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


- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Fix available:** Yes
- **Published:** 23 Oct 2026

## What is RUSTSEC-2026-0387?

RUSTSEC-2026-0387 is a vulnerability in crypto-bigint, published on 23 Oct 2026. 

### Official description

## 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.

## Which versions are affected by RUSTSEC-2026-0387?

1 open source package ship an affected version, including crypto-bigint.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `crypto-bigint` | crates.io | From 0.0.0-0 | 0.7.6 |

## How do I fix RUSTSEC-2026-0387?

Upgrade crypto-bigint to 0.7.6 or later.

## Is RUSTSEC-2026-0387 being exploited?

There is no known exploitation of RUSTSEC-2026-0387 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for RUSTSEC-2026-0387 and fix it if it is affected.

What it is: RUSTSEC-2026-0387 is a vulnerability in crypto-bigint, published on 23 Oct 2026. 
Exploitation: There is no known exploitation of RUSTSEC-2026-0387 in the wild and no public exploit code.

Affected packages and fixed versions:
- crypto-bigint (crates.io): affected from 0.0.0-0; fixed in 0.7.6

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/RUSTSEC-2026-0387/ (Markdown: https://insights.sbomify.com/RUSTSEC-2026-0387/index.md)
```

## Sources

- OSV: https://osv.dev/vulnerability/RUSTSEC-2026-0387

---
From sbomify Insights: https://insights.sbomify.com/RUSTSEC-2026-0387/
Summaries are generated and may contain mistakes; the official description above is the record.