Tornado: Quadratic DoS via Crafted Multipart Parameters

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-2025-67726: 0.53% estimated exploitation activity in the next 30 days; percentile 45.22 (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 The parseparam function in Tornado's httputil.py is used to parse specific HTTP header values, such as those in multipart/form-data. This function uses an inefficient algorithm that repeatedly calls string.count() within a nested loop while processing quoted semicolons (e.g., param=";"). As a result, if an attacker sends a request with a large number of maliciously crafted parameters in a Content-Disposition header, the server's CPU usage increases quadratically (O(n²)) during parsing. Due to Tornado's single event loop architecture, a single malicious request can cause the entire server to become unresponsive for an extended period, leading to a Denial of Service (DoS). Severity: High

Published by the source

20 Jul 2026, 18:57 UTC

Last modified by the source

20 Jul 2026, 19:15 UTC

Source last checked

26 Oct 2026, 13:49 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-jhmp-mqwm-3gq8

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-jhmp-mqwm-3gq8

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# Patch Next advisory brief: CVE-2025-67726

Advisory: Tornado: Quadratic DoS via Crafted Multipart Parameters
IDs: CVE-2025-67726, GHSA-jhmp-mqwm-3gq8
Record: https://patchnext.com/advisories/cve-2025-67726 (revision 2, source checked 26 Oct 2026, 13:49 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 6.5.3. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI tornado: affected < 6.5.3; fixed in 6.5.3

## 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-jhmp-mqwm-3gq8 (OSV record GHSA-jhmp-mqwm-3gq8 lists affected ranges: PyPI tornado >=0 <6.5.3 (fixed))
- Verified: https://github.com/advisories/GHSA-jhmp-mqwm-3gq8 (GitHub-reviewed advisory GHSA-jhmp-mqwm-3gq8 lists affected ranges: PyPI tornado >=0 <6.5.3)

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