CVE-2026-53538: Fastapiexpert Python-Multipart

Low severity, CVSS 3.7. EPSS: 0.3% chance of exploitation in the next 30 days.

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 products

• Fastapiexpert Python-Multipart: before 0.0.30 (fixed in 0.0.30)

Published 2026-06-22. Last modified 2026-06-26.