Low [CVE-2026-53537] Information disclosure via header parsing discrepancy

Python-Multipart is a streaming multipart parser for Python. Prior to 0.0.30, parse_options_header 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 filename*0/filename*1 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. This vulnerability is fixed in 0.0.30.

This vulnerability allows a remote attacker to bypass security controls by exploiting a difference in how Content-Disposition and Content-Type headers are parsed. Specifically, the parse_options_header function incorrectly applies RFC 2231/5987 decoding, which is forbidden for multipart/form-data.

This Low impact information disclosure flaw in `python-multipart` stems from a parsing discrepancy in `Content-Disposition` and `Content-Type` headers.