Tornado is vulnerable to DoS due to too many multipart parts

Exploitation

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

CVSS 4.0

8.7 · CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N · score source

EPSS

• CVE-2026-31958: 0.39% estimated exploitation activity in the next 30 days; percentile 33.15 (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.

In versions of Tornado prior to 6.5.5, the only limit on the number of parts in multipart/form-data is the maxbodysize setting (default 100MB). Since parsing occurs synchronously on the main thread, this creates the possibility of denial-of-service due to the cost of parsing very large multipart bodies with many parts. Tornado 6.5.5 introduces new limits on the size and complexity of multipart bodies, including a default limit of 100 parts per request. These limits are configurable if needed; see tornado.httputil.ParseMultipartConfig. It is also now possible to disable multipart/form-data parsing entirely if it is not required for the application.

Published by the source

12 Mar 2026, 14:19 UTC

Last modified by the source

10 Sep 2026, 03:50 UTC

Source last checked

26 Oct 2026, 13:44 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-qjxf-f2mg-c6mc

2. Revision 2 · amended · 25 Oct 2026

   Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

3. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-qjxf-f2mg-c6mc

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# Patch Next advisory brief: CVE-2026-31958

Advisory: Tornado is vulnerable to DoS due to too many multipart parts
IDs: CVE-2026-31958, GHSA-qjxf-f2mg-c6mc
Record: https://patchnext.com/advisories/cve-2026-31958 (revision 3, source checked 26 Oct 2026, 13:44 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.7)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: tornado 6.5.5. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI tornado: affected <= 6.5.4; fixed in 6.5.5

## Suggested inspection (read-only)
- `python -m pip show tornado`: 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 tornado.
- 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-qjxf-f2mg-c6mc (OSV record GHSA-qjxf-f2mg-c6mc lists affected ranges: PyPI tornado >=0 <6.5.5 (fixed))
- Verified: https://github.com/advisories/GHSA-qjxf-f2mg-c6mc (GitHub-reviewed advisory GHSA-qjxf-f2mg-c6mc lists affected ranges: PyPI tornado >=0 <=6.5.4)

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