CVE-2026-53539

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 3.1

7.5CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS

0.5% chance of exploitation in the next 30 days; higher than 39% of CVEs (scores of 2026-10-25)

CWEs

CWE-400, CWE-407

Published

2026-06-22

Modified

2026-10-25

Record from

the NVD, synced 2026-10-25

Affected

• fastapiexpert:python-multipart

Packages (OSV)

References

• https://github.com/Kludex/python-multipart/security/advisories/GHSA-5rvq-cxj2-64vf

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