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

Summary

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.

Timeline

1. Jun 22, 2026, 4:55 PM UTCCVE published
2. Jun 23, 2026, 2:05 PM UTCCISA SSVC assessment

   Exploitation: none · Automatable: yes · Technical impact: partial

3. Jun 23, 2026, 4:08 PM UTCRecord last updated

Affected versions

• Kludex python-multipart

  fixed in 0.0.30

• python-multipartPyPI

  fixed in 0.0.30

References 1