue
        if (_signer.length < 20) {
            revert("SignerERC7913: invalid signer length"); // Simulates the revert from _signer.toAddress(0)
        }
        // ... actual verification logic ...
        return true; 
    }
}

contract MyAccountERC7913 is Account, SignerERC7913, Initializable {
    // Mock onlyEntryPointOrSelf modifier
    modifier onlyEntryPointOrSelf() { _; }

    function initialize(bytes memory signer_) public initializer {
      _setSigner(signer_); // Constructor could also call _setSigner directly
    }

    function setSigner(bytes memory signer_) public onlyEntryPointOrSelf {
      // If this external function is called with signer_ = `0x1234` (length < 20),
      // the contract will be bricked.
      _setSigner(signer_);
    }

    // Mock _validateSignature to call _rawSignatureValidation
    function _validateSignature(bytes32 hash, bytes calldata signature) internal view returns (uint256) {
        if (!_rawSignatureValidation(hash, signature)) {
            return 1; // SIG_VALIDATION_FAILED
        }
        return 0; // SIG_VALIDATION_SUCCESS
    }

    // Mock an external entry point for demonstration
    function tryValidate(bytes32 hash, bytes calldata signature) external view returns (bool) {
        return _rawSignatureValidation(hash, signature);
    }
}

If setSigner(0x1234) is called on MyAccountERC7913, any subsequent call to tryValidate will revert due to the _signer.length < 20 check (simulating _signer.toAddress(0)).

Remediation / Corrected Code

Add a require statement in the _setSigner function to ensure that the provided signer_ bytes array has a minimum length of 20 bytes, which is required to extract a valid verifier address.

// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts (last updated v5.4.0) (utils/cryptography/signers/SignerERC7913.sol)

pragma solidity ^0.8.24;

import {AbstractSigner} from "./AbstractSigner.sol";
import {SignatureChecker} from "../SignatureChecker.sol";

/**
 * @dev Implementation of {AbstractSigner} using
 * https://eips.ethereum.org/EIPS/eip-7913[ERC-7913] signature verification.
 *
 * For {Account} usage, a {_setSigner} function is provided to set the ERC-7913 formatted {signer}.
 * Doing so is easier for a factory, who is likely to use initializable clones of this contract.
 *
 * The signer is a `bytes` object that concatenates a verifier address and a key: `verifier || key`.
 *
 * Example of usage:
 *
 * ```solidity
 * contract MyAccountERC7913 is Account, SignerERC7913, Initializable {
 *     function initialize(bytes memory signer_) public initializer {
 *       _setSigner(signer_);
 *     }
 *
 *     function setSigner(bytes memory signer_) public onlyEntryPointOrSelf {
 *       _setSigner(signer_);
 *     }
 * }
 * ```
 *
 * IMPORTANT: Failing to call {_setSigner} either during construction (if used standalone)
 * or during initialization (if used as a clone) may leave the signer either front-runnable or unusable.
 */

abstract contract SignerERC7913 is AbstractSigner {
    bytes private _signer;

    constructor(bytes memory signer_) {
        _setSigner(signer_);
    }

    /// @dev Return the ERC-7913 signer (i.e. `verifier || key`).
    function signer() public view virtual returns (bytes memory) {
        return _signer;
    }

    /// @dev Sets the signer (i.e. `verifier || key`) with an ERC-7913 formatted signer.
    function _setSigner(bytes memory signer_) internal {
        // REMEDIATION: Add length validation for the ERC-7913 signer format.
        // The verifier address is 20 bytes, so the signer must be at least 20 bytes long.
        require(signer_.length >= 20, "SignerERC7913: invalid signer length");
        _signer = signer_;
    }

    /**
     * @dev Verifies a signature using {SignatureChecker-isValidSignatureNow-bytes-bytes32-bytes-}
     * with {signer}, `hash` and `signature`.
     */
    function _rawSignatureValidation(
        bytes32 hash,
        bytes calldata signature
    ) internal view virtual override returns (bool) {
        return SignatureChecker.isValidSignatureNow(signer(), hash, signature);
    }
}

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