CVE-2026-53539 Kludex CVE debrief

Who should care

Developers and administrators using Python-Multipart in their applications should be aware of this vulnerability and take steps to mitigate it. This includes updating to version 0.0.30 or later, and being cautious of potential denial of service attacks. Additionally, users of applications that rely on Python-Multipart should monitor their systems for potential performance issues.

Technical summary

The vulnerability in Python-Multipart is caused by the QuerystringParser's two-step lookup for field separators. When parsing application/x-www-form-urlencoded bodies, the parser first scans the entire remaining buffer for '&', and only when no '&' exists does it fall back to scanning for ';'. This can lead to O(B^2) byte comparisons per chunk, where B is the size of the chunk. An attacker can exploit this by submitting a small crafted body with multiple semicolon-separated fields, causing the parser to spend excessive CPU time. The fix in version 0.0.30 addresses this issue.

Defensive priority

High priority should be given to updating Python-Multipart to version 0.0.30 or later. Additionally, defenders should monitor their systems for potential performance issues and be cautious of potential denial of service attacks.

Recommended defensive actions

• Update Python-Multipart to version 0.0.30 or later
• Monitor systems for potential performance issues
• Be cautious of potential denial of service attacks
• Review application code for potential vulnerabilities
• Implement additional security measures to prevent denial of service attacks

Evidence notes

The vulnerability is documented in the CVE record and the NVD detail page. The fix is available in version 0.0.30 of Python-Multipart. The vulnerability has a CVSS score of 7.5 and a HIGH severity rating.

Official resources