CVE-2026-53538

Name	CVE-2026-53538
Description	Python-Multipart is a streaming multipart parser for Python. Prior to 0.0.30, QuerystringParser treated ; as a field separator in application/x-www-form-urlencoded bodies, in addition to &. The WHATWG URL standard, modern browsers, and Python's urllib.parse (since the CVE-2021-23336 fix) treat only & as a separator. This creates a parser differential: the same bytes are tokenized into different fields than a WHATWG compliant intermediary would produce, allowing an attacker to smuggle extra form fields past an upstream body inspecting component. This vulnerability is fixed in 0.0.30.
Source	CVE (at NVD; CERT, ENISA, LWN, oss-sec, fulldisc, Debian ELTS, Red Hat, Ubuntu, Gentoo, SUSE bugzilla/CVE, GitHub advisories/code/issues, web search, more)

Vulnerable and fixed packages

The table below lists information on source packages.

Source Package	Release	Version	Status
python-multipart (PTS)	bullseye	0.0.5-2	vulnerable
	bookworm	0.0.5-3	vulnerable
	trixie	0.0.20-1.1~deb13u1	vulnerable
	forky, sid	0.0.26-1	vulnerable

The information below is based on the following data on fixed versions.

Package	Type	Release	Fixed Version	Urgency	Origin	Debian Bugs
python-multipart	source	(unstable)	(unfixed)

Notes

[trixie] - python-multipart <no-dsa> (Minor issue)
https://github.com/Kludex/python-multipart/security/advisories/GHSA-6jv3-5f52-599m
Fixed by: https://github.com/Kludex/python-multipart/commit/d69df35cd2cad9c72794c2c340db646afae957d8 (0.0.30)