CVE-2026-53538 — Python-Multipart: Semicolon treated as querystring field separator enables parameter smuggling

Description (CNA)

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.

Affected (CNA)

Vendor / product	Versions
Kludex / python-multipart	affected < 0.0.30

CWE

CWE-436, CWE-444

References

https://github.com/Kludex/python-multipart/security/advisories/GHSA-6jv3-5f52-599m

ADP enrichment

CISA-ADP

No English description supplied.

No references supplied.