CVE-2025-67725 — Tornado is Vulnerable to Quadratic DoS via Repeated Header Coalescing
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 f…
Published: 2025-12-12 · CVSS: 7.5 · CVSS 3.1
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.
Source: CVE Program · Revision 8b16575c39f1e1810c5e863010da073d2d3285e7