CVE-2024-32649: vyper performs double eval of the argument of sqrt

Summary Using the sqrt builtin can result in multiple eval evaluation of side effects when the argument has side-effects. The bug is more difficult (but not impossible!) to trigger as of 0.3.4, when the unique symbol fence was introduced (https://github.com/vyperlang/vyper/pull/2914).

A contract search was performed and no vulnerable contracts were found in production.

Details It can be seen that the buildIR function of the sqrt builtin doesn't cache the argument to the stack: https://github.com/vyperlang/vyper/blob/4595938734d9988f8e46e8df38049ae0559abedb/vyper/builtins/functions.py#L2151

As such, it can be evaluated multiple times (instead of retrieving the value from the stack).

PoC With at least Vyper version 0.2.15+commit.6e7dba7 the following contract: vyper c: uint256

@internal def somedecimal() -> decimal: self.c += 1 return 1.0

@external def foo() -> uint256: k: decimal = sqrt(self.somedecimal()) return self.c passes the following test: solidity // SPDX-License-Identifier: MIT pragma solidity >=0.8.13;

import "../../lib/ds-test/test.sol"; import "../../lib/utils/Console.sol"; import "../../lib/utils/VyperDeployer.sol";

import "../ITest.sol";

contract ConTest is DSTest { VyperDeployer vyperDeployer = new VyperDeployer();

ITest t;

function setUp() public { t = ITest(vyperDeployer.deployContract("Test")); }

function testFoo() public { uint256 val = t.foo(); console.log(val); assert (val == 4); } } Patches Patched in https://github.com/vyperlang/vyper/pull/3976.

Impact No vulnerable production contracts were found.