{"id":"CVE-2026-80585","state":"PUBLISHED","title":"mptcp: fastopen: only mark MPTFO subflows with SYN data","description":"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\".","publishedAt":"2026-08-26T14:37:40.901Z","updatedAt":"2026-10-16T19:26:28.485Z","severity":{"score":9.4,"label":"Critical","version":"3.1","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/A:H","source":"Linux"},"assigner":"Linux","products":["Linux Linux"],"affected":[{"vendor":"Linux","product":"Linux","defaultStatus":"unaffected","versions":[{"version":"36b122baf6a8bd46b4a591f12f4ed17b22257408","status":"affected","lessThan":"f75f174edc865738522e514d042cf5627f084859","versionType":"git"},{"version":"36b122baf6a8bd46b4a591f12f4ed17b22257408","status":"affected","lessThan":"fca7e444c04689fe4cc6b56f2725f804a4bb1ef9","versionType":"git"},{"version":"36b122baf6a8bd46b4a591f12f4ed17b22257408","status":"affected","lessThan":"75e564b2ced1cc3d9a8904c7d2d2bb448fffb8b5","versionType":"git"},{"version":"36b122baf6a8bd46b4a591f12f4ed17b22257408","status":"affected","lessThan":"72b4a0c51a4b550d40301d60a366b429b8c8e78d","versionType":"git"},{"version":"36b122baf6a8bd46b4a591f12f4ed17b22257408","status":"affected","lessThan":"e00b63056fb4f261455b3e5df5268a1f8ce47a87","versionType":"git"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["net/mptcp/fastopen.c"]},{"vendor":"Linux","product":"Linux","defaultStatus":"affected","versions":[{"version":"6.2","status":"affected"},{"version":"0","status":"unaffected","lessThan":"6.2","versionType":"semver"},{"version":"6.6.153","status":"unaffected","lessThanOrEqual":"6.6.*","versionType":"semver"},{"version":"6.12.105","status":"unaffected","lessThanOrEqual":"6.12.*","versionType":"semver"},{"version":"6.18.46","status":"unaffected","lessThanOrEqual":"6.18.*","versionType":"semver"},{"version":"7.1.10","status":"unaffected","lessThanOrEqual":"7.1.*","versionType":"semver"},{"version":"7.2","status":"unaffected","lessThanOrEqual":"*","versionType":"original_commit_for_fix"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["net/mptcp/fastopen.c"]}],"references":["https://git.kernel.org/stable/c/f75f174edc865738522e514d042cf5627f084859","https://git.kernel.org/stable/c/fca7e444c04689fe4cc6b56f2725f804a4bb1ef9","https://git.kernel.org/stable/c/75e564b2ced1cc3d9a8904c7d2d2bb448fffb8b5","https://git.kernel.org/stable/c/72b4a0c51a4b550d40301d60a366b429b8c8e78d","https://git.kernel.org/stable/c/e00b63056fb4f261455b3e5df5268a1f8ce47a87"],"weaknesses":[],"ingestedAt":"2026-10-29T20:13:07.220Z","source":{"repository":"CVEProject/cvelistV5","path":"cves/2026/80xxx/CVE-2026-80585.json","commit":"556c9d3a8cd17e218bb8e452ba7cd619a384ff47"},"raw":{"dataType":"CVE_RECORD","dataVersion":"5.2","cveMetadata":{"cveId":"CVE-2026-80585","state":"PUBLISHED","datePublished":"2026-08-26T14:37:40.901Z","dateUpdated":"2026-10-16T19:26:28.485Z","assignerShortName":"Linux","assignerOrgId":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","dateReserved":"2026-08-26T14:34:25.769Z"},"containers":{"cna":{"title":"mptcp: fastopen: only mark MPTFO subflows with SYN data","descriptions":[{"lang":"en","value":"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\"."}],"affected":[{"vendor":"Linux","product":"Linux","defaultStatus":"unaffected","versions":[{"version":"36b122baf6a8bd46b4a591f12f4ed17b22257408","status":"affected","lessThan":"f75f174edc865738522e514d042cf5627f084859","versionType":"git"},{"version":"36b122baf6a8bd46b4a591f12f4ed17b22257408","status":"affected","lessThan":"fca7e444c04689fe4cc6b56f2725f804a4bb1ef9","versionType":"git"},{"version":"36b122baf6a8bd46b4a591f12f4ed17b22257408","status":"affected","lessThan":"75e564b2ced1cc3d9a8904c7d2d2bb448fffb8b5","versionType":"git"},{"version":"36b122baf6a8bd46b4a591f12f4ed17b22257408","status":"affected","lessThan":"72b4a0c51a4b550d40301d60a366b429b8c8e78d","versionType":"git"},{"version":"36b122baf6a8bd46b4a591f12f4ed17b22257408","status":"affected","lessThan":"e00b63056fb4f261455b3e5df5268a1f8ce47a87","versionType":"git"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["net/mptcp/fastopen.c"]},{"vendor":"Linux","product":"Linux","defaultStatus":"affected","versions":[{"version":"6.2","status":"affected"},{"version":"0","status":"unaffected","lessThan":"6.2","versionType":"semver"},{"version":"6.6.153","status":"unaffected","lessThanOrEqual":"6.6.*","versionType":"semver"},{"version":"6.12.105","status":"unaffected","lessThanOrEqual":"6.12.*","versionType":"semver"},{"version":"6.18.46","status":"unaffected","lessThanOrEqual":"6.18.*","versionType":"semver"},{"version":"7.1.10","status":"unaffected","lessThanOrEqual":"7.1.*","versionType":"semver"},{"version":"7.2","status":"unaffected","lessThanOrEqual":"*","versionType":"original_commit_for_fix"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["net/mptcp/fastopen.c"]}],"references":[{"url":"https://git.kernel.org/stable/c/f75f174edc865738522e514d042cf5627f084859"},{"url":"https://git.kernel.org/stable/c/fca7e444c04689fe4cc6b56f2725f804a4bb1ef9"},{"url":"https://git.kernel.org/stable/c/75e564b2ced1cc3d9a8904c7d2d2bb448fffb8b5"},{"url":"https://git.kernel.org/stable/c/72b4a0c51a4b550d40301d60a366b429b8c8e78d"},{"url":"https://git.kernel.org/stable/c/e00b63056fb4f261455b3e5df5268a1f8ce47a87"}],"metrics":[{"cvssV3_1":{"version":"3.1","vectorString":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/A:H","baseScore":9.4,"baseSeverity":"CRITICAL"},"scenarios":[{"lang":"en","value":"AV:N - The bug is reached by sending crafted TCP/MPTCP segments to a listening socket over the network (ip4/ip6 receive -> tcp_conn_request/tcp_rcv_state_process -> subflow_prep_synack or mptcp_incoming_options -> check_fully_established); per kernel CNA guidance, net stack packet-handling paths are Network.\nAC:L - In the intended passive-MPTFO deployment (MPTCP listener with net.ipv4.tcp_fastopen server bit set), the attacker controls obtaining a valid TFO cookie and then sending a data-less SYN followed by an out-of-order MPC_ACK to deterministically hit the stale is_mptfo path in check_fully_established().\nPR:N - Exploitation requires only unauthenticated inbound TCP connections to an MPTCP listener; the attacker obtains the TFO cookie from their own prior connection and needs no local account, capability, or namespace privileges on the target host.\nUI:N - No victim interaction is required beyond the server normally accepting TCP connections; the attacker programmatically sends the SYN and follow-up MPC_ACK packets without anyone opening files, clicking links, or performing other actions.\nS:U - Impact is confined to kernel MPTCP/TCP connection state on the target host and does not cross VM, container, IOMMU, or other security boundaries; it is not a sandbox or guest-to-host escape.\nC:L - Premature fully_established with attacker-chosen MPTCP keys desynchronizes handshake and DSS/map state, enabling misdelivery or premature exposure of connection-level data/metadata to the remote peer before normal MP_CAPABLE validation completes.\nI:H - Stale is_mptfo lets an out-of-order MPC_ACK force __mptcp_subflow_fully_established(), writing attacker-controlled sndr_key/ack_seq into msk and subflow crypto state and corrupting subsequent DSS/ACK handling on the server multipath socket.\nA:H - Data-less passive-TFO SYNs hit WARN_ON_ONCE(!skb) on every trigger, and the resulting inconsistent is_mptfo/fully_established state can cause repeatable kernel WARN/oops (panic_on_warn panic) plus connection reset storms against internet-facing MPTCP servers."}]}],"providerMetadata":{"shortName":"Linux","orgId":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","dateUpdated":"2026-08-27T05:02:06.112Z"},"cpeApplicability":[{"nodes":[{"operator":"OR","negate":false,"cpeMatch":[{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"6.2","versionEndExcluding":"6.6.153"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"6.2","versionEndExcluding":"6.12.105"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"6.2","versionEndExcluding":"6.18.46"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"6.2","versionEndExcluding":"7.1.10"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"6.2","versionEndExcluding":"7.2"}]}]}],"x_generator":{"engine":"bippy-1.2.0"}},"adp":[{"title":"CISA ADP Vulnrichment","metrics":[{"other":{"type":"ssvc","content":{"id":"CVE-2026-80585","role":"CISA Coordinator","options":[{"Exploitation":"none"},{"Automatable":"yes"},{"Technical Impact":"partial"}],"version":"2.0.3","timestamp":"2026-10-16T19:17:56.710913Z"}}}],"providerMetadata":{"shortName":"CISA-ADP","orgId":"134c704f-9b21-4f2e-91b3-4a467353bcc0","dateUpdated":"2026-10-16T19:26:28.485Z"}}]}}}