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

Vulnerability summary

CVE-2025-67726 is a high (CVSS 7.5) Uncontrolled Resource Consumption vulnerability in tornadoweb tornado. EPSS estimates a 0.53% chance of exploitation in the next 30 days, higher than 45.2% of scored vulnerabilities.

Timeline

1. 12-12-2025
   PublishedVulnerability first disclosed.
2. 18-12-2025
   Last modifiedVulnerability information updated.

Description

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.

• CWE-400Uncontrolled Resource ConsumptionThe product does not properly control the allocation and maintenance of a limited resource.
• CWE-834Excessive IterationThe product performs an iteration or loop without sufficiently limiting the number of times that the loop is executed.