CVE-2026-53539

CVE-2026-53539 · Python-Multipart: Quadratic-time querystring parsing with semicolon separators causes CPU denial of service

This page contains facts from public vulnerability sources. Check the original advisory before updating.

Python-Multipart is a streaming multipart parser for Python. Prior to 0.0.30, when parsing application/x-www-form-urlencoded bodies, QuerystringParser located the field separator with a two step lookup: it first scanned the entire remaining buffer for &, and only when no & existed anywhere ahead did it fall back to scanning for ;. For a body that uses ; as the separator and contains no &, every field iteration performed a full failed & scan over the entire remaining buffer before locating the nearby ;. With N semicolon separated fields in a chunk of size B, this yields O(B^2) byte comparisons per chunk. An attacker can submit a small crafted body of the form a;a;a;... and cause the parser to spend seconds of CPU per request. A handful of concurrent requests can exhaust worker processes. This vulnerability is fixed in 0.0.30.

CVSS 7.5 · High · EPSS 0.5% · Fix available

Published

2026-06-22T16:55:42.787Z

Updated

2026-06-23T16:08:36.474Z

Stack: system · python

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