Tornado: Quadratic DoS via Repeated Header Coalescing

Summary

The HTTPHeaders.add method in Tornado accumulates values using string concatenation when the same header name is repeated. Due to Python string immutability, each concatenation copies the entire string, resulting in O(n²) time complexity.

Given Tornado's single event loop architecture, a single maliciously crafted HTTP request can block the server's event loop for an extended period, causing a Denial of Service (DoS).

Severity: High if max_header_size has been increased from its default, low if it has its default value of 64KB.