# CVE-2026-53539: Kludex Python-Multipart denial of service in QuerystringParser

Source: https://junglewise.ai/threats/cve-2026-53539-kludex-python-multipart-denial-of-service-in-querystringparser  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-06-22. Updated: 2026-06-23  
License: CC BY 4.0 with a link back

CVE-2026-53539 · Severity: high · CVSS 7.5 · Published 2026-06-22

A vulnerability in the python-multipart library, commonly used by web frameworks like FastAPI and Starlette, allows attackers to crash or slow down web services. By sending a specially crafted web form, an attacker can force the server to spend excessive amounts of processing power on a single request. This can lead to a denial-of-service (DoS) condition where the server becomes unresponsive to legitimate users.

Technologies: [python-multipart (PyPI)](https://junglewise.ai/threats/technologies/python-multipart). Vendors: [PyPI](https://junglewise.ai/threats/vendors/pypi).

## Quick answers

**Is CVE-2026-53539 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-53539 being exploited in the wild.

**How severe is CVE-2026-53539?** High severity, with a CVSS score of 7.5, 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-2026-53539?** 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-2026-53539 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-53539?** 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

  ```sh
  pip install "python-multipart>=0.0.30"
  ```


## Technical details

The QuerystringParser in python-multipart (prior to version 0.0.30) implements an inefficient two-step lookup for field separators when parsing 'application/x-www-form-urlencoded' bodies. It first scans the entire remaining buffer for '&' before falling back to ';'. If a body uses ';' as a separator and contains no '&', the parser performs a full failed scan of the remaining buffer for every field iteration, resulting in O(B^2) complexity. A remote, unauthenticated attacker can send a small body (e.g., "a;a;a;...") to trigger high CPU consumption, potentially exhausting worker processes in synchronous environments like FastAPI or Starlette. The issue is resolved in version 0.0.30 by adopting a single bounded scan that only recognizes '&' as a separator.

## Affected products

- Kludex python-multipart < 0.0.30

## Identifiers

- CVE-2026-53539
- GHSA-5rvq-cxj2-64vf
- PYSEC-2026-3036

## Timeline

- 2026-06-04: advisory: GitHub advisory published by maintainer
- 2026-06-22: disclosed: CVE published to NVD

## References

- [https://github.com/Kludex/python-multipart/security/advisories/GHSA-5rvq-cxj2-64vf](https://github.com/Kludex/python-multipart/security/advisories/GHSA-5rvq-cxj2-64vf)

## Related threats

- [PYSEC-2026-1851 - Denial of service (DoS) via deformation `multipart/form-data` boundary](https://junglewise.ai/threats/cve-2024-53981-denial-of-service-dos-via-deformation-multipart-form-data)
- [PYSEC-2026-1850 - python-multipart vulnerable to Content-Type Header ReDoS](https://junglewise.ai/threats/cve-2024-24762-python-multipart-content-type-header-redos)
- [Kludex Python-Multipart denial of service via negative Content-Length](https://junglewise.ai/threats/cve-2026-53540-kludex-python-multipart-denial-of-service-via-negative-content)
- [Kludex Python-Multipart parameter smuggling via semicolon separator](https://junglewise.ai/threats/cve-2026-53538-kludex-python-multipart-parameter-smuggling-via-semicolon)
- [Kludex Python-Multipart parameter smuggling in Content-Disposition header](https://junglewise.ai/threats/cve-2026-53537-kludex-python-multipart-parameter-smuggling-in-content)