RUSTSEC-2026-0387: Multiple timing variability issues in operations intended to be constant-time

What to do

1. See which version you have: cargo tree -i crypto-bigint (it also shows which of your dependencies pulls it in).
2. Update within your version requirement: cargo update -p crypto-bigint. If the fix is a newer major or minor than your requirement allows, raise it in Cargo.toml (for example crypto-bigint = "0.7.6") 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

Several operations intended to be constant-time could execute secret-dependent branches, depending on the compiler and target CPU. These include subtraction, zero counting, division, and Montgomery multiplication.

Additionally, UintRef equality could terminate at the first unequal limb, and BoxedMontyForm::invert branched on whether an inverse exists.

Impact

Applications using affected operations on secret integers may leak information through timing or microarchitectural side channels. This includes information about intermediate arithmetic values, matching limbs, and invertibility.

Such leakage could potentially enable recovery of cryptographic secrets. Practical exploitability depends on the downstream algorithm, compiler, target CPU, and an attacker's ability to observe the side channel.

Resolution

Upgrade to v0.7.6, which removes the identified branches, updates ctutils to include its constant-time fixes, and adds BINSEC-based regression checks using cargo checkct for arithmetic on multiple targets.

References

• Package: crates.io/crates/crypto-bigint
• Advisory: rustsec.org/advisories/RUSTSEC-2026-0387.html
• Web: github.com/RustCrypto/crypto-bigint/pull/1365

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