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PatchSiren cyber security CVE debrief

CVE-2026-53539 Kludex CVE debrief

CVE-2026-53539 is a denial of service vulnerability in Python-Multipart, a streaming multipart parser for Python. Prior to version 0.0.30, the parser is vulnerable to a denial of service attack when parsing application/x-www-form-urlencoded bodies. An attacker can submit a small crafted body and cause the parser to spend seconds of CPU per request, potentially exhausting worker processes. The vulnerability is fixed in version 0.0.30. This issue has a CVSS score of 7.5 and a HIGH severity rating.

Vendor
Kludex
Product
python-multipart
CVSS
HIGH 7.5
CISA KEV
Not listed in stored evidence
Original CVE published
2026-06-22
Original CVE updated
2026-06-26
Advisory published
2026-06-22
Advisory updated
2026-06-26

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.

Sources and references

Verified primary and authoritative sources

  • CVE-2026-53539 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2026-53539

    CVE Program - Official CVE Program record with source-provided CVE metadata.

  • CVE-2026-53539 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2026-53539

    NIST National Vulnerability Database - Official NIST NVD detail page and source-specific vulnerability assessment.

Supplemental references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.