CVE-2025-67726: Tornado: Quadratic DoS via Crafted Multipart Parameters

The _parseparam function in Tornado’s httputil.py is used to parse specific HTTP header values, such as those in multipart/form-data. This function uses an inefficient algorithm that repeatedly calls string.count() within a nested loop while processing quoted semicolons (e.g., param=";").

As a result, if an attacker sends a request with a large number of maliciously crafted parameters in a Content-Disposition header, the server’s CPU usage increases quadratically (O(n²)) during parsing. Due to Tornado’s single event loop architecture, a single malicious request can cause the entire server to become unresponsive for an extended period, leading to a Denial of Service (DoS).

Severity: High