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High, CVSS 7.5amended

python-multipart has Denial of Service via unbounded multipart part headers

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

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (python-multipart 0.0.27), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

Sources

Severity and exploitation
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-2026-42561: 0.74% estimated exploitation activity in the next 30 days; percentile 54.67 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

Summary python-multipart has a denial of service vulnerability in multipart part header parsing. When parsing multipart/form-data, MultipartParser previously had no limit on the number of part headers or the size of an individual part header. An attacker could send a request with either many repeated headers without terminating the header block or a single very large header value, causing excessive CPU work before request rejection or completion. Impact Applications that parse attacker-controlled multipart/form-data with affected versions of python-multipart can experience CPU exhaustion. ASGI applications using Starlette, FastAPI, or other frameworks that invoke python-multipart may have worker or event-loop delays while processing malicious upload requests. Details The affected parser states are HEADERFIELDSTART, HEADERFIELD, HEADERVALUESTART, HEADERVALUE, and HEADERVALUEALMOSTDONE. The issue can be triggered by: - A multipart part with an oversized individual header value. - A…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 2 · amended ·

    Amended from ghsa record GHSA-pp6c-gr5w-3c5g

  2. Revision 1 · created ·

    Published from osv record GHSA-pp6c-gr5w-3c5g

Agent brief

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# Patch Next advisory brief: CVE-2026-42561

Advisory: python-multipart has Denial of Service via unbounded multipart part headers
IDs: CVE-2026-42561, GHSA-pp6c-gr5w-3c5g
Record: https://patchnext.com/advisories/cve-2026-42561 (revision 2, source checked 26 Oct 2026, 13:41 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 0.0.27. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI python-multipart: affected < 0.0.27; fixed in 0.0.27

## 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-pp6c-gr5w-3c5g (OSV record GHSA-pp6c-gr5w-3c5g lists affected ranges: PyPI python-multipart >=0 <0.0.27 (fixed))
- Verified: https://github.com/advisories/GHSA-pp6c-gr5w-3c5g (GitHub-reviewed advisory GHSA-pp6c-gr5w-3c5g lists affected ranges: PyPI python-multipart >=0 <0.0.27)

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