# RUSTSEC-2026-0306: `hex_decode_unchecked` AVX2 path reads past `src`

> A memory safety bug in the faster_hex library allows out-of-bounds reads on x86 systems with AVX2, potentially causing crashes or information disclosure.

- **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:** 20 Sep 2026

## What is RUSTSEC-2026-0306?

RUSTSEC-2026-0306 is a memory corruption vulnerability in faster-hex, published on 20 Sep 2026. 

### Official description

`faster_hex::hex_decode_unchecked` is a safe function. On x86 and x86_64, when AVX2 is enabled, `hex_decode_avx2` keeps decoding while `dst.len() >= 32` and then loads 32 bytes from `src` and 32 bytes from `src[32..]`. Nothing in that loop requires `src` to contain 64 bytes.

A safe caller can pass a short `src` and a `dst` of at least 32 bytes. The AVX2 load then reads outside `src`.

The function has had this shape since 0.3.0. 0.10.0 is the last release with the unbounded AVX2 read. The maintainer agreed to this advisory and shipped the backport in 0.10.1 (pull request 74): the AVX2 loop now requires `src.len() >= 64` before those loads. 1.0.0 rewrites `hex_decode_unchecked` so the decoded length is `min(src.len() / 2, dst.len())`. Both are covered by `>= 0.10.1`. Other architectures take the scalar fallback and are outside this advisory.

## Which versions are affected by RUSTSEC-2026-0306?

1 open source package ship an affected version, including faster-hex.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `faster-hex` | crates.io | From 0.3.0 | 0.10.1 |

## How do I fix RUSTSEC-2026-0306?

Upgrade faster-hex to 0.10.1 or later.

## Is RUSTSEC-2026-0306 being exploited?

There is no known exploitation of RUSTSEC-2026-0306 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for RUSTSEC-2026-0306 and fix it if it is affected.

What it is: RUSTSEC-2026-0306 is a memory corruption vulnerability in faster-hex, published on 20 Sep 2026. 
Exploitation: There is no known exploitation of RUSTSEC-2026-0306 in the wild and no public exploit code.

Affected packages and fixed versions:
- faster-hex (crates.io): affected from 0.3.0; fixed in 0.10.1

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-2026-0306/ (Markdown: https://insights.sbomify.com/RUSTSEC-2026-0306/index.md)
```

## Sources

- OSV: https://osv.dev/vulnerability/RUSTSEC-2026-0306

---
From sbomify Insights: https://insights.sbomify.com/RUSTSEC-2026-0306/
Summaries are generated and may contain mistakes; the official description above is the record.