# RUSTSEC-2026-0208: Potential Panic in AVX2 SHAKE-256

> A panic in the AVX2 SHAKE-256 implementation allows denial of service when using specific output buffer lengths outside standard ML-KEM/ML-DSA usage.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Not stated
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 21 May 2026

## What is RUSTSEC-2026-0208?

RUSTSEC-2026-0208 is a denial of service vulnerability in libcrux-sha3, published on 21 May 2026. 

### Official description

The AVX2-optimized implementation of SHAKE-256 intended for use in
ML-KEM and ML-DSA would panic if the length of the output buffers was
greater than 32 and not a multiple of 8, due to an out-of-bounds
indexing operation.

## Impact
This bug impacts users on AVX2 platforms that use the
`libcrux_sha3::avx2::x4::shake256` API outside of ML-KEM or ML-DSA
with output buffers of length `> 32` and not divisible by `8`. It does
not impact the use in ML-KEM or ML-DSA because there output buffer
lengths are always divisible by `8`.

## Mitigation
Starting from version `0.0.10`, the AVX2-optimized SHAKE-256 will no
longer panic on output buffer lengths `> 32` that are not divisible by
`8`.

## Which versions are affected by RUSTSEC-2026-0208?

1 open source package ship an affected version, including libcrux-sha3.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `libcrux-sha3` | crates.io | From 0.0.0-0 | 0.0.10 |

## How do I fix RUSTSEC-2026-0208?

Upgrade libcrux-sha3 to 0.0.10 or later.

## Is RUSTSEC-2026-0208 being exploited?

There is no known exploitation of RUSTSEC-2026-0208 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for RUSTSEC-2026-0208 and fix it if it is affected.

What it is: RUSTSEC-2026-0208 is a denial of service vulnerability in libcrux-sha3, published on 21 May 2026. 
Exploitation: There is no known exploitation of RUSTSEC-2026-0208 in the wild and no public exploit code.

Affected packages and fixed versions:
- libcrux-sha3 (crates.io): affected from 0.0.0-0; fixed in 0.0.10

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-2026-0208/ (Markdown: https://insights.sbomify.com/RUSTSEC-2026-0208/index.md)
```

## Sources

- OSV: https://osv.dev/vulnerability/RUSTSEC-2026-0208

---
From sbomify Insights: https://insights.sbomify.com/RUSTSEC-2026-0208/
Summaries are generated and may contain mistakes; the official description above is the record.