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Low, CVSS 3.7amended

python-multipart: Semicolon treated as querystring field separator enables parameter smuggling

CVE-2026-53538GHSA-6jv3-5f52-599m

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (python-multipart 0.0.30), 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
3.7 · CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N
EPSS
  • CVE-2026-53538: 0.26% estimated exploitation activity in the next 30 days; percentile 17.24 (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 QuerystringParser treated ; as a field separator in application/x-www-form-urlencoded bodies, in addition to &. The WHATWG URL standard, modern browsers, and Python's urllib.parse (since the CVE-2021-23336 fix) treat only & as a separator. This creates a parser differential: the same bytes are tokenized into different fields than a WHATWG compliant intermediary would produce, allowing an attacker to smuggle extra form fields past an upstream body inspecting component. Details In pythonmultipart/multipart.py, the FIELDNAME and FIELDDATA states located the next separator by scanning for & and, failing that, for ;: As a result, ; acted as a field boundary. Because the fallback only triggered when no & remained in the current chunk, tokenization also depended on unrelated bytes later in the buffer and on how the body was split across write() calls. This is the same class of issue as CVE-2021-23336 in CPython's urllib.parse. For example, a body inspecting WAF or gateway that…

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

    Amended from ghsa record GHSA-6jv3-5f52-599m

  2. Revision 1 · created ·

    Published from osv record GHSA-6jv3-5f52-599m

Agent brief

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# Patch Next advisory brief: CVE-2026-53538

Advisory: python-multipart: Semicolon treated as querystring field separator enables parameter smuggling
IDs: CVE-2026-53538, GHSA-6jv3-5f52-599m
Record: https://patchnext.com/advisories/cve-2026-53538 (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 0.0.30. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI python-multipart: affected < 0.0.30; fixed in 0.0.30

## 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-6jv3-5f52-599m (OSV record GHSA-6jv3-5f52-599m lists affected ranges: PyPI python-multipart >=0 <0.0.30 (fixed))
- Verified: https://github.com/advisories/GHSA-6jv3-5f52-599m (GitHub-reviewed advisory GHSA-6jv3-5f52-599m lists affected ranges: PyPI python-multipart >=0 <0.0.30)

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