Tornado: Quadratic DoS via Repeated Header Coalescing

## Summary The `HTTPHeaders.add` method in Tornado accumulates values using string concatenation when the same header name is repeated. Due to Python string immutability, each concatenation copies the entire string, resulting in O(n²) time complexity. Given Tornado's single event loop architecture, a single maliciously crafted HTTP request can block the server's event loop for an extended period, causing a Denial of Service (DoS). **Severity:** **High** if `max_header_size` has been increased from its default, **low** if it has its default value of 64KB. 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-c98p-7wgm-6p64 - https://nvd.nist.gov/vuln/detail/CVE-2025-67725 - https://github.com/tornadoweb/tornado/commit/771472cfdaeebc0d89a9cc46e249f8891a6b29cd - https://github.com/pypa/advisory-database/tree/main/vulns/tornado/PYSEC-2025-266.yaml - https://github.com/tornadoweb/tornado/releases/tag/v6.5.3 - https://github.com/advisories/GHSA-c98p-7wgm-6p64