CVE-2025-67726: Tornadoweb Tornado

High severity, CVSS 7.5. EPSS: 0.5% chance of exploitation in the next 30 days.

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 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. This issue is fixed in version 6.5.3.

Affected products

• Tornadoweb Tornado: before 6.5.3 (fixed in 6.5.3)

Published 2025-12-12. Last modified 2026-10-07.