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…

PriorityP341high7.5CVSS 3.1

AVNACLPRNUINSUCNINAH

EPSS

0.26%

17.6th percentile

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.

Affected

3 ranges

Vendor	Product	Version range	Fixed in
fastapiexpert	python-multipart	< 0.0.30	0.0.30
kludex	python-multipart	< 0.0.30	0.0.30
kludex	python-multipart	>= 0 < 0.0.30	0.0.30

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