CVE-2026-53537: Python-Multipart: Content-Disposition parameter smuggling via RFC 2231/5987 extended parameters

Summary

parseoptionsheader parsed Content-Disposition (and Content-Type) headers with email.message.Message, which transparently applies RFC 2231/5987 decoding. The extended parameter syntax (filename=charset'lang'value, name=..., and the filename0/filename1 continuation form) is decoded and surfaced under the bare filename/name key, and overrides the plain parameter when both are present. RFC 7578 §4.2 explicitly forbids the filename form in multipart/form-data.

Components that follow RFC 7578, or that do not implement RFC 2231/5987 decoding for multipart/form-data (WAFs, proxies, gateways), may interpret such a header differently. An attacker can exploit that difference to smuggle a different field name or filename past an upstream inspector to the backend.

Details

Given both a plain and an extended parameter, the extended value won. For example:

Content-Disposition: form-data; name="comment"; name=utf-8''role

An inspector following RFC 7578 sees the field comment, while the returned value was name=role. The same applies to filenames:

Content-Disposition: form-data; name="upload"; filename="safe.txt"; filename=utf-8''evil.php

The inspector sees safe.txt, while the returned value was filename=evil.php. Continuation parameters (filename0, filename1, and so on) were likewise reassembled into a filename invisible to a plain filename= match, and percent encoded sequences in the extended value were decoded (so ..%2F, %00, and similar appeared in the returned filename).

This affects the high level parseoptionsheader, FormParser, createformparser, and parseform APIs, and reaches Starlette/FastAPI through request.form(), where the smuggled value is exposed as the form field name or UploadFile.filename.

Impact

This is an interpretation conflict (CWE-436) with other multipart/form-data parsers. An attacker able to submit multipart/form-data can present a different field name or filename to an upstream body inspecting component than the one delivered to the application. Concrete consequences depend on how the application uses these values, and may include bypassing a field name or filename based access/upload control, or, for an application that builds filesystem paths from the parsed filename without sanitization, path traversal via decoded ..%2F sequences. Decoded control bytes such as %00 can likewise cause confusion between an upstream validator and the backend. The File class applies os.path.basename, so file writing through it is not directly affected.

Mitigation

Upgrade to python-multipart 0.0.30 or later, which ignores RFC 2231/5987 extended parameters (name, filename, and their continuations) so the plain name/filename parameter remains authoritative. RFC 7578 §4.2 forbids filename for multipart/form-data; name and the continuation forms are dropped for the same reason, since they are not valid multipart/form-data parameters either.