Potential hash collision when using abi.encodePacked() with multiple variable-length arguments

[indeqs]

Checklist

• [✅] I have searched the existing issues and pull requests for duplicates.

Type of Issue

• [✅] New vulnerability addition
• Feature request
• Update existing vulnerability

Description

Using abi.encodePacked() with multiple variable-length arguments can, in certain situations, lead to a hash collision. Since abi.encodePacked() packs all elements in order regardless of whether they're part of an array, you can move elements between arrays and, so long as all elements are in the same order, it will return the same encoding. In a signature verification situation, an attacker could exploit this by modifying the position of elements in a previous function call to effectively bypass authorization.

Sources

• Smart Contract Weakness Classification #133
• Solidity Non-standard Packed Mode