CVE-2026-53537

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.

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 (filename*0, filename*1, 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 parse_options_header, FormParser, create_form_parser, and parse_form APIs, and reaches Starlette/FastAPI through request.form(), where the smuggled value is exposed as the form field name or UploadFile.filename.

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.

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.

The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths. Typical impact: varies by context: data corruption, logic bypass, or denial of service.

CVE-2026-53537 has a CVSS score of 3.7 (Low). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (0.0.30); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-53537? CVE-2026-53537 is a low-severity improper input validation vulnerability in python-multipart (pip), affecting versions < 0.0.30. It is fixed in 0.0.30. The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths.
2. How severe is CVE-2026-53537? CVE-2026-53537 has a CVSS score of 3.7 (Low). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of python-multipart are affected by CVE-2026-53537? python-multipart (pip) versions < 0.0.30 is affected.
4. Is there a fix for CVE-2026-53537? Yes. CVE-2026-53537 is fixed in 0.0.30. Upgrade to this version or later.
5. Is CVE-2026-53537 exploitable, and should I be worried? Whether CVE-2026-53537 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-53537 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-53537? Upgrade python-multipart to 0.0.30 or later.