# RUSTSEC-2026-0402: Heap buffer overflow in `CCtx::try_clone` on a multithreaded context


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

## What is RUSTSEC-2026-0402?

RUSTSEC-2026-0402 is a vulnerability in zstd-safe, published on 16 Oct 2026. 

### Official description

`CCtx::try_clone` (`experimental` feature, deprecated) copies tables sized
from the context's applied parameters. Starting a multithreaded frame
(`zstdmt` feature) replaces those parameters with the workers' without
resizing the context's own tables, so after an abandoned single-threaded
frame, the copy wrote up to 512 KiB past a heap allocation, or read past
one. This became reachable with the bundled zstd 1.5.6.

The flaw was corrected by refusing `try_clone` (and the unsafe
`compress_block`) once `NbWorkers` was set on the context.

## Which versions are affected by RUSTSEC-2026-0402?

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


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `zstd-safe` | crates.io | From 4.1.4 | 8.2.0 |

## How do I fix RUSTSEC-2026-0402?

Upgrade zstd-safe to 8.2.0 or later.

## Is RUSTSEC-2026-0402 being exploited?

There is no known exploitation of RUSTSEC-2026-0402 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for RUSTSEC-2026-0402 and fix it if it is affected.

What it is: RUSTSEC-2026-0402 is a vulnerability in zstd-safe, published on 16 Oct 2026. 
Exploitation: There is no known exploitation of RUSTSEC-2026-0402 in the wild and no public exploit code.

Affected packages and fixed versions:
- zstd-safe (crates.io): affected from 4.1.4; 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-2026-0402/ (Markdown: https://insights.sbomify.com/RUSTSEC-2026-0402/index.md)
```

## Sources

- OSV: https://osv.dev/vulnerability/RUSTSEC-2026-0402

---
From sbomify Insights: https://insights.sbomify.com/RUSTSEC-2026-0402/
Summaries are generated and may contain mistakes; the official description above is the record.