# zstd-safe out-of-bounds read in train_from_buffer with sample size overflow

Source: https://junglewise.ai/threats/rustsec-2026-0401-out-of-bounds-read-in-train-from-buffer-when-sample-b8e1d058  
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 for compression and decompression using the zstd algorithm. A flaw in the dictionary training function allows an attacker who controls sample sizes to read memory beyond the allocated buffer, potentially exposing sensitive data from memory.

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 over the network.

**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

An integer overflow in the `train_from_buffer` function fails to detect when the sum of sample sizes overflows in release builds (where overflow checks are disabled). An attacker providing crafted sample sizes (e.g., totaling 2^64 plus the buffer length) can bypass the bounds check and cause the trainer to read past the end of the buffer. The vulnerability requires untrusted sample size input and is fixed by adding explicit overflow checks during summation.

## Affected products

- zstd-rs zstd-safe before 8.2.0

## Timeline

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

## References

- [https://crates.io/crates/zstd-safe](https://crates.io/crates/zstd-safe)
- [https://rustsec.org/advisories/RUSTSEC-2026-0401.html](https://rustsec.org/advisories/RUSTSEC-2026-0401.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)