# CVE-2026-116577: Tornado HTTP header parser quadratic CPU consumption via line folding

Source: https://junglewise.ai/threats/cve-2026-116577-tornado-is-a-python-web-framework-and-asynchronous-networking  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-10-26. Updated: 2026-10-26  
License: CC BY 4.0 with a link back

CVE-2026-116577 · Severity: medium · CVSS 5.3 · Published 2026-10-26

Tornado is a popular Python web framework used to build high-performance web applications. An attacker can send specially crafted HTTP requests with obsolete line-folding continuations to trigger excessive CPU usage on the server's processing thread, preventing the application from serving legitimate requests. This denial-of-service attack requires no authentication and can significantly impact application availability.

Technologies: [tornado (PyPI)](https://junglewise.ai/threats/technologies/tornado), [Tornado](https://junglewise.ai/threats/technologies/tornado-tornado). Vendors: [PyPI](https://junglewise.ai/threats/vendors/pypi), [Tornado](https://junglewise.ai/threats/vendors/tornado).

## Quick answers

**Is CVE-2026-116577 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-116577 being exploited in the wild.

**How severe is CVE-2026-116577?** Medium severity, with a CVSS score of 5.3, published 26 October 2026.

**Which products and versions are affected?** tornado (PyPI) < 6.6, ECOSYSTEM:introduced=0,fixed=6.6; Tornado before 6.6.

**Is there a fix for CVE-2026-116577?** Yes. tornado (PyPI): upgrade to 6.6 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-116577 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-116577?** Check whether you run tornado (PyPI) or Tornado at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- tornado (PyPI): upgrade to 6.6 or later

  ```sh
  pip install "tornado>=6.6"
  ```


## Technical details

Tornado's HTTP header parser prior to version 6.6 accepts obsolete HTTP line folding syntax and repeatedly concatenates continuation lines, causing quadratic CPU consumption. An unauthenticated client can submit a malicious header block before application code executes, blocking the IOLoop thread and other connections. The attack cost scales with the configured max_header_size parameter.

## Affected products

- Tornado Tornado before 6.6

## Identifiers

- CVE-2026-116577
- GHSA-pcg2-g6cj-q92c

## Timeline

- 2026-10-26: disclosed
- 2026-10-06: patched

## References

- [https://github.com/tornadoweb/tornado/commit/1b7a1dc9f340f52de929852e8125aa12f25297c9](https://github.com/tornadoweb/tornado/commit/1b7a1dc9f340f52de929852e8125aa12f25297c9)
- [https://github.com/tornadoweb/tornado/pull/3765](https://github.com/tornadoweb/tornado/pull/3765)

## Related threats

- [Tornado cookie attribute injection in set_cookie max_age](https://junglewise.ai/threats/cve-2026-123535-tornado-before-6-6-does-not-validate-the-max-age-argument-of)
- [Tornado open redirect in RedirectHandler URL templates](https://junglewise.ai/threats/cve-2026-116578-tornado-is-a-python-web-framework-and-asynchronous-networking)
- [Tornado HTTP/1.1 chunked encoding denial of service](https://junglewise.ai/threats/cve-2026-115927-tornado-is-a-python-web-framework-and-asynchronous-networking)
- [Tornado StaticFileHandler path traversal via symlinks](https://junglewise.ai/threats/cve-2026-103263-tornado-staticfilehandler-follows-symlinks-outside-static-root)
- [Tornado CurlAsyncHTTPClient unbounded memory accumulation in decompression](https://junglewise.ai/threats/cve-2026-103262-tornado-curlasynchttpclient-enforces-no-response-size-limit)