From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-97523][MODERATE REGULAR] mptcp: close race between scheduler and state change Date: Fri, 25 Sep 2026 09:27:44 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-97523 X-AL-KERNEL-Priority: MODERATE REGULAR X-AL-KERNEL-Severity: MODERATE REGULAR X-AL-KERNEL-Base-Severity: MODERATE X-AL-KERNEL-KPANIC: NO X-AL-KERNEL-ActionableScore: 3 X-AL-KERNEL-ActionableScore-Lower: unknown X-AL-KERNEL-Commit: 8f11430d51ff8365bc51b670bc3002b65ae4c6b7 List-Id: CVE: CVE-2026-97523 Priority: MODERATE REGULAR AL-KERNEL base severity: MODERATE KPANIC flag: NO Patch: mptcp: close race between scheduler and state change Commit: 8f11430d51ff8365bc51b670bc3002b65ae4c6b7 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8f11430d51ff8365bc51b670bc3002b65ae4c6b7 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97523 Analysis date: Fri, 25 Sep 2026 09:27:44 -0400 ActionableScore: 3 ActionableScore lower bound: unknown Actionable bucket: Borderline / manual review recommended Manual review required: YES Summary: No one-sentence report phrase was found. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-97523 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97523 The original announcement does not normally provide a security severity estimate, CVSS assessment, or enough information to determine whether the reported kernel bug represents a practically relevant security issue. This report was generated by AL-KERNEL, an AI-assisted Linux kernel vulnerability analysis system developed by Alexander Larkin. It combines an autonomous classifier with LLM-assisted technical analysis and a separate ActionableScore mechanism. The purpose of this report is to prioritize Linux kernel CVEs before manual review, identify cases that require prompt investigation, and support automatic closure of issues that are unlikely to have meaningful security impact. Published priority for this report: MODERATE REGULAR Manual review required: YES A detailed explanation of the methodology and priority rules is included at the end of this message. ====================================================================== AL-KERNEL CLASSIFICATION RESULT ====================================================================== CVE-2026-97523 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-97523 CHECK WITH IMPACT FROM ORIG NN MODERATE Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';CWE-362;CWE-369;CWE-754;BEST CVSS score: '5.9';DESCR 'A race exists between MPTCP scheduler subflow selection and a subflow state change. A selected subflow can fail transmission and reset mss_now to zero before mptcp_push_release calls tcp_push. A remote peer participating in an active MPTCP connection may influence subflow closure or state transitions and potentially trigger a divide by zero or kernel crash. For the CVSS the PR:N is used because no privileges on the target are required beyond network access to an exposed MPTCP service. Exploitation requires winning a timing dependent race, so AC:H is appropriate. Impact is denial of service only. The patch context does not show a use after free, out of bounds access, information leak, or write primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3) YES REMOTE OOB SIMPLEFIX RACE NETWORK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 NO NO checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=3 PrimitiveClass=CLASS_C RealUAF=NO HostAdminRequired=NO NamespaceScopedPrivilege=UNKNOWN ImportantCandidate=NO ManualReview=RECOMMENDED AutoDowngradeAllowed=YES 1. ActionableScore * Raw conservative score: 3 * Conservative score: 3 * Raw paranoid score: 3 * Paranoid score: 3 * Final recommended bucket: **Borderline / manual review recommended**:: No floor or cap was applied. The patch establishes an invalid zero MSS value reaching a TCP transmission helper after a scheduler and subflow-state race. It does not establish memory corruption, stale lifetime use, disclosure, integrity modification, or privilege escalation. 2. Signal breakdown Positive signals: * +2 Remote reachable / directly network-triggerable. A remote peer connected to an exposed MPTCP service can influence subflow state transitions, including closure or failure conditions, while the target schedules data transmission. * +1 Reliable kernel crash / strong DoS. The patch describes a path where `mss_now` is reset to zero and subsequently used by `tcp_push()` for a divide operation. A zero divisor in this kernel path can cause a kernel fault or crash. Negative signals: * No hard-race penalty applied. The bug is a race, but the supplied evidence does not establish that it is a one-shot, externally uncontrollable, low-probability race. The peer may repeatedly create connection and subflow state transitions. * No niche-subsystem penalty applied. MPTCP deployment is less universal than ordinary TCP, but the supplied evidence does not establish that the attacker cannot reach an exposed MPTCP listener. * No host-admin penalty applied. The trigger is network-originated and does not require attacker privileges on the target host. Important non-awarded signals: * No UAF or stale post-lifetime object use is established. * No controlled reclaim, object replacement, arbitrary write, directed write, callback control, or refcount mutation is established. * No confidentiality or integrity signal is awarded. A zero MSS used in a divide operation establishes invalid-state and crash risk, not disclosure or modification of protected state. * No attacker-influenced active-object lifetime manipulation signal is awarded. The attacker may influence network state, but the patch does not establish attacker control of kernel object lifetime, reuse, replacement, or post-free contents. * No additional Availability point is awarded because the kernel crash is the same consequence already counted by the strong DoS signal. * No Local unprivileged trigger point is awarded because the evaluated attacker is remote, not a local user. 3. Reachability analysis The evaluated attacker is a remote peer that can establish or participate in an active MPTCP connection to a target service. The vulnerable sequence requires a scheduler-selected subflow to undergo a state transition or transmission failure before `mptcp_push_release()` invokes `tcp_push()` with the saved MSS value. Host administrator privileges are not required for the attacker if the target already exposes an MPTCP-capable service. This supports `HostAdminRequired=NO`. The target must have MPTCP enabled and must accept an MPTCP connection reachable by the attacker. This is an operational prerequisite, but the supplied patch does not establish that an administrator-only setup step is needed for the attacker to reach an already exposed listener. No namespace-specific trigger chain is supplied, so `NamespaceScopedPrivilege=UNKNOWN`. The exact race reliability is not demonstrated by the small patch. The attacker may be able to generate repeated connection and subflow transition attempts, but the evidence does not establish deterministic triggering. This is sufficient for manual review but does not create a memory-corruption primitive. 4. Primitive and severity interpretation The exact primitive is an invalid-state use of `info->mss_now == 0` in `tcp_push()` after a race between MPTCP subflow selection and a socket state change. The patch prevents the call to `tcp_push()` when the MSS value is zero. This is neither an active structured-object UAF nor fixed stale storage. No object is shown to be freed, reused, dereferenced after lifetime end, or released twice. There is no concrete repeated release, refcount mutation, or post-free operation. The demonstrated consequence is a possible divide-by-zero-related kernel failure and denial of service. The speculative consequence is only broader service disruption from a kernel crash. There is no patch-supported mechanism for confidentiality impact, integrity impact, privilege escalation, arbitrary kernel read or write, or code execution. The issue is `CLASS_C` because the demonstrated behavior is crash or invalid-state handling without an independently useful corruption primitive. `ImportantCandidate=NO` because the patch does not establish meaningful exploitation leverage beyond denial of service. `AutoDowngradeAllowed=YES` because this is a constrained invalid-state race with no unresolved lifetime, ownership, authorization, semantic reinterpretation, or memory-corruption primitive. Likely operational severity is an externally relevant remote DoS for exposed MPTCP services. It merits confirmation of target exposure and reproducer behavior, but it is not a strong Important-class kernel vulnerability based on the supplied evidence. 5. One-sentence report phrase A remote peer connected to an exposed MPTCP service may race subflow state changes with scheduler transmission handling, causing a zero MSS value to reach tcp_push and potentially crash the kernel for denial of service. 6. Manual review recommendation MANUAL CHECK RECOMMENDED The issue is network-reachable for exposed MPTCP services and may cause a kernel crash, but the patch establishes only a zero-value divide or invalid-state DoS path and does not demonstrate memory corruption or privilege escalation. 7. Call-site confidence Call-site confidence: medium ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: mptcp: close race between scheduler and state change Commit: 8f11430d51ff8365bc51b670bc3002b65ae4c6b7 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8f11430d51ff8365bc51b670bc3002b65ae4c6b7 Commit description: The mptcp scheduler may race with subflow sockets state change: data transmission on the selected socket may fail and a later release could try to use mss_now reset to 0 for a divide operation. Address the issue by explicitly checking for the critical scenario. Fixes: c886d70 ("mptcp: do not queue data on closed subflows") Cc: stable@vger.kernel.org Reported-by: Shardul Bankar Reported-by: Xinyang Ge Closes: https://lore.kernel.org/20260525194828.1137119-1-shardul.b@mpiricsoftware.com Signed-off-by: Paolo Abeni Reviewed-by: Matthieu Baerts (NGI0) Signed-off-by: Matthieu Baerts (NGI0) Link: https://patch.msgid.link/20260917-net-mptcp-misc-fixes-7-3-rc4-v2-2-0cf5c72667c8@kernel.org Signed-off-by: Jakub Kicinski Signed-off-by: Matthieu Baerts (NGI0) Signed-off-by: Greg Kroah-Hartman Changed files: net/mptcp/protocol.c Diff excerpt: Not included in this email. See the upstream URL for the full patch. Full patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8f11430d51ff8365bc51b670bc3002b65ae4c6b7 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-97523 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97523 The original CVE announcement normally does not include a security-level estimate. In particular, it may not contain a CVSS assessment, an impact level, or enough information to determine whether the reported bug is a practically relevant security issue. One purpose of this parallel CVE list is to provide that missing technical and prioritization information. The original goal of the AL-KERNEL project was to prioritize Linux kernel CVE analysis automatically before manual review. The system can also help identify non-security issues that may be suitable for automatic closure. This report was generated by AL-KERNEL, an AI-assisted Linux kernel vulnerability analysis system developed by Alexander Larkin. The first analysis stage combines an autonomous classifier with additional LLM-based analysis. The autonomous classifier runs locally on a CPU and is based on a backpropagation neural network. Together, these mechanisms produce a technical vulnerability description, identify likely weakness types, estimate CVSS severity, and provide input for ActionableScore. Two CVSS estimates are retained because incomplete kernel vulnerability information often permits more than one defensible interpretation: Conservative CVSS vector: Not available The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H The Best / paranoid CVSS score: 5.9 The conservative vector represents a lower-impact interpretation. The Best/paranoid vector intentionally represents a plausible upper-bound interpretation and should not automatically be treated as demonstrated real-world impact. CVSS may also need to be adjusted for a particular Linux deployment, because actual reachability, privileges, enabled kernel configuration, hardware, namespaces, exposed device nodes, and other environmental conditions can differ significantly between systems. A separate ActionableScore mechanism evaluates practical remediation urgency. Its analysis may include reachability, attack prerequisites, subsystem exposure, memory-corruption characteristics, denial-of-service reliability, and possible confidentiality, integrity, or privilege-escalation impact. Conservative ActionableScore: unknown Paranoid ActionableScore: 3 The final base severity is taken directly from the second tab-separated field of the AL-KERNEL classification result. ActionableScore does not replace or independently override that final AL-KERNEL decision, and if ActionableScore adjusted impact level of ALKERNEL, then you would see self-readable flags above like INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7. For an AL-KERNEL result of MODERATE, this report uses the following additional presentation split: ActionableScore below 5 -> MODERATE REGULAR ActionableScore 5 or more -> MODERATE 7.0 The distinction between MODERATE REGULAR and MODERATE 7.0 makes it possible to identify Moderate issues that should receive manual analysis and fixes before lower-priority MODERATE REGULAR issues. In many cases, MODERATE REGULAR fixes may wait for a later rebase or routine update. There is one override in which MODERATE REGULAR becomes MODERATE 7.0 even when the ActionableScore is below 5. When the AL-KERNEL result contains the KPANIC flag, a MODERATE result is always presented as MODERATE 7.0. The KPANIC flag selected with few regexps without usage of AI at all, so it helps to detect cases when Kernel Crash happens and similar (to filter False-Negative results from the LLM usage). KPANIC indicates that a reliable kernel crash, kernel panic, or similarly serious kernel availability impact was identified by the classification workflow. AL-KERNEL base severity for this report: MODERATE KPANIC detected for this report: NO Published priority for this report (same as in Subject): MODERATE REGULAR These results are intended to support engineering triage. They are machine-generated estimates, and cases marked for manual review should be validated by a human security engineer before final disposition. For more info read docs linked from here: https://kernelcve.org/ (and you can submit you own patch there to generate such a report for non-existant CVE-id yet). Note that in many cases this AI tool selects higher severity, than real is (means you can expect Importants instead of Moderate 7.0 or Moderates 7.0 instead of regular Moderates). If you see such cases, please use reply email interface to add additional manual analyses info to this particular CVE. And please, please, let me know when you see Lows instead of Importants or Important instead of Low (because particular for such cases I need to tune this AI tool to make it better for this one and next similar). My contact email for such notifications is alexanjelausa@gmail.com (and both send reply to CVE record itself too and see "reply" button below for howto reply).