CVE-2026-53538: Kludex Python-Multipart parameter smuggling via semicolon separator

CVE-2026-53538 · Severity: low · CVSS 3.7 · Published 2026-06-22

Python-Multipart is a library used by web frameworks like FastAPI and Starlette to process form data sent by users. A flaw in how it handles form data allows attackers to bypass security filters by "smuggling" hidden parameters using semicolons. This could allow an attacker to override important settings or permissions that a security firewall or gateway was supposed to block.

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

Quick answers

Is CVE-2026-53538 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-53538 being exploited in the wild.
How severe is CVE-2026-53538?
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-2026-53538?
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-2026-53538 attacked?
It is attacked over the network, in a software library.
Am I affected by CVE-2026-53538?
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

A parser differential exists in Python-Multipart's QuerystringParser because it treats the semicolon (;) as a field separator in application/x-www-form-urlencoded bodies, contradicting the WHATWG URL standard which only recognizes the ampersand (&). This inconsistency allows for HTTP parameter smuggling when the library is deployed behind a WHATWG-compliant intermediary, such as a Web Application Firewall (WAF) or proxy. An attacker can craft a request where the intermediary sees one set of parameters while the backend Python-Multipart parser sees another, potentially overriding security-critical fields (e.g., 'role=user' becoming 'role=admin'). The vulnerability is fixed in version 0.0.30 by removing semicolon support as a separator.

Affected products

  • Kludex python-multipart < 0.0.30

Identifiers

  • CVE-2026-53538
  • GHSA-6jv3-5f52-599m
  • PYSEC-2026-3037

Timeline

  • 2026-06-04: advisory: GitHub advisory published by maintainer
  • 2026-06-22: disclosed: NVD publication date

References

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