# CVE-2025-67725: Tornado quadratic DoS via repeated header coalescing

Source: https://junglewise.ai/threats/cve-2025-67725-tornado-quadratic-dos-via-repeated-header-coalescing  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2025-12-12. Updated: 2026-07-20  
License: CC BY 4.0 with a link back

CVE-2025-67725 · Severity: high · CVSS 7.5 · Published 2025-12-12

Tornado is a Python web framework and asynchronous networking library. A vulnerability in how it handles web request headers allows an attacker to slow down or crash the server by sending a specially crafted request with many repeated headers. This can lead to a denial-of-service (DoS) condition, making the application unavailable to legitimate users.

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

## Quick answers

**Is CVE-2025-67725 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2025-67725 being exploited in the wild.

**How severe is CVE-2025-67725?** High severity, with a CVSS score of 7.5, published 12 December 2025.

**Which products and versions are affected?** tornado (PyPI) < 6.5.3, ECOSYSTEM:introduced=0,fixed=6.5.3.

**Is there a fix for CVE-2025-67725?** Yes. tornado (PyPI): upgrade to 6.5.3 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2025-67725 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2025-67725?** Check whether you run tornado (PyPI) or Tornadoweb 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 6.5.3 or later

  ```sh
  pip install "tornado>=6.5.3"
  ```


## Technical details

The HTTPHeaders.add method in Tornado's web server implementation uses string concatenation to coalesce values when the same header name is repeated in a request. Because Python strings are immutable, each concatenation requires copying the existing string, leading to O(n²) time complexity relative to the number of repeated headers. Since Tornado operates on a single-threaded event loop, an attacker can exploit this by sending a large number of repeated headers to block the loop for an extended period. The impact is significantly higher if the default 'max_header_size' (64KB) has been increased. This issue is fixed in version 6.5.3.

## Affected products

- Tornadoweb Tornado < 6.5.3

## Identifiers

- CVE-2025-67725
- GHSA-c98p-7wgm-6p64
- PYSEC-2025-266

## Timeline

- 2025-12-11: disclosed
- 2025-12-12: advisory: NVD publication date
- 2026-07-20: patched: GitHub Advisory published/updated date

## References

- [https://api.github.com/users/Finder16](https://api.github.com/users/Finder16)
- [https://github.com/Finder16](https://github.com/Finder16)
- [https://api.github.com/users/Finder16/gists%7B/gist_id%7D](https://api.github.com/users/Finder16/gists%7B/gist_id%7D)
- [https://api.github.com/users/Finder16/repos](https://api.github.com/users/Finder16/repos)
- [https://avatars.githubusercontent.com/u/101492831?v=4](https://avatars.githubusercontent.com/u/101492831?v=4)
- [https://api.github.com/users/Finder16/events%7B/privacy%7D](https://api.github.com/users/Finder16/events%7B/privacy%7D)

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- [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 header parser quadratic CPU consumption via line folding](https://junglewise.ai/threats/cve-2026-116577-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)