{"id":"GHSA-pcg2-g6cj-q92c","summary":"Tornado: Quadratic CPU usage when parsing HTTP headers with line folding","details":"### Summary\n\nTornado's HTTP header parser takes quadratic time (O(n²)) when a header value is continued across many \"obsolete line folding\" (obs-fold) lines. An unauthenticated attacker can send a request whose header block is mostly short continuation lines. Parsing it uses a large amount of CPU on the IOLoop thread and blocks every other connection that process is serving.\n\nThis issue is closely related to CVE-2025-67725 (GHSA-c98p-7wgm-6p64). The fix for that advisory covered repeated headers but not line-folded continuations.\n\n### Impact\n\nAll Tornado HTTP servers are affected. Header parsing runs before any application code, so no authentication or particular handler is required.\n\nHow bad it gets depends on `max_header_size`:\n\n- **Default (64 KB):** each malicious request blocks the event loop for tens of milliseconds. Repeated requests can degrade service.\n- **Raised `max_header_size`:** the cost grows with the square of the header size. A single 1 MB request can block the process for several seconds.\n\nOnly availability is affected. Confidentiality and integrity are not.\n\n### Patches\n\nFixed in Tornado 6.6 by removing support for obsolete line folding completely; header lines that begin with whitespace are now rejected.\n\n### Workarounds\n\n- Keep `max_header_size` at its default value or lower it. This limits the impact but doesn't remove it.\n- Run Tornado behind a reverse proxy that rejects or normalizes obsolete line folding before forwarding requests.\n\n### Credit\n\nReported by @manus-pi.","aliases":["CVE-2026-116577"],"modified":"2026-10-28T23:00:04.070057045Z","published":"2026-10-28T22:43:41Z","database_specific":{"github_reviewed_at":"2026-10-28T22:43:41Z","nvd_published_at":"2026-10-26T21:18:14Z","cwe_ids":["CWE-407","CWE-770"],"severity":"MODERATE","github_reviewed":true},"references":[{"type":"WEB","url":"https://github.com/tornadoweb/tornado/security/advisories/GHSA-pcg2-g6cj-q92c"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-116577"},{"type":"WEB","url":"https://github.com/tornadoweb/tornado/pull/3765"},{"type":"WEB","url":"https://github.com/tornadoweb/tornado/commit/1b7a1dc9f340f52de929852e8125aa12f25297c9"},{"type":"PACKAGE","url":"https://github.com/tornadoweb/tornado"},{"type":"WEB","url":"https://github.com/tornadoweb/tornado/releases/tag/v6.6.0"}],"affected":[{"package":{"name":"tornado","ecosystem":"PyPI","purl":"pkg:pypi/tornado"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"6.6"}]}],"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","6.5.10","6.5.2","6.5.3","6.5.4","6.5.5","6.5.6","6.5.7","6.5.8","6.5.9","6.5b1","6.6a1","6.6b1","6.6rc1"],"database_specific":{"source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/10/GHSA-pcg2-g6cj-q92c/GHSA-pcg2-g6cj-q92c.json"}}],"schema_version":"1.9.2","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L"}]}