{"cve":"CVE-2025-67725","nvd":{"published":"2025-12-12T06:15:41.380","modified":"2026-10-07T20:10:01.970","status":"Analyzed","description":"Tornado is a Python web framework and asynchronous networking library. In versions 6.5.2 and below, a single maliciously crafted HTTP request can block the server's event loop for an extended period, caused by the HTTPHeaders.add method. The function accumulates values using string concatenation when the same header name is repeated, causing a Denial of Service (DoS).  Due to Python string immutability, each concatenation copies the entire string, resulting in O(n²) time complexity. The severity can vary from high if max_header_size has been increased from its default, to low if it has its default value of 64KB. This issue is fixed in version 6.5.3.","cvss_v4":null,"cvss_v3":{"version":"3.1","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","score":7.5},"cwes":["CWE-400"],"references":["https://github.com/tornadoweb/tornado/commit/771472cfdaeebc0d89a9cc46e249f8891a6b29cd","https://github.com/tornadoweb/tornado/releases/tag/v6.5.3","https://github.com/tornadoweb/tornado/security/advisories/GHSA-c98p-7wgm-6p64"],"products":["tornadoweb:tornado"],"source":"nvd","synced_at":"2026-10-19T22:16:40.427100871+00:00"},"epss":{"cve":"CVE-2025-67725","score":0.00559,"percentile":0.46276,"date":"2026-10-25"},"kev":null,"osv":[{"id":"GHSA-c98p-7wgm-6p64","url":"https://osv.dev/vulnerability/GHSA-c98p-7wgm-6p64","summary":"Tornado: Quadratic DoS via Repeated Header Coalescing","modified":"2026-07-20T19:15:27.567094965Z","packages":[{"ecosystem":"PyPI","name":"tornado","purl":"pkg:pypi/tornado","ranges":["<6.5.3"],"fixed":["6.5.3"]}]},{"id":"PYSEC-2025-266","url":"https://osv.dev/vulnerability/PYSEC-2025-266","summary":"","modified":"2026-07-13T07:15:44.064352714Z","packages":[{"ecosystem":"PyPI","name":"tornado","purl":"pkg:pypi/tornado","ranges":["<6.5.3"],"fixed":["6.5.3"]}]}],"attribution":["This product uses the NVD API but is not endorsed or certified by the NVD.","EPSS scores: FIRST Exploit Prediction Scoring System, https://www.first.org/epss","Known exploited vulnerabilities: CISA Known Exploited Vulnerabilities Catalog, https://www.cisa.gov/known-exploited-vulnerabilities-catalog","Package advisories: OSV (https://osv.dev), from the GitHub Advisory Database, PyPA, Go, RustSec and other ecosystem databases, under CC-BY 4.0 or CC0","CVE records: the CVE Program's CVE List (https://www.cve.org), under its terms of use (https://www.cve.org/Legal/TermsOfUse). CVE is a registered trademark of The MITRE Corporation; this product is not affiliated with the CVE Program."]}