python-multipart: Negative Content-Length in parse_form buffers the entire body in memory

Exploitation

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

CVSS

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

EPSS

• CVE-2026-53540: 0.34% estimated exploitation activity in the next 30 days; percentile 27.79 (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 parseform() did not validate the Content-Length header before using it to bound its chunked read of the request body. A negative Content-Length turned the bounded read into a read-until-EOF, so the entire body was loaded into memory in a single read instead of in fixed-size chunks. Details parseform() reads the input stream in chunks, never reading more than the remaining Content-Length at a time. The per-chunk size is computed as min(contentlength - bytesread, chunksize). The header value was parsed to an integer without checking its sign, so a Content-Length of -1 made this expression negative, and inputstream.read(-1) reads until end of stream. The intended bounded, chunked read therefore collapsed into a single unbounded read of the whole stream. The amount read is still bounded by what the client actually sends. Impact This only affects code that calls parseform() directly with a Content-Length header taken from attacker-controlled input and without normalizing a…

Published by the source

15 Jun 2026, 20:23 UTC

Last modified by the source

10 Sep 2026, 03:50 UTC

Source last checked

26 Oct 2026, 13:45 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-v9pg-7xvm-68hf

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-v9pg-7xvm-68hf

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# Patch Next advisory brief: CVE-2026-53540

Advisory: python-multipart: Negative Content-Length in parse_form buffers the entire body in memory
IDs: CVE-2026-53540, GHSA-v9pg-7xvm-68hf
Record: https://patchnext.com/advisories/cve-2026-53540 (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: Low (CVSS 3.7)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: python-multipart 0.0.31. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI python-multipart: affected < 0.0.31; fixed in 0.0.31

## 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-v9pg-7xvm-68hf (OSV record GHSA-v9pg-7xvm-68hf lists affected ranges: PyPI python-multipart >=0 <0.0.31 (fixed))
- Verified: https://github.com/advisories/GHSA-v9pg-7xvm-68hf (GitHub-reviewed advisory GHSA-v9pg-7xvm-68hf lists affected ranges: PyPI python-multipart >=0 <0.0.31)

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