From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-89631][IMPORTANT] smb: client: reject a tree connect response whose byte count is too small [ Upstream Date: Sat, 12 Sep 2026 00:14:50 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-89631 X-AL-KERNEL-Priority: IMPORTANT X-AL-KERNEL-Severity: IMPORTANT X-AL-KERNEL-Base-Severity: IMPORTANT X-AL-KERNEL-KPANIC: YES X-AL-KERNEL-ActionableScore: 7 X-AL-KERNEL-ActionableScore-Lower: 6 X-AL-KERNEL-Commit: 3be89e8039a85fa3b6cd5f9dcc4c1459eb356c2d List-Id: CVE: CVE-2026-89631 Priority: IMPORTANT AL-KERNEL base severity: IMPORTANT KPANIC flag: YES Patch: smb: client: reject a tree connect response whose byte count is too small [ Upstream Commit: 3be89e8039a85fa3b6cd5f9dcc4c1459eb356c2d Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3be89e8039a85fa3b6cd5f9dcc4c1459eb356c2d Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89631 Analysis date: Sat, 12 Sep 2026 00:14:50 -0400 ActionableScore: 7 ActionableScore lower bound: 6 Actionable bucket: Strong Important candidate Manual review required: YES Summary: No one-sentence report phrase was found. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-89631 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89631 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: IMPORTANT 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-2026-89631 IMPORTANT https://www.kernelcve.org/list/?q=CVE-2026-89631 CHECK WITH IMPACT FROM ORIG NN MODERATE Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H';CWE-125;CWE-190;*CWE-200;*CWE-20;Other CVSS 'AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:L';BEST CVSS score: '9.1';DESCR 'An out-of-bounds read in the SMB/CIFS client tree connect response parser can occur when a server returns ByteCount 0 or 1. The parser subtracts 2 from the byte count and the value can wrap into a much larger bound, allowing nativeFileSystem parsing to read beyond the SMB response buffer and copy kernel memory into tcon nativeFileSystem. A malicious or compromised SMB server can trigger this during tree connect after the client connects or reconnects to the share. For the CVSS the PR:N is used because the attacker needs control of the SMB server response but no privileges on the client. The issue is network reachable from server to client, but practical exploitation usually requires the victim system to mount or already trust an attacker controlled SMB server. Impact is kernel memory disclosure through /proc/fs/cifs/DebugData and possible denial of service from the out-of-bounds read. YES REQUIRES MANUAL CHECK';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate (with actual score 7) YES DISK SIMPLEFIX SERVERTOCLIENT LINUS KPANIC KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS NO NO checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=7 ActionableScoreLower=6 PrimitiveClass=CLASS_A RealUAF=NO HostAdminRequired=NO NamespaceScopedPrivilege=NO ImportantCandidate=YES ManualReview=REQUIRED AutoDowngradeAllowed=NO 1. ActionableScore * Raw conservative score: 6 * Conservative score: 6. No floor or cap applied. * Raw paranoid score: 7 * Paranoid score: 7. No floor or cap applied. * Final recommended bucket: **Strong Important candidate**:: 2. Signal breakdown Conservative signals: * +2 Remote reachable / server-triggered input. A malicious or compromised SMB server can return a malformed Tree Connect response with ByteCount 0 or 1 when the client connects or reconnects. * +2 CLASS_A substantial kernel disclosure primitive. The underflow and wraparound can make CIFSTCon read beyond the SMB response buffer and copy kernel memory into tcon->nativeFileSystem. * +1 Confidentiality plausible. The commit explicitly states that the bytes reach userspace through /proc/fs/cifs/DebugData. * +1 Practical exploit chain demonstrated. The supplied evidence connects malformed server response -> OOB read -> copied nativeFileSystem string -> userspace-visible DebugData. Paranoid additional signal: * +1 Possible strong DoS. The OOB read can run beyond a roughly 16 KB cifs_req_poolp slab object with a bound up to 65535, so a kernel fault or crash is technically plausible. Important non-awarded signals: * RealUAF not awarded. No stale lifetime object or use-after-free is established. * Arbitrary write not awarded. The patch establishes an out-of-bounds read, not a write primitive. * Integrity impact not awarded. There is no unauthorized modification of kernel state or protected data. * Controlled reclaim not demonstrated. * Strong LPE not awarded. The evidence supports disclosure and possible crash, not privilege escalation. * Extra availability not awarded beyond the paranoid +1 because no distinct persistent availability mechanism is shown. * No host-admin penalty applied because the attacker-controlled input is a server response and no victim host-admin privilege is required by the attacker once the client reaches the SMB server. * No rare-subsystem penalty applied. SMB/CIFS client mounts are common enough in enterprise environments, even though the vulnerable path requires an SMB tree connect. 3. Reachability analysis The evaluated attacker is a malicious or compromised SMB server, or an attacker able to influence the SMB server response. The vulnerable operation is reached when the Linux SMB/CIFS client processes a Tree Connect response in CIFSTCon. This is not an arbitrary Internet packet vulnerability against every host. The victim client must connect, reconnect, or already trust the attacker-controlled SMB server or share. In many deployments SMB/CIFS mounts are configured by an administrator, autofs, fstab, desktop integration, or a service, but the attacker does not need host-admin privileges on the victim to send the malformed response after the connection path exists. Namespace-scoped privilege is not relevant here. There is no netns capability chain to evaluate. No special hardware, firmware mediation, or race timing is required. The trigger is an ordered parser path, not a timing race. 4. Primitive and severity interpretation The exact primitive is an attacker-controlled out-of-bounds read in the SMB/CIFS client Tree Connect response parser. ByteCount 0 or 1 makes the parser compute bytes_left - 2 incorrectly, producing an excessive bound that can read beyond the response buffer. The data is then copied into tcon->nativeFileSystem and exposed to userspace through /proc/fs/cifs/DebugData. This is not a UAF, not a repeated release/refcount case, and not active structured stale-object reuse. It is a concrete OOB read with an established disclosure path. Because the disclosure path is explicit and network-controlled server input can trigger it, this is CLASS_A for actionable triage. The demonstrated consequence is kernel memory disclosure to userspace. A crash is plausible due to the large OOB read beyond the slab object, but privilege escalation is not established. ImportantCandidate is YES because this is a network-reachable kernel memory disclosure primitive. AutoDowngradeAllowed is NO because automatic Moderate closure would lose a concrete disclosure primitive requiring security review. Likely operational severity is Important-class or at least high-end Actionable Moderate, depending on deployment exposure and readability of /proc/fs/cifs/DebugData in the affected product. 5. One-sentence report phrase A malicious SMB server can send a Tree Connect response with a too-small ByteCount, causing the Linux CIFS client to perform an out-of-bounds read and expose kernel memory through DebugData, with possible kernel crash. 6. Manual review recommendation MANUAL CHECK REQUIRED Reason: the issue is server-triggerable over SMB/CIFS and includes a concrete kernel memory disclosure path, not merely a crash-only parser failure. 7. Call-site confidence Call-site confidence: high ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: smb: client: reject a tree connect response whose byte count is too small [ Upstream Commit: 3be89e8039a85fa3b6cd5f9dcc4c1459eb356c2d Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3be89e8039a85fa3b6cd5f9dcc4c1459eb356c2d Changed files: fs/smb/client/cifssmb.c fs/smb/client/trace.h 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=3be89e8039a85fa3b6cd5f9dcc4c1459eb356c2d ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-89631 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89631 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: AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:L The Best / paranoid CVSS vector: AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H The Best / paranoid CVSS score: 9.1 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: 6 Paranoid ActionableScore: 7 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: IMPORTANT KPANIC detected for this report: YES Published priority for this report (same as in Subject): IMPORTANT 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).