python-multipart: Quadratic-time querystring parsing with semicolon separators causes CPU denial of service

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

7.5 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS

• CVE-2026-53539: 0.46% estimated exploitation activity in the next 30 days; percentile 39.92 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary When parsing application/x-www-form-urlencoded bodies, QuerystringParser located the field separator with a two step lookup: it first scanned the entire remaining buffer for &, and only when no & existed anywhere ahead did it fall back to scanning for ;. For a body that uses ; as the separator and contains no &, every field iteration performed a full failed & scan over the entire remaining buffer before locating the nearby ;. With N semicolon separated fields in a chunk of size B, this yields O(B^2) byte comparisons per chunk. An attacker can submit a small crafted body of the form a;a;a;... and cause the parser to spend seconds of CPU per request. A handful of concurrent requests can exhaust worker processes. Details In pythonmultipart/multipart.py, both the FIELDNAME and FIELDDATA states located the next separator like this: data.find(b"&", i) scans from i to the end of the buffer and returns -1 only when there is no & anywhere in the remainder. For a ; separated body with…

Published by the source

15 Jun 2026, 20:24 UTC

Last modified by the source

10 Sep 2026, 03:50 UTC

Source last checked

26 Oct 2026, 13:45 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-5rvq-cxj2-64vf

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-5rvq-cxj2-64vf

Preview the exact text before you copy it. Commands are read-only suggestions; review them before running anything.

# Patch Next advisory brief: CVE-2026-53539

Advisory: python-multipart: Quadratic-time querystring parsing with semicolon separators causes CPU denial of service
IDs: CVE-2026-53539, GHSA-5rvq-cxj2-64vf
Record: https://patchnext.com/advisories/cve-2026-53539 (revision 2, source checked 26 Oct 2026, 13:45 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: python-multipart 0.0.30. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI python-multipart: affected < 0.0.30; fixed in 0.0.30

## Suggested inspection (read-only)
- `python -m pip show python-multipart`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on python-multipart.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-5rvq-cxj2-64vf (OSV record GHSA-5rvq-cxj2-64vf lists affected ranges: PyPI python-multipart >=0 <0.0.30 (fixed))
- Verified: https://github.com/advisories/GHSA-5rvq-cxj2-64vf (GitHub-reviewed advisory GHSA-5rvq-cxj2-64vf lists affected ranges: PyPI python-multipart >=0 <0.0.30)

Commands are suggestions: review each one before running it.