{"schema_version":"1.7.5","id":"GHSA-c98p-7wgm-6p64","published":"2026-07-20T18:57:34Z","modified":"2026-07-20T19:15:27.567094965Z","aliases":["CVE-2025-67725","PYSEC-2025-266"],"summary":"Tornado: Quadratic DoS via Repeated Header Coalescing","details":"## Summary\n\nThe `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.\n\nGiven 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).\n\n**Severity:**  **High** if `max_header_size` has been increased from its default, **low** if it has its default value of 64KB.","affected":[{"package":{"name":"tornado","ecosystem":"PyPI","purl":"pkg:pypi/tornado"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"6.5.3"}]}],"versions":["0.2","1.0","1.1","1.1.1","1.2","1.2.1","2.0","2.1","2.1.1","2.2","2.2.1","2.3","2.4","2.4.1","3.0","3.0.1","3.0.2","3.1","3.1.1","3.2","3.2.1","3.2.2","4.0","4.0.1","4.0.2","4.1","4.1b2","4.2","4.2.1","4.2b1","4.3","4.3b1","4.3b2","4.4","4.4.1","4.4.2","4.4.3","4.4b1","4.5","4.5.1","4.5.2","4.5.3","4.5b1","4.5b2","5.0","5.0.1","5.0.2","5.0a1","5.0b1","5.1","5.1.1","5.1b1","6.0","6.0.1","6.0.2","6.0.3","6.0.4","6.0a1","6.0b1","6.1","6.1b1","6.1b2","6.2","6.2b1","6.2b2","6.3","6.3.1","6.3.2","6.3.3","6.3b1","6.4","6.4.1","6.4.2","6.4b1","6.5","6.5.1","6.5.2","6.5b1"],"database_specific":{"source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/07/GHSA-c98p-7wgm-6p64/GHSA-c98p-7wgm-6p64.json"}}],"references":[{"type":"WEB","url":"https://github.com/tornadoweb/tornado/security/advisories/GHSA-c98p-7wgm-6p64"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2025-67725"},{"type":"WEB","url":"https://github.com/tornadoweb/tornado/commit/771472cfdaeebc0d89a9cc46e249f8891a6b29cd"},{"type":"WEB","url":"https://github.com/pypa/advisory-database/tree/main/vulns/tornado/PYSEC-2025-266.yaml"},{"type":"PACKAGE","url":"https://github.com/tornadoweb/tornado"},{"type":"WEB","url":"https://github.com/tornadoweb/tornado/releases/tag/v6.5.3"}],"database_specific":{"cwe_ids":["CWE-400"],"github_reviewed":true,"github_reviewed_at":"2026-07-20T18:57:34Z","nvd_published_at":"2025-12-12T06:15:41Z","severity":"HIGH"},"severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"}]}