Tornado Uncontrolled Resource Consumption Vulnerability - CVE-2025-67726

Tornado is a Python web framework and asynchronous networking library. Versions 6.5.2 and below use an inefficient algorithm when parsing parameters for HTTP header values potentially causing a DoS. The _parseparam function in httputil.py is used to parse specific HTTP header values such as those in multipart/form-data and repeatedly calls string.count() within a nested loop while processing quoted semicolons. If an attacker sends a request with a large number of maliciously crafted parameters in a Content-Disposition header the server39s CPU usage increases quadratically (O(n)) during parsing. Due to Tornado39s single event loop architecture a single malicious request can cause the entire server to become unresponsive for an extended period. This issue is fixed in version 6.5.3.