# RUSTSEC-2026-0406: Use-after-free when changing a multithreaded context with an unfinished frame


- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Fix available:** Yes
- **Published:** 16 Oct 2026

## What is RUSTSEC-2026-0406?

RUSTSEC-2026-0406 is a vulnerability in zstd-safe, published on 16 Oct 2026. 

### Official description

When a multithreaded frame (`zstdmt` feature, `CParameter::NbWorkers`) is
left unfinished, its jobs may still be running on the worker threads: the C
library only waits for them when the next frame starts, or when the context
is freed. A session reset makes the context accept changes again in the
meantime, so:

- unloading the dictionary (`load_dictionary`, `ref_cdict`, `ref_prefix`,
  `disable_dictionary`, `init*`, or `reset` with parameters) freed the
  dictionary a worker was still compressing against;
- changing `NbWorkers` made the next frame resize the job table before
  waiting for the old jobs.

The flaw was corrected by refusing these calls while a multithreaded frame
is unfinished. The new `CCtx::wait_for_workers()` waits for the workers
first.

## Which versions are affected by RUSTSEC-2026-0406?

1 open source package ship an affected version, including zstd-safe.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `zstd-safe` | crates.io | From 0.0.0-0 | 8.2.0 |

## How do I fix RUSTSEC-2026-0406?

Upgrade zstd-safe to 8.2.0 or later.

## Is RUSTSEC-2026-0406 being exploited?

There is no known exploitation of RUSTSEC-2026-0406 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for RUSTSEC-2026-0406 and fix it if it is affected.

What it is: RUSTSEC-2026-0406 is a vulnerability in zstd-safe, published on 16 Oct 2026. 
Exploitation: There is no known exploitation of RUSTSEC-2026-0406 in the wild and no public exploit code.

Affected packages and fixed versions:
- zstd-safe (crates.io): affected from 0.0.0-0; fixed in 8.2.0

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-2026-0406/ (Markdown: https://insights.sbomify.com/RUSTSEC-2026-0406/index.md)
```

## Sources

- OSV: https://osv.dev/vulnerability/RUSTSEC-2026-0406

---
From sbomify Insights: https://insights.sbomify.com/RUSTSEC-2026-0406/
Summaries are generated and may contain mistakes; the official description above is the record.