{"id":"CVE-2026-89554","title":"kernel: mptcp: fix uninitialized local_id in syncookie MP_JOIN reconstruction (CVE-2026-89554)","summary":"A flaw was found in the Linux kernel's Multipath TCP (MPTCP) implementation. When reconstructing a Multipath TCP (MPTCP) join request under SYN cookies, the `local_id` field is not properly initialized. An off-path attacker can influence t…","severity":"medium","cvss":5.5,"cvssVector":"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H","cvssSource":"vendor","cwe":"CWE-824","vendor":"Red Hat","product":"Red Hat Enterprise Linux 9","affected":["enterprise_linux 10","enterprise_linux 8","enterprise_linux 9","openshift_container_platform 4"],"published":"2026-09-11","updated":"2026-09-15","sourceUpdated":"2026-09-15T06:19:13+00:00","source":"CSAF","sourceUrl":"https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-89554.json","references":[{"url":"https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-89554.json"},{"url":"https://access.redhat.com/security/cve/CVE-2026-89554"},{"url":"https://bugzilla.redhat.com/show_bug.cgi?id=2532504"},{"url":"https://www.cve.org/CVERecord?id=CVE-2026-89554"},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-89554"},{"url":"https://git.kernel.org/pub/scm/linux/security/vulns.git/plain/cve/published/2026/CVE-2026-89554.mbox"},{"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"}],"tags":["csaf","vex","red-hat","cve.org","score-dispute"],"epss":0.00606,"epssPercentile":0.47387,"scores":{"vendor":5.5,"cna":8.2},"ingestedAt":"2026-09-14T15:23:07.451Z","slug":"CVE-2026-89554","body":"## Overview\n\nA flaw was found in the Linux kernel's Multipath TCP (MPTCP) implementation. When reconstructing a Multipath TCP (MPTCP) join request under SYN cookies, the `local_id` field is not properly initialized. An off-path attacker can influence this uninitialized field by sending concurrent MP_JOIN SYN requests, leading to corruption of the path manager's subflow bookkeeping. This can result in a denial of service for affected MPTCP connections.\n\n## Vendor advisories\n\n- **Red Hat VEX** · Moderate · affected: Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 8, Red Hat Enterprise Linux 9, Red Hat OpenShift Container Platform 4 · no fix planned: Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 8, Red Hat Enterprise Linux 9, Red Hat OpenShift Container Platform 4 · updated 2026-09-15 · [vex](https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-89554.json)\n\n**kernel: mptcp: fix uninitialized local_id in syncookie MP_JOIN reconstruction** — rated Moderate by Red Hat. Released 2026-09-11, updated 2026-09-15.\n\nAffected:\n\n- Red Hat Enterprise Linux 10\n- Red Hat Enterprise Linux 8\n- Red Hat Enterprise Linux 9\n- Red Hat OpenShift Container Platform 4\n\nNo fix planned:\n\n- Red Hat Enterprise Linux 10\n- Red Hat Enterprise Linux 8\n- Red Hat Enterprise Linux 9\n- Red Hat OpenShift Container Platform 4\n\nNot affected:\n\n- Red Hat Enterprise Linux 6\n- Red Hat Enterprise Linux 7\n\n## Remediation\n\nFix deferred","depth":"sunlit","depthScore":30,"depthScoreParts":{"impact":30.3,"likelihood":0.1,"exploitation":0,"ransomware":0},"changes":[{"seq":203947,"id":"CVE-2026-89554","ts":1789490230744,"field":"cvss","old":"8.2","new":"5.5"},{"seq":203946,"id":"CVE-2026-89554","ts":1789490230744,"field":"severity","old":"high","new":"medium"},{"seq":197928,"id":"CVE-2026-89554","ts":1789384320997,"field":"cvss","old":"6.5","new":"8.2"},{"seq":197927,"id":"CVE-2026-89554","ts":1789384320997,"field":"severity","old":"medium","new":"high"},{"seq":183479,"id":"CVE-2026-89554","ts":1789356675535,"field":"cvss","old":"8.2","new":"6.5"},{"seq":183478,"id":"CVE-2026-89554","ts":1789356675535,"field":"severity","old":"high","new":"medium"},{"seq":153362,"id":"CVE-2026-89554","ts":1789285350204,"field":"cvss","old":null,"new":"8.2"},{"seq":153361,"id":"CVE-2026-89554","ts":1789285350204,"field":"severity","old":"none","new":"high"},{"seq":147095,"id":"CVE-2026-89554","ts":1789270195148,"field":"cvss","old":null,"new":"6.5"},{"seq":147094,"id":"CVE-2026-89554","ts":1789270195148,"field":"severity","old":"none","new":"medium"},{"seq":108854,"id":"CVE-2026-89554","ts":1789183729265,"field":"cvss","old":null,"new":"6.5"},{"seq":108853,"id":"CVE-2026-89554","ts":1789183729265,"field":"severity","old":"none","new":"medium"}]}