Tornado: Quadratic DoS via Crafted Multipart Parameters

## 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** Source Code Location: https://github.com/tornadoweb/tornado Affected Packages: - pip:tornado, affected < 6.5.3, patched in 6.5.3 CWEs: - CWE-400: Uncontrolled Resource Consumption CVSS: - Primary: score 7.5, CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H - CVSS_V3: score 7.5, CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H References: - 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 - https://github.com/advisories/GHSA-jhmp-mqwm-3gq8