Vyper: reversed order of side effects for some operations

Vulnerability

Description

The Vyper smart contract compiler evaluates arguments from right to left for a specific set of built-in operators and functions, including unsafe_add, unsafe_sub, unsafe_mul, unsafe_div, pow_mod256, bitwise operators, raw_call, comparison operators, and in/not in when used with enums [1]. This behavior contradicts the expected left-to-right evaluation order and becomes problematic when an argument produces side effects that other arguments depend on [3].

Exploitation

Conditions

An attacker can exploit this issue by crafting a contract where one argument to such an expression performs a state-modifying operation (e.g., a state-changing external or internal call, raw_call, pop() on a storage dynamic array, or proxy creation functions) while another argument reads or depends on the modified state [1]. Because the rightmost argument is evaluated first, the side effect occurs before the left argument is evaluated, leading to unexpected state changes.

Impact

If exploited, this evaluation order discrepancy can result in incorrect contract behavior, such as wrong arithmetic results, unintended state mutations, or logic errors that could be leveraged to drain funds or manipulate contract state. The issue affects any contract using the listed operators with side-effect-producing arguments [3].

Mitigation

Status

As of the publication date, the issue has not been patched [1]. A pull request (PR #4157) has been opened to add a compiler panic for potential evaluation order issues, but a full codegen fix is still pending [4]. Users are advised to ensure that arguments to the affected expressions do not produce side effects, or if they do, that no other argument depends on those side effects [1].