# CVE-2026-42561: Kludex python-multipart denial of service in multipart header parsing

Source: https://junglewise.ai/threats/cve-2026-42561-kludex-python-multipart-denial-of-service-in-multipart-header  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-05-13. Updated: 2026-06-30  
License: CC BY 4.0 with a link back

CVE-2026-42561 · Severity: high · CVSS 7.5 · Published 2026-05-13

Python-Multipart is a software library used by web applications to process file uploads and form data. A vulnerability in this library allows an attacker to crash or slow down a web service by sending specially crafted, malicious upload requests. This can lead to a denial of service, preventing legitimate users from accessing the application and impacting business operations.

Technologies: [python-multipart (PyPI)](https://junglewise.ai/threats/technologies/python-multipart), [Red Hat AI Inference Server](https://junglewise.ai/threats/technologies/ai-inference-server), [Red Hat OpenShift Lightspeed](https://junglewise.ai/threats/technologies/openshift-lightspeed). Vendors: [PyPI](https://junglewise.ai/threats/vendors/pypi), [Red Hat](https://junglewise.ai/threats/vendors/red-hat).

## Quick answers

**Is CVE-2026-42561 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-42561 being exploited in the wild.

**How severe is CVE-2026-42561?** High severity, with a CVSS score of 7.5, published 13 May 2026.

**Which products and versions are affected?** python-multipart (PyPI) < 0.0.27, ECOSYSTEM:introduced=0,fixed=0.0.27; Red Hat AI Inference Server 3; Red Hat Exploit Intelligence 0; Red Hat OpenShift Lightspeed any.

**Is there a fix for CVE-2026-42561?** Yes. python-multipart (PyPI): upgrade to 0.0.27 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-42561 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-42561?** Check whether you run python-multipart (PyPI), Red Hat AI Inference Server, Red Hat Exploit Intelligence or one of the others listed 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 0.0.27 or later

  ```sh
  pip install "python-multipart>=0.0.27"
  ```


## Technical details

A denial of service (DoS) vulnerability exists in python-multipart's MultipartParser due to a lack of resource limits during header parsing. Specifically, the parser does not restrict the number of part headers or the size of individual header values when processing multipart/form-data. An unauthenticated remote attacker can exploit this by sending a request containing many repeated headers or an exceptionally large header value, leading to excessive CPU consumption and event-loop delays in frameworks like FastAPI or Starlette. The issue is rooted in the HEADER_FIELD and HEADER_VALUE parser states. This vulnerability is resolved in version 0.0.27 by enforcing default limits on header counts and sizes.

## Affected products

- Kludex python-multipart < 0.0.27
- Red Hat Exploit Intelligence 0
- Red Hat Migration Toolkit for Applications 8 8
- Red Hat OpenShift Lightspeed any
- Red Hat Red Hat AI Inference Server 3
- Red Hat Red Hat Ansible Automation Platform 2 2
- Red Hat Red Hat Enterprise Linux AI (RHEL AI) 3 3
- Red Hat Red Hat OpenShift AI (RHOAI) any
- Red Hat Red Hat Satellite 6 6

## Identifiers

- CVE-2026-42561
- GHSA-pp6c-gr5w-3c5g
- PYSEC-2026-3039

## Timeline

- 2026-04-29: advisory: GitHub Advisory GHSA-pp6c-gr5w-3c5g published by Kludex
- 2026-05-13: disclosed: CVE-2026-42561 published
- 2026-06-29: other: Red Hat updated their VEX data for affected products

## References

- [https://github.com/Kludex/python-multipart/security/advisories/GHSA-pp6c-gr5w-3c5g](https://github.com/Kludex/python-multipart/security/advisories/GHSA-pp6c-gr5w-3c5g)
- [https://access.redhat.com/security/cve/CVE-2026-42561](https://access.redhat.com/security/cve/CVE-2026-42561)
- [https://bugzilla.redhat.com/show_bug.cgi?id=2477309](https://bugzilla.redhat.com/show_bug.cgi?id=2477309)
- [https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-42561.json](https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-42561.json)

## 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 denial of service in QuerystringParser](https://junglewise.ai/threats/cve-2026-53539-kludex-python-multipart-denial-of-service-in-querystringparser)
- [Kludex Python-Multipart parameter smuggling via semicolon separator](https://junglewise.ai/threats/cve-2026-53538-kludex-python-multipart-parameter-smuggling-via-semicolon)