---
id: CVE-2025-67725
aliases:
  - PYSEC-2025-266
  - GHSA-c98p-7wgm-6p64
title: >-
  Tornado is a Python web framework and asynchronous networking library. In
  versions 6.5.2 and below, a single maliciously crafted HTTP req…
summary: >-
  Tornado is a Python web framework and asynchronous networking library. In
  versions 6.5.2 and below, a single maliciously crafted HTTP request can block
  the server's event loop for an extended period, caused by the HTTPHeaders.add
  method.…
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-2025-266'
references:
  - url: 'https://github.com/tornadoweb/tornado/releases/tag/v6.5.3'
  - url: >-
      https://github.com/tornadoweb/tornado/security/advisories/GHSA-c98p-7wgm-6p64
  - url: >-
      https://github.com/tornadoweb/tornado/commit/771472cfdaeebc0d89a9cc46e249f8891a6b29cd
tags:
  - osv
  - pip
epss: 0.00559
epssPercentile: 0.44304
ingestedAt: '2026-07-13T18:58:06.924Z'
---

## Overview

Tornado is a Python web framework and asynchronous networking library. In versions 6.5.2 and below, a single maliciously crafted HTTP request can block the server's event loop for an extended period, caused by the HTTPHeaders.add method. The function accumulates values using string concatenation when the same header name is repeated, causing a Denial of Service (DoS).  Due to Python string immutability, each concatenation copies the entire string, resulting in O(n²) time complexity. The severity can vary from high if max_header_size has been increased from its default, to low if it has its default value of 64KB. 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`