Tornado: Urlencoded body parsing omits max_num_fields, so one request can stall the event loop

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-2026-82397: 0.35% estimated exploitation activity in the next 30 days; percentile 28.22 (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 Tornado parses application/x-www-form-urlencoded bodies with urllib.parse.parseqs and does not pass maxnumfields. A body made almost entirely of separators produces tens of millions of fields, and the parse happens on the event loop before the handler runs, so a single request stalls the whole server. Where it is tornado/escape.py, at HEAD e530031405e2154654dedc4c84d5656b557ea310: maxnumfields is the parameter CPython added for exactly this, and it is absent. The path to it is entirely server-side and pre-dispatch. RequestHandler.execute parses the body at tornado/web.py:1821, which reaches HTTPServerRequest.parsebody at tornado/httputil.py:636, and the urlencoded branch of parsebodyarguments calls parseqsbytes at tornado/httputil.py:1030. The size that reaches it is bounded only by the body cap, which defaults to the stream's maxbuffersize of 104857600 at tornado/iostream.py:239, applied as the request body default at tornado/http1connection.py:136-140. A 100 MB body of…

Published by the source

2 Sep 2026, 14:38 UTC

Last modified by the source

2 Sep 2026, 14:38 UTC

Source last checked

26 Oct 2026, 13:51 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-mpf4-983q-p7j4

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# Patch Next advisory brief: CVE-2026-82397

Advisory: Tornado: Urlencoded body parsing omits max_num_fields, so one request can stall the event loop
IDs: CVE-2026-82397, GHSA-mpf4-983q-p7j4
Record: https://patchnext.com/advisories/cve-2026-82397 (revision 1, source checked 26 Oct 2026, 13:51 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: tornado 6.5.8. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI tornado: affected <= 6.5.7; fixed in 6.5.8

## 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://github.com/advisories/GHSA-mpf4-983q-p7j4 (GitHub-reviewed advisory GHSA-mpf4-983q-p7j4 lists affected ranges: PyPI tornado >=0 <=6.5.7)

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