Tornado: Quadratic CPU usage when parsing HTTP headers with line folding

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-2026-116577: 0.38% estimated exploitation activity in the next 30 days; percentile 32.50 (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's HTTP header parser takes quadratic time (O(n²)) when a header value is continued across many "obsolete line folding" (obs-fold) lines. An unauthenticated attacker can send a request whose header block is mostly short continuation lines. Parsing it uses a large amount of CPU on the IOLoop thread and blocks every other connection that process is serving. This issue is closely related to CVE-2025-67725 (GHSA-c98p-7wgm-6p64). The fix for that advisory covered repeated headers but not line-folded continuations.

Impact All Tornado HTTP servers are affected. Header parsing runs before any application code, so no authentication or particular handler is required. How bad it gets depends on `max_header_size`: - Default (64 KB): each malicious request blocks the event loop for tens of milliseconds. Repeated requests can degrade service. - Raised `max_header_size`: the cost grows with the square of the header size. A single 1 MB request can block the process for…

Published by the source

28 Oct 2026, 22:43 UTC

Last modified by the source

28 Oct 2026, 23:00 UTC

Source last checked

29 Oct 2026, 06:10 UTC

1. Revision 2 · amended · 29 Oct 2026

   Amended from osv record GHSA-pcg2-g6cj-q92c

2. Revision 1 · created · 28 Oct 2026

   Published from ghsa record GHSA-pcg2-g6cj-q92c

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# Patch Next advisory brief: CVE-2026-116577

Advisory: Tornado: Quadratic CPU usage when parsing HTTP headers with line folding
IDs: CVE-2026-116577, GHSA-pcg2-g6cj-q92c
Record: https://patchnext.com/advisories/cve-2026-116577 (revision 2, source checked 29 Oct 2026, 06:10 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 6.6. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI tornado: affected < 6.6; fixed in 6.6

## 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-pcg2-g6cj-q92c (OSV record GHSA-pcg2-g6cj-q92c lists affected ranges: PyPI tornado >=0 <6.6 (fixed))
- Verified: https://github.com/advisories/GHSA-pcg2-g6cj-q92c (GitHub-reviewed advisory GHSA-pcg2-g6cj-q92c lists affected ranges: PyPI tornado >=0 <6.6)

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