Tornado: Unbounded query-string argument count allows event-loop-stalling DoS

Exploitation

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

CVSS

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

EPSS

• CVE-2026-103261: 0.47% estimated exploitation activity in the next 30 days; percentile 40.49 (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 HTTPServerRequest.init in tornado/httputil.py parses the URL query string via parseqsbytes() with no field-count limit — while the sibling POST-body parsing path (parsebodyarguments) received a maxnumfields=1000 cap added earlier in this exact same release (v6.5.8, commit 8d6363ed), explicitly to bound parsing cost for the identical underlying primitive. This leaves the query-string path with the resource-exhaustion exposure the body-path fix was meant to close. File: tornado/httputil.py, line 553 (HTTPServerRequest.init) Root Cause Compare with the POST-body path fixed one commit earlier in the same release: Both call sites funnel through the same tornado.escape.parseqsbytes (a thin wrapper over urllib.parse.parseqs), which is exactly why maxnumfields was added to urllib.parse.parseqsl upstream — to let frameworks bound field count. The fix was applied only to the body path; the query-string path was missed. The request line + headers together are capped at maxheadersize…

Published by the source

30 Sep 2026, 23:49 UTC

Last modified by the source

2 Oct 2026, 03:56 UTC

Source last checked

26 Oct 2026, 13:54 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-3hv7-mjh2-fv65

2. Revision 2 · amended · 24 Oct 2026

   Amended from osv record GHSA-3hv7-mjh2-fv65

3. Revision 1 · created · 23 Oct 2026

   Published from ghsa record GHSA-3hv7-mjh2-fv65

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# Patch Next advisory brief: CVE-2026-103261

Advisory: Tornado: Unbounded query-string argument count allows event-loop-stalling DoS
IDs: CVE-2026-103261, GHSA-3hv7-mjh2-fv65
Record: https://patchnext.com/advisories/ghsa-3hv7-mjh2-fv65 (revision 3, source checked 26 Oct 2026, 13:54 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: tornado 6.5.9. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI tornado: affected <= 6.5.8; fixed in 6.5.9

## 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-3hv7-mjh2-fv65 (GitHub-reviewed advisory GHSA-3hv7-mjh2-fv65 lists affected ranges: PyPI tornado >=0 <=6.5.8)
- Verified: https://osv.dev/vulnerability/GHSA-3hv7-mjh2-fv65 (OSV record GHSA-3hv7-mjh2-fv65 lists affected ranges: PyPI tornado >=0 <6.5.9 (fixed))

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