Tornado: Quadratic DoS via Repeated Header Coalescing

NVD Description

## 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.

Affected Packages (1)