CVE-2026-53538

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.

Score

CVSS 3.1 3.7 Low

Status

Analyzed - security-advisories@github.com

Published

2026-06-22

Last modified

2026-10-25

Weaknesses

CWE-436, CWE-444

Affected products

• Python-multipart (Fastapiexpert)
• Python-multipart (Kludex)

References

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