CVE-2025-67725

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.

The following products are affected by CVE-2025-67725 vulnerability.

Here, you will find a curated list of external links that provide in-depth information to CVE-2025-67725.

URL	Resource
https://github.com/tornadoweb/tornado/commit/771472cfdaeebc0d89a9cc46e249f8891a6b29cd
https://github.com/tornadoweb/tornado/releases/tag/v6.5.3
https://github.com/tornadoweb/tornado/security/advisories/GHSA-c98p-7wgm-6p64
https://nvd.nist.gov/vuln/detail/CVE-2025-67725
https://www.cve.org/CVERecord?id=CVE-2025-67725