# zstd-safe heap buffer overflow in CCtx::try_clone

Source: https://junglewise.ai/threats/rustsec-2026-0402-heap-buffer-overflow-in-cctx-try-clone-on-a-8f6c7847  
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 library that provides bindings to the Zstandard compression algorithm. A heap buffer overflow flaw in the CCtx::try_clone function can allow an attacker to write or read up to 512 KiB past a heap allocation when using multithreaded compression, potentially leading to memory corruption or information disclosure.

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=4.1.4,fixed=8.2.0; Zstd-Rs Zstd-Safe 4.1.4 through 8.1.x.

**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

The vulnerability exists in CCtx::try_clone (an experimental, deprecated function) which copies internal tables based on context parameters. When a multithreaded compression frame (zstdmt) is started, it replaces those parameters with worker values without resizing the context's tables. After an abandoned single-threaded frame, the try_clone operation writes or reads up to 512 KiB past the heap allocation boundary. The flaw requires a Rust application to explicitly call try_clone and use both single-threaded and multithreaded compression modes on the same context.

## Affected products

- zstd-rs zstd-safe 4.1.4 through 8.1.x

## Timeline

- 2026-10-16: disclosed
- 2026-10-26: patched

## References

- [https://crates.io/crates/zstd-safe](https://crates.io/crates/zstd-safe)
- [https://rustsec.org/advisories/RUSTSEC-2026-0402.html](https://rustsec.org/advisories/RUSTSEC-2026-0402.html)

## Related threats

- [zstd-safe use-after-free in multithreaded compression context](https://junglewise.ai/threats/rustsec-2026-0406-use-after-free-when-changing-a-multithreaded-context-0c474512)
- [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)