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Not in CISA KEV
HIGH severity

Tornado: Quadratic DoS via Repeated Header CoalescingGHSA-c98p-7wgm-6p64

HIGHFix: tornadoweb/tornado@771472c

GHSA-c98p-7wgm-6p64 is a high-severity (CVSS 7.5) Uncontrolled Resource Consumption vulnerability in tornado. A fix is available for tornado — see the affected versions and patch details below.

Also known asCVE-2025-67725PYSEC-2025-266
Published
Updated
Affected
1 pkg
Patched
1 / 1
Exploits
None indexed
Exploitation data as of Oct 23, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

  • CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
  • CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for GHSA-c98p-7wgm-6p64.

EPSS Exploitation Probability

via FIRST.org ↗
0.6%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs46th percentile — riskier than 46% of all scored CVEsHighest risk
0.00%0.35%0.71%1.06%0.3%0.6%Apr 26Aug 26Oct 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

GHSA-c98p-7wgm-6p64 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 395,249 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

1 pkg affected
🐍tornado

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.

Description

Tornado is a Python web framework and asynchronous networking library. In versions 6.5.2 and below, a single maliciously crafted HTTP request can block the server's event loop for an extended period, caused by the HTTPHeaders.add method. The function accumulates values using string concatenation when the same header name is repeated, causing a Denial of Service (DoS). Due to Python string immutability, each concatenation copies the entire string, resulting in O(n²) time complexity. The severity can vary from high if max_header_size has been increased from its default, to low if it has its default value of 64KB. This issue is fixed in version 6.5.3.

Affected Packages

1 total 1 fixed
EcosystemPackageVulnerable rangeFix
🐍PyPItornadoall versions6.5.3pip install --upgrade 'tornado==6.5.3'

Affected Products

1 product · 1 configurations
Application
tornadotornadoweb
< 6.5.3
range

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for tornado, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update tornado to 6.5.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-c98p-7wgm-6p64 is resolved across your whole dependency graph.

  3. Workarounds

    Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

Red HatModerate

The availability impact of this flaw is partially mitigated on Red Hat products as the host systems are not at risk.

Workaround published by Red Hat
Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base or stability.
Source: Red Hat security advisory for GHSA-c98p-7wgm-6p64 (CC BY 4.0)
ProductFixed inAdvisory
Red Hat Enterprise Linux 8pcs-0:0.10.18-2.el8_10.8RHSA-2026:0930
Red Hat Enterprise Linux 8.4 Advanced Mission Critical Update Supportpcs-0:0.10.8-1.el8_4.10RHSA-2026:2462
Red Hat Enterprise Linux 8.6 Telecommunications Update Servicepcs-0:0.10.12-6.el8_6.12RHSA-2026:2469
Red Hat Enterprise Linux 8.8 Telecommunications Update Servicepcs-0:0.10.15-4.el8_8.10RHSA-2026:2465
Red Hat Enterprise Linux 9.0 Update Services for SAP Solutionspcs-0:0.11.1-10.el9_0.10RHSA-2026:2484
Red Hat OpenShift AI 2.25rhoai/odh-llama-stack-core-rhel9:1770237759RHSA-2026:2106
Red Hat OpenShift AI 3.3rhoai/odh-llama-stack-core-rhel9:1771415661RHSA-2026:3713
UbuntuHIGH
USN-7950-1Ubuntu

Frequently Asked Questions

## Summary The `HTTPHeaders.add` method in Tornado accumulates values using string concatenation when the same header name is repeated. Due to Python string immutability, each concatenation copies the entire string, resulting in O(n²) time complexity. Given Tornado's single event loop architecture, a single maliciously crafted HTTP request can block the server's event loop for an extended period, causing a Denial of Service (DoS). **Severity:** **High** if `max_header_size` has been increased from its default, **low** if it has its default value of 64KB.
O3 Security · Impact-Aware SCA

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Tornado: Quadratic DoS via Repeated Header Coalescing