Starlette has possible denial-of-service vector when parsing large files in multipart forms

Exploitation

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

CVSS

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

EPSS

• CVE-2025-54121: 0.66% estimated exploitation activity in the next 30 days; percentile 51.61 (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 a multi-part form with large files (greater than the default max spool size) starlette will block the main thread to roll the file over to disk. This blocks the event thread which means we can't accept new connections. Details Please see this discussion for details: https://github.com/encode/starlette/discussions/2927#discussioncomment-13721403. In summary the following UploadFile code (copied from here) has a minor bug. Instead of just checking for self.inmemory we should also check if the additional bytes will cause a rollover. I have already created a PR which fixes the problem: https://github.com/encode/starlette/pull/2962 PoC See the discussion here for steps on how to reproduce. Impact To be honest, very low and not many users will be impacted. Parsing large forms is already CPU intensive so the additional IO block doesn't slow down starlette that much on systems with modern HDDs/SSDs. If someone is running on tape they might see a greater impact.

Published by the source

21 Jul 2025, 19:34 UTC

Last modified by the source

10 Sep 2026, 03:50 UTC

Source last checked

26 Oct 2026, 13:32 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-2c2j-9gv5-cj73

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-2c2j-9gv5-cj73

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# Patch Next advisory brief: CVE-2025-54121

Advisory: Starlette has possible denial-of-service vector when parsing large files in multipart forms
IDs: CVE-2025-54121, GHSA-2c2j-9gv5-cj73
Record: https://patchnext.com/advisories/cve-2025-54121 (revision 2, source checked 26 Oct 2026, 13:32 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: starlette 0.47.2. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI starlette: affected < 0.47.2; fixed in 0.47.2

## Suggested inspection (read-only)
- `python -m pip show starlette`: 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 starlette.
- 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-2c2j-9gv5-cj73 (OSV record GHSA-2c2j-9gv5-cj73 lists affected ranges: PyPI starlette >=0 <0.47.2 (fixed))
- Verified: https://github.com/advisories/GHSA-2c2j-9gv5-cj73 (GitHub-reviewed advisory GHSA-2c2j-9gv5-cj73 lists affected ranges: PyPI starlette >=0 <0.47.2)

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