# CVE-2026-116591: Klaus Post compress zstd decoder panic on crafted input

Source: https://junglewise.ai/threats/cve-2026-116591-compress-provides-various-compression-algorithms-prior-to-1-20-2  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-10-26. Updated: 2026-10-26  
License: CC BY 4.0 with a link back

CVE-2026-116591 · Severity: high · CVSS 4 · Published 2026-10-26

The compress library provides data compression algorithms used by Go applications. A specially crafted zstd-compressed file can crash applications using the decoder, causing a denial of service. An attacker could exploit this by sending malicious compressed data to any application that decompresses untrusted zstd streams.

Technologies: [github.com/klauspost/compress (Go)](https://junglewise.ai/threats/technologies/github-com-klauspost-compress). Vendors: [Go](https://junglewise.ai/threats/vendors/go).

## Quick answers

**Is CVE-2026-116591 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-116591 being exploited in the wild.

**How severe is CVE-2026-116591?** High severity, with a CVSS score of 4, published 26 October 2026.

**Which products and versions are affected?** github.com/klauspost/compress (Go) SEMVER:introduced=0,fixed=1.20.2; github.com/klauspost/compress/zstd (Go) <= 1.20.1, SEMVER:introduced=0,fixed=1.20.2; Klaus Post Compress prior to 1.20.2.

**Is there a fix for CVE-2026-116591?** Yes. github.com/klauspost/compress (Go): upgrade to 1.20.2 or later; github.com/klauspost/compress/zstd (Go): upgrade to 1.20.2 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-116591 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-116591?** Check whether you run github.com/klauspost/compress (Go), github.com/klauspost/compress/zstd (Go) or Klaus Post Compress 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

- github.com/klauspost/compress (Go): upgrade to 1.20.2 or later

  ```sh
  go get github.com/klauspost/compress@v1.20.2
  ```

- github.com/klauspost/compress/zstd (Go): upgrade to 1.20.2 or later

  ```sh
  go get github.com/klauspost/compress/zstd@v1.20.2
  ```


## Technical details

The zstd DecodeAll and streaming decoder paths panic when processing a crafted two-block frame where the first block poisons the Huffman table state via an FSE normalized count of 32768 stored as -32768 in int16, and the second block reuses this stale state to derive an out-of-range index. This requires both blocks in sequence and impacts the huff0.ReadTable component. An attacker with ability to provide untrusted zstd input can trigger an unrecoverable panic.

## Affected products

- Klaus Post compress prior to 1.20.2

## Identifiers

- CVE-2026-116591
- GHSA-fpmh-frq8-7rj8
- GO-2026-6865

## Timeline

- 2026-10-26: disclosed
- 2026-10-12: patched

## References

- [https://github.com/klauspost/compress/commit/7f5d8528720c895a6106f201719f87eb030a565d](https://github.com/klauspost/compress/commit/7f5d8528720c895a6106f201719f87eb030a565d)
- [https://github.com/klauspost/compress/releases/tag/v1.20.2](https://github.com/klauspost/compress/releases/tag/v1.20.2)

## Related threats

- [klauspost compress signed integer overflow in s2.NewDict](https://junglewise.ai/threats/cve-2026-63209-compress-provides-various-compression-algorithms-prior-to-version)
- [GO-2026-5841 - OOB read in github.com/klauspost/compress/s2](https://junglewise.ai/threats/go-2026-5841-oob-read-in-github-com-klauspost-compress-s2-bc43eef4)