Starlette: request.form() limits silently ignored for application/x-www-form-urlencoded enable DoS

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-2026-54283: 0.48% estimated exploitation activity in the next 30 days; percentile 41.51 (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 request.form() accepts maxfields and maxpartsize to bound resource consumption while parsing form data. These limits are enforced for multipart/form-data, but silently ignored for application/x-www-form-urlencoded. An unauthenticated attacker can therefore send a urlencoded body with an arbitrarily large number of fields or an arbitrarily large field, even when the application configured limits it believed would apply. Details request.form() dispatches to a different parser depending on the Content-Type. For multipart/form-data the maxfiles, maxfields, and maxpartsize limits are forwarded to the parser, but for application/x-www-form-urlencoded the parser is constructed without them. It has no maxfields or maxpartsize parameter to receive them, and it appends every field with no count check and accumulates each field's name and value with no size check. The configured limits are therefore both unreachable and unenforced for url-encoded bodies. Because the url-encoded parser…

Published by the source

15 Jun 2026, 20:39 UTC

Last modified by the source

10 Sep 2026, 03:51 UTC

Source last checked

26 Oct 2026, 13:45 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-82w8-qh3p-5jfq

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-82w8-qh3p-5jfq

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# Patch Next advisory brief: CVE-2026-54283

Advisory: Starlette: request.form() limits silently ignored for application/x-www-form-urlencoded enable DoS
IDs: CVE-2026-54283, GHSA-82w8-qh3p-5jfq
Record: https://patchnext.com/advisories/cve-2026-54283 (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: starlette 1.3.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI starlette: affected >= 0.4.1, < 1.3.1; fixed in 1.3.1

## 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-82w8-qh3p-5jfq (OSV record GHSA-82w8-qh3p-5jfq lists affected ranges: PyPI starlette >=0.4.1 <1.3.1 (fixed))
- Verified: https://github.com/advisories/GHSA-82w8-qh3p-5jfq (GitHub-reviewed advisory GHSA-82w8-qh3p-5jfq lists affected ranges: PyPI starlette >=0.4.1 <1.3.1)

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