RUSTSEC-2026-0003: Non-constant-time code generation on ARM32 targets

What to do

1. See which version you have: cargo tree -i cmov (it also shows which of your dependencies pulls it in).
2. Update within your version requirement: cargo update -p cmov. If the fix is a newer major or minor than your requirement allows, raise it in Cargo.toml (for example cmov = "0.4.4") or update the dependency that pulls it in.
3. Check again with cargo audit, or paste your Cargo.lock into our Cargo.toml checker.

Details from the advisory

Summary

While the cmov crate has a special backend for aarch64 which uses special CSEL instructions, on 32-bit ARM it uses a portable pure Rust fallback implementation. This implementation uses a combination of bitwise arithmetic and core::hint::black_box to attempt to coerce constant-time code generation out of the optimizer, but the implementation in v0.4.3 and earlier failed to do this on 32-bit ARM targets.

Impact

Branch instructions inserted by the LLVM optimizer on 32-bit targets can be leveraged using various microarchitectural sidechannels like cache timing attacks to learn secret information that cmov is designed to protect.

Details

The following assembly was emitted when using Cmov::cmovnz, a function which implements a conditional move when a provided value is non-zero:

    bne  .LBB0_2
    mvns r3, r3

This includes a branch instruction bne: Branch if Not Equal.

PoC

The following code reproduces the issue:

#![no_std]
use cmov::Cmov;

#[inline(never)]
pub fn test_ct_cmov(a: &mut u8, b: u8, c: u8) {
    a.cmovnz(&b, c);
}

Resolution

cmov v0.4.4 includes a portable black_box-based tactical mitigation for the issue which coerced the compiler into producing the expected codegen, and additionally v0.4.5 added an asm! reimplementation of the problematic mask generation function for ARM32 targets which should guarantee that particular function never contains a branch on such targets.

References

• Package: crates.io/crates/cmov
• Advisory: rustsec.org/advisories/RUSTSEC-2026-0003.html
• Web: github.com/RustCrypto/utils/commit/55977257e7c82a309d5e8abfdd380a774f0f9778

From the RustSec Advisory Database (CC0-1.0) via the OSV export, downloaded Oct 24, 2026. Severity score computed from the published CVSS vector.

Other advisories for cmov

• VulnerabilityGHSA-3rjw-m598-pq24 · 2026-07-02Cmov/CmovEq on aarch64 can produce wrong results if high-bits of registers are set

Full security history of cmov