{"id":"CVE-2026-89554","state":"PUBLISHED","title":"mptcp: fix uninitialized local_id in syncookie MP_JOIN reconstruction","description":"In the Linux kernel, the following vulnerability has been resolved:\n\nmptcp: fix uninitialized local_id in syncookie MP_JOIN reconstruction\n\nmptcp_token_join_cookie_init_state() restores remote_nonce, local_nonce,\nbackup, join_id, token and msk from the saved cookie entry when rebuilding\nthe request socket for a MP_JOIN 4th-ACK handled under SYN cookies, but it\ndoes not restore local_id, even though the SYN path saved it.\nsubflow_ulp_clone() then reads that uninitialized field and stores it as\nthe joined subflow's address-ID. Because the request-sock slab is\nSLAB_TYPESAFE_BY_RCU and not zeroed on allocation, the value is the stale\nbyte of a previously freed request socket, which an off-path peer can\ninfluence by sending concurrent MP_JOIN SYNs. This corrupts the path\nmanager's id-based subflow bookkeeping for the connection.\n\nRestore subflow_req->local_id from the cookie entry, as done for the other\nfields.","publishedAt":"2026-09-11T19:44:27.586Z","updatedAt":"2026-10-19T08:48:47.068Z","severity":{"score":8.2,"label":"High","version":"3.1","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:H","source":"Linux"},"assigner":"Linux","products":["Linux Linux"],"affected":[{"vendor":"Linux","product":"Linux","defaultStatus":"unaffected","versions":[{"version":"9466a1ccebbe54ac57fb8a89c2b4b854826546a8","status":"affected","lessThan":"4534e0eead692bb069ad1e6a5a245fc2e2078300","versionType":"git"},{"version":"9466a1ccebbe54ac57fb8a89c2b4b854826546a8","status":"affected","lessThan":"63cacb05e51a1c6e1349d2e593d00fd6ef43def4","versionType":"git"},{"version":"9466a1ccebbe54ac57fb8a89c2b4b854826546a8","status":"affected","lessThan":"954b5ea836eb118d188975ced803448537ab8479","versionType":"git"},{"version":"9466a1ccebbe54ac57fb8a89c2b4b854826546a8","status":"affected","lessThan":"9df36a4846375a9b75bf42d77bfed216b0f366f9","versionType":"git"},{"version":"9466a1ccebbe54ac57fb8a89c2b4b854826546a8","status":"affected","lessThan":"64f2c5dd49b956a542c8c02b8dad5d262a462bba","versionType":"git"},{"version":"9466a1ccebbe54ac57fb8a89c2b4b854826546a8","status":"affected","lessThan":"51887ccd88791ddaa8755a7c614fda031ecf7982","versionType":"git"},{"version":"9466a1ccebbe54ac57fb8a89c2b4b854826546a8","status":"affected","lessThan":"bf19d166337c6488b39cb03eeeffb30a941d6326","versionType":"git"},{"version":"9466a1ccebbe54ac57fb8a89c2b4b854826546a8","status":"affected","lessThan":"b878dfdd12d7a5b8722a78d35e313506140ca3d9","versionType":"git"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["net/mptcp/syncookies.c"]},{"vendor":"Linux","product":"Linux","defaultStatus":"affected","versions":[{"version":"5.9","status":"affected"},{"version":"0","status":"unaffected","lessThan":"5.9","versionType":"semver"},{"version":"5.10.270","status":"unaffected","lessThanOrEqual":"5.10.*","versionType":"semver"},{"version":"5.15.221","status":"unaffected","lessThanOrEqual":"5.15.*","versionType":"semver"},{"version":"6.1.188","status":"unaffected","lessThanOrEqual":"6.1.*","versionType":"semver"},{"version":"6.6.157","status":"unaffected","lessThanOrEqual":"6.6.*","versionType":"semver"},{"version":"6.12.109","status":"unaffected","lessThanOrEqual":"6.12.*","versionType":"semver"},{"version":"6.18.50","status":"unaffected","lessThanOrEqual":"6.18.*","versionType":"semver"},{"version":"7.2.4","status":"unaffected","lessThanOrEqual":"7.2.*","versionType":"semver"},{"version":"7.3","status":"unaffected","lessThanOrEqual":"*","versionType":"original_commit_for_fix"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["net/mptcp/syncookies.c"]}],"references":["https://git.kernel.org/stable/c/4534e0eead692bb069ad1e6a5a245fc2e2078300","https://git.kernel.org/stable/c/63cacb05e51a1c6e1349d2e593d00fd6ef43def4","https://git.kernel.org/stable/c/954b5ea836eb118d188975ced803448537ab8479","https://git.kernel.org/stable/c/9df36a4846375a9b75bf42d77bfed216b0f366f9","https://git.kernel.org/stable/c/64f2c5dd49b956a542c8c02b8dad5d262a462bba","https://git.kernel.org/stable/c/51887ccd88791ddaa8755a7c614fda031ecf7982","https://git.kernel.org/stable/c/bf19d166337c6488b39cb03eeeffb30a941d6326","https://git.kernel.org/stable/c/b878dfdd12d7a5b8722a78d35e313506140ca3d9"],"weaknesses":[],"ingestedAt":"2026-10-29T20:13:26.108Z","source":{"repository":"CVEProject/cvelistV5","path":"cves/2026/89xxx/CVE-2026-89554.json","commit":"556c9d3a8cd17e218bb8e452ba7cd619a384ff47"},"raw":{"dataType":"CVE_RECORD","dataVersion":"5.2","cveMetadata":{"cveId":"CVE-2026-89554","state":"PUBLISHED","datePublished":"2026-09-11T19:44:27.586Z","dateUpdated":"2026-10-19T08:48:47.068Z","assignerShortName":"Linux","assignerOrgId":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","dateReserved":"2026-09-11T19:38:34.724Z"},"containers":{"cna":{"title":"mptcp: fix uninitialized local_id in syncookie MP_JOIN reconstruction","descriptions":[{"lang":"en","value":"In the Linux kernel, the following vulnerability has been resolved:\n\nmptcp: fix uninitialized local_id in syncookie MP_JOIN reconstruction\n\nmptcp_token_join_cookie_init_state() restores remote_nonce, local_nonce,\nbackup, join_id, token and msk from the saved cookie entry when rebuilding\nthe request socket for a MP_JOIN 4th-ACK handled under SYN cookies, but it\ndoes not restore local_id, even though the SYN path saved it.\nsubflow_ulp_clone() then reads that uninitialized field and stores it as\nthe joined subflow's address-ID. Because the request-sock slab is\nSLAB_TYPESAFE_BY_RCU and not zeroed on allocation, the value is the stale\nbyte of a previously freed request socket, which an off-path peer can\ninfluence by sending concurrent MP_JOIN SYNs. This corrupts the path\nmanager's id-based subflow bookkeeping for the connection.\n\nRestore subflow_req->local_id from the cookie entry, as done for the other\nfields."}],"affected":[{"vendor":"Linux","product":"Linux","defaultStatus":"unaffected","versions":[{"version":"9466a1ccebbe54ac57fb8a89c2b4b854826546a8","status":"affected","lessThan":"4534e0eead692bb069ad1e6a5a245fc2e2078300","versionType":"git"},{"version":"9466a1ccebbe54ac57fb8a89c2b4b854826546a8","status":"affected","lessThan":"63cacb05e51a1c6e1349d2e593d00fd6ef43def4","versionType":"git"},{"version":"9466a1ccebbe54ac57fb8a89c2b4b854826546a8","status":"affected","lessThan":"954b5ea836eb118d188975ced803448537ab8479","versionType":"git"},{"version":"9466a1ccebbe54ac57fb8a89c2b4b854826546a8","status":"affected","lessThan":"9df36a4846375a9b75bf42d77bfed216b0f366f9","versionType":"git"},{"version":"9466a1ccebbe54ac57fb8a89c2b4b854826546a8","status":"affected","lessThan":"64f2c5dd49b956a542c8c02b8dad5d262a462bba","versionType":"git"},{"version":"9466a1ccebbe54ac57fb8a89c2b4b854826546a8","status":"affected","lessThan":"51887ccd88791ddaa8755a7c614fda031ecf7982","versionType":"git"},{"version":"9466a1ccebbe54ac57fb8a89c2b4b854826546a8","status":"affected","lessThan":"bf19d166337c6488b39cb03eeeffb30a941d6326","versionType":"git"},{"version":"9466a1ccebbe54ac57fb8a89c2b4b854826546a8","status":"affected","lessThan":"b878dfdd12d7a5b8722a78d35e313506140ca3d9","versionType":"git"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["net/mptcp/syncookies.c"]},{"vendor":"Linux","product":"Linux","defaultStatus":"affected","versions":[{"version":"5.9","status":"affected"},{"version":"0","status":"unaffected","lessThan":"5.9","versionType":"semver"},{"version":"5.10.270","status":"unaffected","lessThanOrEqual":"5.10.*","versionType":"semver"},{"version":"5.15.221","status":"unaffected","lessThanOrEqual":"5.15.*","versionType":"semver"},{"version":"6.1.188","status":"unaffected","lessThanOrEqual":"6.1.*","versionType":"semver"},{"version":"6.6.157","status":"unaffected","lessThanOrEqual":"6.6.*","versionType":"semver"},{"version":"6.12.109","status":"unaffected","lessThanOrEqual":"6.12.*","versionType":"semver"},{"version":"6.18.50","status":"unaffected","lessThanOrEqual":"6.18.*","versionType":"semver"},{"version":"7.2.4","status":"unaffected","lessThanOrEqual":"7.2.*","versionType":"semver"},{"version":"7.3","status":"unaffected","lessThanOrEqual":"*","versionType":"original_commit_for_fix"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["net/mptcp/syncookies.c"]}],"references":[{"url":"https://git.kernel.org/stable/c/4534e0eead692bb069ad1e6a5a245fc2e2078300"},{"url":"https://git.kernel.org/stable/c/63cacb05e51a1c6e1349d2e593d00fd6ef43def4"},{"url":"https://git.kernel.org/stable/c/954b5ea836eb118d188975ced803448537ab8479"},{"url":"https://git.kernel.org/stable/c/9df36a4846375a9b75bf42d77bfed216b0f366f9"},{"url":"https://git.kernel.org/stable/c/64f2c5dd49b956a542c8c02b8dad5d262a462bba"},{"url":"https://git.kernel.org/stable/c/51887ccd88791ddaa8755a7c614fda031ecf7982"},{"url":"https://git.kernel.org/stable/c/bf19d166337c6488b39cb03eeeffb30a941d6326"},{"url":"https://git.kernel.org/stable/c/b878dfdd12d7a5b8722a78d35e313506140ca3d9"}],"metrics":[{"cvssV3_1":{"version":"3.1","vectorString":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:H","baseScore":8.2,"baseSeverity":"HIGH"},"scenarios":[{"lang":"en","value":"AV:N - The flaw is on the MPTCP MP_JOIN SYN-cookie receive path (tcp_v4_rcv/cookie_v4_check/mptcp_subflow_init_cookie_req/mptcp_token_join_cookie_init_state then subflow_ulp_clone). Any remote peer that can reach an MPTCP listener over routed IP triggers it with MP_JOIN packets when SYN cookies are in use.\nAC:L - An attacker can fill the SYN queue to force cookies (tcp_syncookies defaults to 1), spray concurrent MP_JOIN SYNs to seed the SLAB_TYPESAFE_BY_RCU request-sock cache with chosen local_id values, then complete a cookie MP_JOIN ACK. The attacker drives both the flood and the handshake; success does not depend on uninfluencable victim state.\nPR:N - No local account or capability is required. Any unauthenticated remote client obtains a token by completing MP_CAPABLE to a public MPTCP listener, then drives cookie JOIN; HMAC-valid JOIN from that peer, or slab poisoning of another connection's JOIN, needs no victim credentials.\nUI:N - Cookie MP_JOIN reconstruction runs in TCP receive softirq on attacker-sent segments; no local user mount, click, or other action is required beyond ordinary network reachability of an MPTCP service.\nS:U - Impact is confined to the host kernel's MPTCP path-manager and connection state and does not cross a VM, IOMMU, container, or other security authority.\nC:N - The uninitialized field is a single u8 address-ID leftover from a same-type request-sock slab object. It is not an out-of-bounds or pointer leak and does not disclose kernel memory contents to the attacker.\nI:L - subflow_ulp_clone stores the stale local_id as the joined subflow's address-ID, corrupting path-manager id-based bookkeeping (endpoint availability, ADD_ADDR/REMOVE_ADDR matching, and ID-0 primary-path identity). The modification is limited to that protocol ID and is not an arbitrary kernel write or control-flow primitive.\nA:H - A poisoned local_id of 0 or a colliding endpoint ID makes later REMOVE_ADDR/RMSUBFLOW close the initial or other still-needed subflows via mptcp_close_ssk, and desynchronized PM state can reject further joins or tear down the MPTCP session. An attacker can repeat this against many connections on an internet-facing MPTCP service."}]}],"providerMetadata":{"shortName":"Linux","orgId":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","dateUpdated":"2026-10-19T08:48:47.068Z"},"cpeApplicability":[{"nodes":[{"operator":"OR","negate":false,"cpeMatch":[{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"5.9","versionEndExcluding":"5.10.270"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"5.9","versionEndExcluding":"5.15.221"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"5.9","versionEndExcluding":"6.1.188"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"5.9","versionEndExcluding":"6.6.157"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"5.9","versionEndExcluding":"6.12.109"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"5.9","versionEndExcluding":"6.18.50"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"5.9","versionEndExcluding":"7.2.4"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"5.9","versionEndExcluding":"7.3"}]}]}],"x_generator":{"engine":"bippy-1.2.0"}},"adp":[{"title":"CISA ADP Vulnrichment","metrics":[{"other":{"type":"ssvc","content":{"id":"CVE-2026-89554","role":"CISA Coordinator","options":[{"Exploitation":"none"},{"Automatable":"yes"},{"Technical Impact":"partial"}],"version":"2.0.3","timestamp":"2026-10-16T19:23:57.686804Z"}}}],"providerMetadata":{"shortName":"CISA-ADP","orgId":"134c704f-9b21-4f2e-91b3-4a467353bcc0","dateUpdated":"2026-10-16T19:26:43.610Z"}}]}}}