CVE-2026-116577 is a medium-severity (CVSS 5.3) CWE-407 vulnerability in tornado. A fix is available for tornado — see the affected versions and patch details below.
Tornado: Quadratic CPU usage when parsing HTTP headers with line folding
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2026-116577 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 396,966 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Real-World Exposure
tornadoReal-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
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 several seconds.
Only availability is affected. Confidentiality and integrity are not.
Patches
Fixed in Tornado 6.6 by removing support for obsolete line folding completely; header lines that begin with whitespace are now rejected.
Workarounds
- Keep
max_header_sizeat its default value or lower it. This limits the impact but doesn't remove it. - Run Tornado behind a reverse proxy that rejects or normalizes obsolete line folding before forwarding requests.
Credit
Reported by @manus-pi.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | tornado | all versions | 6.6pip install --upgrade 'tornado==6.6' |
Detection & mitigation playbook
Open-source dependencyDetect
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.
Fix
Update tornado to 6.6 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-116577 is resolved across your whole dependency graph.
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.
Frequently Asked Questions
Is CVE-2026-116577 in your dependencies?
Find it across PyPI, including transitive dependencies.