{"id":"RUSTSEC-2026-0385","title":"Non-constant-time mask generation and `u128` comparisons","summary":"Non-constant-time mask generation and `u128` comparisons","severity":"none","vendor":"ctutils","product":"ctutils","ecosystem":"rust","affected":["ctutils >= 0.0.0-0, < 0.4.4"],"patched":["ctutils 0.4.4"],"published":"2026-10-23","updated":"2026-10-23","sourceUpdated":"2026-10-23T08:30:02.389575143Z","source":"OSV","sourceUrl":"https://osv.dev/vulnerability/RUSTSEC-2026-0385","references":[{"url":"https://crates.io/crates/ctutils"},{"url":"https://rustsec.org/advisories/RUSTSEC-2026-0385.html"},{"url":"https://github.com/RustCrypto/utils/pull/1569"}],"tags":["osv","rust"],"ingestedAt":"2026-10-23T11:58:17.249Z","slug":"RUSTSEC-2026-0385","body":"## Overview\n\n## Summary\n\nThe `ctutils::Choice` type has mask generation functions for each unsigned\ninteger size (`u8`, `u16`, `u32`, `u64`, `u128`). These are `const fn` and\ntherefore can't reuse the `asm!`-backed mask generation in the `cmov` crate\nthat's otherwise used to ensure constant-time.\n\nIn releases prior to v0.4.4, the generated code from these functions could\ninclude a branch on commonly used platforms. This has been corrected in this\nrelease and now verified on several platforms using BINSEC.\n\nAdditionally, `u128` comparison operations provided via `ctutils::Choice` and\nintended to work in constant-time were generating branches on 32-bit targets.\nThis has been resolved via a special case implementation of these functions\nfor such targets.\n\n## Impact\n\nBranch instructions on secret values can potentially leak such values via\ncovert channels. This can enable attackers to potentially recover such secrets.\n\n## Resolution\n\nThis has been resolved by calling `Choice::to_u8()` when generating the mask,\nwhich in turn uses `black_box` as an optimization barrier. Verification that\nthis resulted in the desired change in the generated assembly across a variety\nof target CPU architectures was performed by BINSEC, which now runs\nautomatically in CI using `cargo checkct`.\n\n## Affected packages\n\n- `ctutils >= 0.0.0-0, < 0.4.4`\n\n## Remediation\n\nUpgrade to a patched release:\n\n- `ctutils 0.4.4`","depth":"sunlit","depthScore":3,"depthScoreParts":{"impact":2.8,"likelihood":0,"exploitation":0,"ransomware":0},"changes":[]}