---
id: CVE-2025-67726
aliases:
  - PYSEC-2025-267
  - GHSA-jhmp-mqwm-3gq8
title: >-
  Tornado is a Python web framework and asynchronous networking library.
  Versions 6.5.2 and below use an inefficient algorithm when parsing…
summary: >-
  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.…
severity: high
cvss: 7.5
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H'
vendor: tornado
product: tornado
ecosystem: pip
affected:
  - tornado < 6.5.3
patched:
  - tornado 6.5.3
published: '2025-12-12'
updated: '2026-07-13'
source: OSV
sourceUrl: 'https://osv.dev/vulnerability/PYSEC-2025-267'
references:
  - url: 'https://github.com/tornadoweb/tornado/releases/tag/v6.5.3'
  - url: >-
      https://github.com/tornadoweb/tornado/security/advisories/GHSA-jhmp-mqwm-3gq8
  - url: >-
      https://github.com/tornadoweb/tornado/commit/771472cfdaeebc0d89a9cc46e249f8891a6b29cd
tags:
  - osv
  - pip
epss: 0.00535
epssPercentile: 0.42769
ingestedAt: '2026-07-13T18:58:06.943Z'
---

## Overview

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.

## Affected packages

- `tornado < 6.5.3`

## Remediation

Upgrade to a patched release:

- `tornado 6.5.3`