# zstd-safe use-after-free in multithreaded compression context

Source: https://junglewise.ai/threats/rustsec-2026-0406-use-after-free-when-changing-a-multithreaded-context-0c474512  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-10-16. Updated: 2026-10-28  
License: CC BY 4.0 with a link back

Severity: info · Published 2026-10-16

zstd-safe is a Rust wrapper for the Zstandard compression library that supports parallel compression using multiple worker threads. When a multithreaded compression frame is left incomplete, operations that modify the compression context (such as changing the dictionary or the number of worker threads) can cause the library to access memory that has already been freed, potentially leading to memory corruption or crashes.

Technologies: [zstd-safe (crates.io)](https://junglewise.ai/threats/technologies/zstd-safe), [Zstd-Rs Zstd-Safe](https://junglewise.ai/threats/technologies/zstd-rs-zstd-safe). Vendors: [crates.io](https://junglewise.ai/threats/vendors/crates-io), [Zstd-Rs](https://junglewise.ai/threats/vendors/zstd-rs).

## Quick answers

**Is This vulnerability exploited in the wild?** No. Junglewise Threat Intelligence has no report of This vulnerability being exploited in the wild.

**How severe is This vulnerability?** Informational severity, published 16 October 2026.

**Which products and versions are affected?** zstd-safe (crates.io) SEMVER:introduced=0.0.0-0,fixed=8.2.0; Zstd-Rs Zstd-Safe before 8.2.0.

**Is there a fix for This vulnerability?** Yes. zstd-safe (crates.io): upgrade to 8.2.0 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is This vulnerability attacked?** It is attacked with local access, in a software library.

**Am I affected?** Check whether you run zstd-safe (crates.io) or Zstd-Rs Zstd-Safe at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- zstd-safe (crates.io): upgrade to 8.2.0 or later

  ```sh
  cargo update -p zstd-safe
  ```


## Technical details

A use-after-free vulnerability occurs when multithreaded compression frames (enabled via zstdmt feature or CParameter::NbWorkers) are not fully completed before calling certain context-modification functions. Worker threads may still be using the old dictionary or job table after these functions free or resize them. The vulnerable functions are load_dictionary, ref_cdict, ref_prefix, disable_dictionary, init, reset, and set_parameter. The fix adds wait_for_workers() to ensure all outstanding worker jobs complete before allowing context modifications.

## Affected products

- zstd-rs zstd-safe before 8.2.0

## Timeline

- 2026-10-16: disclosed
- 2026-10-26: advisory

## References

- [https://crates.io/crates/zstd-safe](https://crates.io/crates/zstd-safe)
- [https://rustsec.org/advisories/RUSTSEC-2026-0406.html](https://rustsec.org/advisories/RUSTSEC-2026-0406.html)

## Related threats

- [zstd-safe double free in DCtx::try_clone](https://junglewise.ai/threats/rustsec-2026-0404-double-free-in-dctx-try-clone-108878f4)
- [zstd-safe Seekable decompress buffer overrun on invalid offset](https://junglewise.ai/threats/rustsec-2026-0398-seekable-decompress-sets-a-vec-length-beyond-its-2f278ece)
- [zstd-safe Seekable null or out-of-bounds seek table read](https://junglewise.ai/threats/rustsec-2026-0396-seekable-reads-a-null-or-out-of-bounds-seek-table-4b9e21ae)
- [zstd-safe use-after-free in dictionary decompression context](https://junglewise.ai/threats/rustsec-2026-0400-use-after-free-of-a-dictionary-after-a-one-shot-call-541157eb)
- [zstd-safe out-of-bounds read in seekable stream compression](https://junglewise.ai/threats/rustsec-2026-0405-out-of-bounds-read-with-an-out-of-range-inbuffer-pos-610ef200)