{"id":"CVE-2026-80585","aliases":[],"url":"https://o3.security/vulnerability/CVE-2026-80585","summary":"mptcp: fastopen: only mark MPTFO subflows with SYN data","details":"In the Linux kernel, the following vulnerability has been resolved:\n\nmptcp: fastopen: only mark MPTFO subflows with SYN data\n\nPassive TCP Fast Open accepts a valid-cookie SYN even when it carries\nno data. In that case the child socket's receive queue is intentionally\nleft empty.\n\nmptcp_fastopen_subflow_synack_set_params() set is_mptfo before checking\nfor queued SYN data. That made data-less TFO SYNs hit a WARN and, if\nthe warning was non-fatal, left stale MPTFO state behind. The stale\nflag could later trigger a state-confusion bug in\ncheck_fully_established().\n\nOnly mark the subflow as MPTFO after confirming that an SKB was queued.\nReturn quietly when the receive queue is empty.\n\nNote that mptcp_subflow_context's is_mptfo field is now not just about\nsubflows where the TFO was present, but about MPTFO subflow that\nconsumed SYN data. Only having a valid cookie but not carrying data is\nnot really \"doing TFO\".","published":"2026-08-26T14:37:40.901Z","modified":"2026-08-28T03:47:29.299159794Z","cvss":{"score":9.4,"severity":"CRITICAL","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/A:H"},"epss":null,"cisaKev":null,"exploitsKnown":null,"affectedPackages":[{"ecosystem":"Linux","name":"Kernel","fixedVersion":"6.6.153"}],"fix":null,"references":[{"type":"WEB","url":"https://git.kernel.org/stable/c/72b4a0c51a4b550d40301d60a366b429b8c8e78d"},{"type":"WEB","url":"https://git.kernel.org/stable/c/75e564b2ced1cc3d9a8904c7d2d2bb448fffb8b5"},{"type":"WEB","url":"https://git.kernel.org/stable/c/e00b63056fb4f261455b3e5df5268a1f8ce47a87"},{"type":"WEB","url":"https://git.kernel.org/stable/c/f75f174edc865738522e514d042cf5627f084859"},{"type":"WEB","url":"https://git.kernel.org/stable/c/fca7e444c04689fe4cc6b56f2725f804a4bb1ef9"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/80xxx/CVE-2026-80585.json"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-80585"},{"type":"PACKAGE","url":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-08-28T03:47:29.299159794Z"}}