Summary
Tornado: Quadratic DoS via Crafted Multipart Parameters
Advisory details
Summary
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
References
- https://github.com/advisories/GHSA-jhmp-mqwm-3gq8
- https://github.com/tornadoweb/tornado/security/advisories/GHSA-jhmp-mqwm-3gq8
- https://nvd.nist.gov/vuln/detail/CVE-2025-67726
- https://github.com/tornadoweb/tornado/commit/771472cfdaeebc0d89a9cc46e249f8891a6b29cd
- https://github.com/pypa/advisory-database/tree/main/vulns/tornado/PYSEC-2025-267.yaml
- https://github.com/tornadoweb/tornado/releases/tag/v6.5.3
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