Tornado Uncontrolled Resource Consumption Vulnerability - 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 server39s 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(n) 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.