Project

Summary

In the NativeMetaTransaction.sol contract, the abi.encodePacked() function is used with user-controlled data as input, including bytes arguments. This usage opens the door to potential hash collisions, as abi.encodePacked() does not add length prefixes, leading to ambiguous concatenated byte streams. As a result, certain crafted inputs could generate the same hash and bypass security checks that rely on hash uniqueness.

Vulnerability Details

The abi.encodePacked() function is used in the NativeMetaTransaction.sol contract, where user-controlled data (functionSignature, userAddress) is concatenated without boundaries. This practice can cause hash collisions when specific byte inputs align, leading to ambiguous representations. The issue is particularly concerning when user-controlled byte arrays are passed, as an attacker could craft byte sequences that collide with legitimate inputs.

Line of Code Affected

• https://github.com/Cyfrin/2024-11-one-world/blob/1e872c7ab393c380010a507398d4b4caca1ae32b/contracts/meta-transaction/NativeMetaTransaction.sol#L63

Impact

A hash collision vulnerability could allow an attacker to create specific inputs that generate the same hash, potentially bypassing certain security checks. This is particularly dangerous when user-controlled data is passed into abi.encodePacked() without proper handling of byte boundaries. If exploited, this vulnerability could allow unauthorized users to execute specific functions or access restricted data within the contract.

The likelihood of this vulnerability being exploited depends on several factors, including the usage of abi.encodePacked() with user-controlled data and the lack of boundary markers in byte concatenation. Given that user input is directly involved in the concatenation process, the risk is moderate, especially in scenarios where hash-based security checks are implemented without thorough validation.

1. User-Controlled Inputs: Since functionSignature and userAddress can be user-defined, there is a higher probability of malicious manipulation.

2. Absence of Boundaries in Concatenation: The abi.encodePacked() function lacks length markers, increasing the risk that specific byte sequences will result in collisions.

This vulnerability could allow an attacker to exploit hash collisions to bypass critical checks or validations within the smart contract. Specifically, the attacker could create crafted byte sequences that mimic legitimate input values, bypassing hash-based comparisons and gaining access to restricted functionalities.

For example:

1. Function Access Bypass: If the abi.encodePacked() result is used for signature verification or function access checks, an attacker could craft inputs that result in the same hash as valid inputs, potentially gaining unauthorized access.

2. Impersonation: By creating crafted inputs, the attacker could impersonate other users or execute functions on their behalf without permission.

Proof of Concept (if required)

The proof of concept for this hash collision issue involves crafting different inputs that would produce the same hash output due to the concatenation style of abi.encodePacked().

In this scenario, the attacker could manipulate the input to force a collision between two different sets of inputs, potentially gaining unauthorized access if the contract relies on the uniqueness of the hashed result.

Tools Used

Manual Review

Recommendations

To prevent hash collisions, replace abi.encodePacked() with abi.encode(), which adds length markers to the input data, thereby maintaining clear boundaries between different byte arrays. This ensures the uniqueness of the concatenated output, even when user-controlled data is involved.

Using abi.encode() instead of abi.encodePacked() ensures that each element is prefixed with its length, effectively preventing collisions and preserving the integrity of hash-based checks. Additionally, consider validating input data to prevent crafted byte sequences from bypassing security checks.