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multiple eval of `sqrt()` argument built in

Low
charles-cooper published GHSA-5jrj-52x8-m64h Apr 25, 2024

Package

pip vyper (pip)

Affected versions

<=0.3.10

Patched versions

0.4.0

Description

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 (#2914).

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

Details

It can be seen that the build_IR function of the sqrt builtin doesn't cache the argument to the stack:

def build_IR(self, expr, args, kwargs, context):

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:

c: uint256

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

@external
def foo() -> uint256:
    k: decimal = sqrt(self.some_decimal())
    return self.c

passes the following test:

// 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 #3976.

Impact

No vulnerable production contracts were found.

Severity

Low

CVE ID

CVE-2024-32649

Weaknesses

No CWEs

Credits