CVE-2026-53537

The record

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.

Affected packages

Related vulnerabilities

• CVE-2026-53539 Python-Multipart is a streaming multipart parser for Python. Also affects python-python-multipart
• CVE-2026-53540 Python-Multipart is a streaming multipart parser for Python. Also affects python-python-multipart
• CVE-2026-53538 Python-Multipart is a streaming multipart parser for Python. Also affects python-python-multipart
• CVE-2026-24486 Python-Multipart is a streaming multipart parser for Python. Also affects python-python-multipart
• CVE-2026-42561 Python-Multipart is a streaming multipart parser for Python. Also affects python-python-multipart
• CVE-2026-40347 Python-Multipart is a streaming multipart parser for Python. Also affects python-python-multipart
• CVE-2024-53981 python-multipart is a streaming multipart parser for Python. Also affects python-python-multipart
• CVE-2026-113786 pypdf is a free and open-source pure-python PDF library. CWE-20