Pypi · Python-Multipart · CVE-2026-53537
**Name of the Vulnerable Software and Affected Versions**
Python-Multipart versions prior to 0.0.30
**Description**
The `parse options header` function parsed `Content-Disposition` and `Content-Type` headers using `email.message.Message`, which applies RFC 2231/5987 decoding. This allows extended parameter syntax, such as `filename*`, `name*`, and continuation forms (`filename*0`, `filename*1`), to override plain parameters when both are present. However, RFC 7578 §4.2 explicitly forbids the `filename*` form in `multipart/form-data`.
This creates an interpretation conflict where upstream components (such as WAFs, proxies, or gateways) that follow RFC 7578 or do not implement RFC 2231/5987 decoding may interpret headers differently than the backend. An attacker can exploit this to smuggle a different field name or filename past an inspector. This affects the `parse options header`, `FormParser`, `create form parser`, and `parse form` APIs, and impacts Starlette/FastAPI via `request.form()`, exposing smuggled values as form field names or `UploadFile.filename`.
Potential consequences include bypassing access or upload controls based on field names or filenames, and path traversal if the application builds filesystem paths from the parsed filename without sanitization, as percent-encoded sequences like `..%2F` are decoded.
**Recommendations**
Update to version 0.0.30 or later.