Quadratic CPU usage in Tornado HTTP header parsing with line folding
Key facts
- Attack vector
- Network
- Privileges required
- None
- User interaction
- None
- Attack complexity
- Low
- Impact type
- Denial of service
- Confidentiality · integrity · availability
- None · None · Low
- Fix status
- Partial Red Hat: 11 without a fix
Attack profile from CVSS 3.1 · CNA. Fix status from CNA, GitHub and Red Hat. Impact type from CWE-407.
Summary
Tornado before 6.6 uses quadratic CPU time when its HTTP header parser handles obsolete line folding, because it repeatedly concatenates short continuation lines (CWE-407). An unauthenticated remote client can send a header block with many such continuations, which blocks other connections served by the process. Only availability is affected.
Written with Claude Haiku 5.5 from the sources below · 2026-10-31
Description from the CNA
Tornado is a Python web framework and asynchronous networking library. Prior to 6.6, Tornado's HTTP header parser accepts obsolete line folding and repeatedly concatenates short continuation lines, causing quadratic CPU consumption on the IOLoop thread. An unauthenticated client can submit a header block containing many obsolete line-folding continuations before application code runs, blocking other connections served by the process. The cost grows with max_header_size, and only availability is affected. This issue is fixed in version 6.6.
Why it matters now
SSVC decision points
CISA ADP · SSVC 2.0.3
- Exploitation
- None
- Automatable
- Yes
- Technical impact
- Partial
The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.
Affected products
| Vendor | Product | Affected versions | Fixed in | Status | Source |
|---|---|---|---|---|---|
| tornadoweb | tornado | before 6.6 | 6.6 | Patch available | CNA |
| — | tornado (PyPI) | before 6.6 | 6.6 | Patch available | GHSA-pcg2-g6cj-q92c |
- tornado Vendortornadoweb Affected before 6.6 Fixed in 6.6 Patch available CNA
- tornado (PyPI) Vendor— Affected before 6.6 Fixed in 6.6 Patch available GHSA-pcg2-g6cj-q92c
CPE names Threatist (inferred) · 1
Inferred from how NVD named this product before; not given by a source.
| CPE | Versions | Role | Vendor |
|---|---|---|---|
| cpe:2.3:a:tornadoweb:tornado:*:*:*:*:*:*:*:* | — | — | tornadoweb |
- cpe:2.3:a:tornadoweb:tornado:*:*:*:*:*:*:*:* Versions — — ·tornadoweb
Remediation
Upgrade the PyPI package tornado to version 6.6, which fixes this issue. Red Hat lists Lightspeed Core, OpenShift Lightspeed, Red Hat Ansible Automation Platform 2, Red Hat Enterprise Linux 8, Red Hat Enterprise Linux 9, Red Hat Enterprise Linux 10, Red Hat Enterprise Linux AI (RHEL AI) 3, Red Hat OpenShift AI (RHOAI), Red Hat OpenShift Container Platform 4 and Red Hat OpenShift Container Platform 5 as affected with no fix plan available, and Red Hat OpenStack Platform 16.2 with no fix planned.
Written with Claude Haiku 5.5 from the sources below · 2026-10-31
- Fix · CNA tornado (tornadoweb) Fixed in 6.6
- Fix · GHSA-pcg2-g6cj-q92c tornado (PyPI) Fixed in 6.6 Advisory →
Vendor assessments
Red Hat
Red Hat rating: Moderate. Red Hat's page for this CVE →
| Product | Status | Plan or fix |
|---|---|---|
| Lightspeed Core | Affected | No fix yet: Fix deferred |
| OpenShift Lightspeed | Affected | No fix yet: Fix deferred |
| Red Hat Ansible Automation Platform 2 | Affected | No fix yet: Fix deferred |
| Red Hat Enterprise Linux 10 | Affected | No fix yet: Fix deferred |
| Red Hat Enterprise Linux 8 | Affected | No fix yet: Fix deferred |
| Red Hat Enterprise Linux 9 | Affected | No fix yet: Fix deferred |
| Red Hat Enterprise Linux AI (RHEL AI) 3 | Affected | No fix yet: Fix deferred |
| Red Hat OpenShift AI (RHOAI) | Affected | No fix yet: Fix deferred |
| Red Hat OpenShift Container Platform 4 | Affected | No fix yet: Fix deferred |
| Red Hat OpenShift Container Platform 5 | Affected | No fix yet: Fix deferred |
| Red Hat OpenStack Platform 16.2 | Affected | No fix planned: Out of support scope |
Timeline
- 2026-10-26 Threatist record created from CVE-2026-116577
- 2026-10-27 Affected versions changed · Red Hat
- 2026-10-27 CVSS score added · Red Hat
- 2026-10-28 Alias added: GHSA-pcg2-g6cj-q92c
- 2026-10-28 Fix available: PyPI/[email protected] · GHSA-pcg2-g6cj-q92c
- 2026-10-28 CVSS score added · GHSA-pcg2-g6cj-q92c
- 2026-10-28 Weakness changed · GHSA-pcg2-g6cj-q92c
- 2026-10-28 6 new references · GHSA-pcg2-g6cj-q92c
- 2026-10-30 Affected versions changed · Red Hat
- 2026-10-31 CISA SSVC decision points changed
References
Other 5
- https://github.com/tornadoweb/tornado/security/advisories/GHSA-pcg2-g6cj-q92c
- https://github.com/tornadoweb/tornado/pull/3765
- https://github.com/tornadoweb/tornado/commit/1b7a1dc9f340f52de929852e8125aa12f25297c9
- https://github.com/tornadoweb/tornado/releases/tag/v6.6.0
- https://github.com/tornadoweb/tornado
More tornadoweb vulnerabilities
- TI-2026-65637 Tornado before 6.5.9 Denial of Service via CurlAsyncHTTPClient CVSS 4.0HIGH 8.7
- TI-2026-62120 Tornado before 6.5.8 Memory Amplification DoS via multipart CVSS 4.0HIGH 8.7
- TI-2024-13023 Tornado before 6.4.1 HTTP Request Smuggling via Transfer-Encoding CVSS 4.0CRITICAL 9.0
- TI-2026-65638 Tornado before 6.5.9 StaticFileHandler Path Traversal via Symlink CVSS 4.0HIGH 8.2
- TI-2026-62122 Tornado before 6.5.7 Credential Leak via Handle Reuse CVSS 4.0HIGH 8.2
- TI-2026-62121 Tornado before 6.5.8 Cookie Attribute Injection via Capitalized kwargs CVSS 4.0MEDIUM 6.3
Sources
- CVE-2026-116577 · CVE Program
- CVE-2026-116577 · NVD
- FIRST EPSS
- GHSA-pcg2-g6cj-q92c · GitHub Advisory Database · CC BY 4.0
- CVE-2026-116577 · Red Hat security data · CC BY 4.0
This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences