CVE-2026-53537: Kludex Python-Multipart parameter smuggling in Content-Disposition header

CVE-2026-53537 · Severity: low · CVSS 3.7 · Published 2026-06-22

Python-Multipart is a software library used by web frameworks like FastAPI to process data uploaded through web forms. A security flaw allows attackers to hide or "smuggle" malicious filenames or form field names past security filters (like firewalls) by using specialized formatting that the library interprets differently than the filters. This could allow an attacker to bypass upload restrictions or, in some cases, perform unauthorized file operations on the server.

Technologies: python-multipart (PyPI). Vendors: PyPI.

Quick answers

Is CVE-2026-53537 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-53537 being exploited in the wild.
How severe is CVE-2026-53537?
Low severity, with a CVSS score of 3.7, published 22 June 2026.
Which products and versions are affected?
python-multipart (PyPI) < 0.0.30, ECOSYSTEM:introduced=0,fixed=0.0.30.
Is there a fix for CVE-2026-53537?
Yes. python-multipart (PyPI): upgrade to 0.0.30 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-53537 attacked?
It is attacked over the network, in a software library.
Am I affected by CVE-2026-53537?
Check whether you run python-multipart (PyPI) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

  • python-multipart (PyPI): upgrade to 0.0.30 or later
    pip install "python-multipart>=0.0.30"

Technical details

An interpretation conflict (CWE-436) exists in Python-Multipart's parse_options_header function. Prior to version 0.0.30, the parser used email.message.Message to decode Content-Disposition headers, which incorrectly applied RFC 2231/5987 decoding to multipart/form-data—a practice explicitly forbidden by RFC 7578. This allows extended parameters (like filename*) to override plain parameters (like filename). Because many WAFs and proxies follow RFC 7578 and ignore the extended form, an attacker can craft a header where the security proxy sees a "safe" filename while the Python backend processes a "malicious" one. This can lead to path traversal via decoded sequences like '..%2F' or bypasses of filename-based access controls. The issue is fixed in version 0.0.30 by ignoring extended parameters.

Affected products

  • Kludex python-multipart < 0.0.30

Identifiers

  • CVE-2026-53537
  • GHSA-vffw-93wf-4j4q
  • PYSEC-2026-3041

Timeline

  • 2026-06-04: advisory: GitHub Security Advisory published by Kludex
  • 2026-06-22: disclosed: CVE-2026-53537 published to NVD
  • 2026-06-22: patched: Vulnerability fixed in version 0.0.30

References

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