vyper performs double eval of the argument of sqrt

Vulnerability

Overview The sqrt builtin in Vyper versions 0.3.10 and prior contains a double evaluation vulnerability. The build_IR function does not cache the argument to the stack, causing it to be evaluated multiple times if the argument has side effects [1]. This can lead to unexpected behavior in smart contracts that use sqrt with expressions modifying state (e.g., storage writes or external calls).

Exploitation

Conditions An attacker can exploit this vulnerability if they can influence an argument passed to sqrt that has side effects. The contract must use sqrt in a context where the argument is not a simple variable but an expression that changes state or relies on mutable data. No authentication is required beyond the ability to interact with the vulnerable contract. The double evaluation is deterministic and can be triggered by normal contract operations.

Impact

According to the advisory, no vulnerable production contracts have been found. Additionally, double evaluation of side effects is easily discoverable through client tests, limiting the practical risk. The CVSS score has not yet been assigned by NVD, but the impact is considered low [1].

Mitigation

Status As of the publication date (2024-04-25), no fixed version of Vyper is available. Developers are advised to avoid using sqrt with arguments that have side effects until a patch is released [1][4]. The Vyper team has acknowledged the issue and may address it in a future release (see related pull request [3]).