Executive brief
Python-Multipart is a library used by web frameworks like FastAPI and Starlette to process form data sent by users. A flaw in how it handles form data allows attackers to bypass security filters by "smuggling" hidden parameters using semicolons. This could allow an attacker to override important settings or permissions that a security firewall or gateway was supposed to block.
Technical details
A parser differential exists in Python-Multipart's QuerystringParser because it treats the semicolon (;) as a field separator in application/x-www-form-urlencoded bodies, contradicting the WHATWG URL standard which only recognizes the ampersand (&). This inconsistency allows for HTTP parameter smuggling when the library is deployed behind a WHATWG-compliant intermediary, such as a Web Application Firewall (WAF) or proxy. An attacker can craft a request where the intermediary sees one set of parameters while the backend Python-Multipart parser sees another, potentially overriding security-critical fields (e.g., 'role=user' becoming 'role=admin'). The vulnerability is fixed in version 0.0.30 by removing semicolon support as a separator.
Affected products
- Kludex python-multipart < 0.0.30
Timeline
- 2026-06-04: advisory: GitHub advisory published by maintainer
- 2026-06-22: disclosed: NVD publication date