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Hash Collision when using abi.encodePacked() with Multiple Variable-Length Arguments

In Solidity, the abi.encodePacked() function is used to create tightly packed byte arrays which can then be hashed using keccak256()

However, this function can be dangerous when used with multiple variable-length arguments because it can lead to hash collisions. These collisions can potentially be exploited in scenarios such as signature verification, allowing attackers to bypass authorization mechanisms.

Hash Collision is a situation where two different sets of inputs produce the same hash output. In this context, a hash collision can occur when using abi.encodePacked() with multiple variable-length arguments, allowing an attacker to craft different inputs that produce the same hash.

Understanding the vulnerability

When abi.encodePacked() is used with multiple variable-length arguments (such as arrays and strings), the packed encoding does not include information about the boundaries between different arguments. This can lead to situations where different combinations of arguments result in the same encoded output, causing hash collisions.

For example, consider the following two calls to abi.encodePacked():

abi.encodePacked(["a", "b"], ["c", "d"])
abi.encodePacked(["a"], ["b", "c", "d"])

Both calls could potentially produce the same packed encoding because abi.encodePacked() simply concatenates the elements without any delimiters!

Consider the below example for strings:

abi.encodePacked("foo", "bar") == abi.encodePacked("fo", "obar")

Strings in Solidity are dynamic types and when they are concatenated using abi.encodePacked(), there is no delimiter between them to mark their boundaries, which can lead to hash collisions.

As a matter of fact, the below warning is taken as it is straight from the official solidity language documentation regarding the same.

Warning

If you use keccak256(abi.encodePacked(a, b)) and both a and b are dynamic types, it is easy to craft collisions in the hash value by moving parts of a into b and vice-versa. More specifically, abi.encodePacked("a", "bc") == abi.encodePacked("ab", "c"). If you use abi.encodePacked for signatures, authentication or data integrity, make sure to always use the same types and check that at most one of them is dynamic. Unless there is a compelling reason, abi.encode should be preferred.

Sample Code Analysis

/// INSECURE
function addUsers(address[] calldata admins, address[] calldata regularUsers, bytes calldata signature) external {
    if (!isAdmin[msg.sender]) {
        bytes32 hash = keccak256(abi.encodePacked(admins, regularUsers));
        address signer = hash.toEthSignedMessageHash().recover(signature);
        require(isAdmin[signer], "Only admins can add users.");
    }
    for (uint256 i = 0; i < admins.length; i++) {
        isAdmin[admins[i]] = true;
    }
    for (uint256 i = 0; i < regularUsers.length; i++) {
        isRegularUser[regularUsers[i]] = true;
    }
}

In the provided sample code above, the addUsers function uses abi.encodePacked(admins, regularUsers) to generate a hash. An attacker could exploit this by rearranging elements between the admins and regularUsers arrays, resulting in the same hash and thereby bypassing authorization checks.

/// INSECURE
function verifyMessage(string calldata message1, string calldata message2, bytes calldata signature) external {
    bytes32 hash = keccak256(abi.encodePacked(message1, message2));
    address signer = hash.toEthSignedMessageHash().recover(signature);
    require(isAuthorized[signer], "Unauthorized signer");
}

The above function verifyMessage() could easily be exploited as below:-

verifyMessage("hello", "world", signature);

or

verifyMessage("hell", "oworld", signature);

or a variation of the string hello world

All variations of the string hello world passed to verifyMessage() would produce the same hash, potentially allowing an attacker to bypass the authorization check if they can provide a valid signature for their manipulated inputs.

Fixed Code Using Single User:

function addUser(address user, bool admin, bytes calldata signature) external {
    if (!isAdmin[msg.sender]) {
        bytes32 hash = keccak256(abi.encodePacked(user));
        address signer = hash.toEthSignedMessageHash().recover(signature);
        require(isAdmin[signer], "Only admins can add users.");
    }
    if (admin) {
        isAdmin[user] = true;
    } else {
        isRegularUser[user] = true;
    }
}

This approach eliminates the use of variable-length arrays, thus avoiding the hash collision issue entirely by dealing with a single user at a time.

Fixed Code Using Fixed-Length Arrays:

function addUsers(address[3] calldata admins, address[3] calldata regularUsers, bytes calldata signature) external {
    if (!isAdmin[msg.sender]) {
        bytes32 hash = keccak256(abi.encodePacked(admins, regularUsers));
        address signer = hash.toEthSignedMessageHash().recover(signature);
        require(isAdmin[signer], "Only admins can add users.");
    }
    for (uint256 i = 0; i < admins.length; i++) {
        isAdmin[admins[i]] = true;
    }
    for (uint256 i = 0; i < regularUsers.length; i++) {
        isRegularUser[regularUsers[i]] = true;
    }
}

In this version, fixed-length arrays are used, which mitigates the risk of hash collisions since the encoding is unambiguous.

Remediation Strategies

To prevent this type of hash collision, the below remediation strategies can be employed:

  1. Avoid Variable-Length Arguments: Avoid using abi.encodePacked() with variable-length arguments such as arrays and strings. Instead, use fixed-length arrays to ensure the encoding is unique and unambiguous.

  2. Use abi.encode() Instead: Unlike abi.encodePacked(), abi.encode() includes additional type information and length prefixes in the encoding, making it much less prone to hash collisions. Switching from abi.encodePacked() to abi.encode() is a simple yet effective fix.

Important

Replay Protection does not protect against possible hash collisions!

It is listed here as a defense in depth strategy and SHOULD NOT be solely relied upon to protect against said vulnerability

  1. Replay Protection: Implement replay protection mechanisms to prevent attackers from reusing valid signatures. This can involve including nonces or timestamps in the signed data. However, this does not completely eliminate the risk of hash collisions but adds an additional layer of security. More on this can be found here

Sources