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

June 19, 2026

PackInfo._read() uses an O(n^2) cumulative sum pattern where numstreams is read directly from the archive header. A crafted .7z archive with a large numstreams value causes excessive CPU consumption during SevenZipFile.init() — no extraction is needed. A 50 KB archive takes ~7 seconds of CPU time.

References

github.com/advisories/GHSA-h4gh-22qq-72r7
github.com/miurahr/py7zr/releases/tag/v1.1.3
github.com/miurahr/py7zr/security/advisories/GHSA-h4gh-22qq-72r7
nvd.nist.gov/vuln/detail/CVE-2026-55206

Code Behaviors & Features

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