From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-99031][LOW] net: bridge: avoid recursive multicast port cleanup Date: Tue, 27 Oct 2026 03:19:46 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-99031 X-AL-KERNEL-Priority: LOW X-AL-KERNEL-Severity: LOW X-AL-KERNEL-Base-Severity: LOW X-AL-KERNEL-KPANIC: NO X-AL-KERNEL-ActionableScore: 1 X-AL-KERNEL-ActionableScore-Lower: unknown X-AL-KERNEL-Commit: 695a5a4c69c922d9296b2d7ad5f1ce8c6858fc64 List-Id: CVE: CVE-2026-99031 Priority: LOW AL-KERNEL base severity: LOW KPANIC flag: NO Patch: net: bridge: avoid recursive multicast port cleanup Commit: 695a5a4c69c922d9296b2d7ad5f1ce8c6858fc64 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=695a5a4c69c922d9296b2d7ad5f1ce8c6858fc64 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99031 Analysis date: Tue, 27 Oct 2026 03:19:46 -0400 ActionableScore: 1 ActionableScore lower bound: unknown Actionable bucket: Ordinary Moderate Manual review required: NO Summary: Recursive bridge multicast port-group cleanup can exhaust the kernel stack and crash the host when deleting an S,G entry with many STAR_EXCL ports. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-99031 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99031 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: LOW Manual review required: NO A detailed explanation of the methodology and priority rules is included at the end of this message. ====================================================================== AL-KERNEL CLASSIFICATION RESULT ====================================================================== CVE-2026-99031 LOW https://www.kernelcve.org/list/?q=CVE-2026-99031 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 LEGACY=LOW RESULT=DIFF TEMP_DISTANCE_MERGE_SKIP=AEGIS_NO_POSITIVE_SEVERITY_EVIDENCE (final Legacy and Aegis differ; FINAL: LOW, Aegis: MODERATE_KPANIC) Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';CWE-674;**CWE-400;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.3';DESCR 'A recursive cleanup path in bridge multicast handling can consume one kernel stack frame for every automatically added STAR_EXCL port group. With enough affected ports, deletion of an S,G entry can reach the kernel stack guard page and crash the host. The demonstrated primitive is stack exhaustion and denial of service, with no supported UAF, OOB access, information leak, or write primitive. For the CVSS the PR:N paranoid value applies only when an adjacent actor can influence multicast state across enough bridge ports without host credentials. Reliable triggering otherwise requires local bridge administration and substantial multicast state preparation, so AC:H is appropriate. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 5.3 and the production ActionableScore returned 1 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were final reconciliation stage LOW to LOW because KP specialist did not establish a qualifying panic and the cross model evidence reconciliation rule was applied. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were KPANIC_UNSUPPORTED_REMOVED and V64_AEGIS_UNSUPPORTED_PROMOTION_BLOCKED. The independent weaker Aegis shadow evaluated CVSS 5.3 and ActionableScore 1 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. The models disagreed but the aggregation rules retained the production result. The final reported impact is LOW.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1) MAYBE SIMPLEFIX LINUS KPANIC_UNSUPPORTED_REMOVED V64_AEGIS_UNSUPPORTED_PROMOTION_BLOCKED - - checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=1 PrimitiveClass=CLASS_C RealUAF=NO HostAdminRequired=UNKNOWN NamespaceScopedPrivilege=UNKNOWN ImportantCandidate=NO ManualReview=RECOMMENDED AutoDowngradeAllowed=YES RequiresHostUID0=UNKNOWN AuthorizationDecisionBypass=NO ProtectedResourceImpact=NONE B4PendingReview=NO B4PendingConfirmed=NO 1. ActionableScore * Raw base score: 1. * Conservative score: 1; no floor or cap applied. * Raw paranoid score: 1. * Paranoid score: 1; adjacent unprivileged reachability is conditional rather than established. * Final recommended bucket: **Ordinary Moderate**:: 2. Signal breakdown * +1 Reliable kernel crash / strong DoS: each recursive port-group deletion consumes another kernel stack frame; sufficiently many STAR_EXCL groups can reach the stack guard page. * No additional availability point: it would double-count the same stack-exhaustion crash. * No reachability bonus: neither remote nor below-host-admin completion of the required bridge-state preparation is shown. * No privilege penalty: host-administrator privilege for the complete trigger is also not positively established. * No race penalty: this is deterministic recursion after state preparation, not a narrow timing race. * No corruption bonus: the patch shows unbounded recursive cleanup, not UAF, OOB access, heap overwrite, or corrupted structured-state consumption. * B5-R, B3-PENDING, and B4-PENDING do not apply: there is no OOB write, shared-backing write, or authorization DENY-to-ALLOW transition. 3. Reachability analysis An actor must cause an S,G deletion while many automatically added STAR_EXCL port groups exist. Traffic may influence multicast membership, but the evidence does not establish that an adjacent or ordinary local attacker can create the required bridge/port topology and complete state sequence without bridge administration. The governing capability namespace and whether all setup works using attacker-obtainable namespace-scoped CAP_NET_ADMIN are unresolved. No special hardware or timing race is required; substantial state preparation is required. 4. Primitive and severity reading The exact primitive is recursive kernel-stack consumption during active multicast cleanup. The fix retains full deletion while suppressing re-entry into exclude-port cleanup for the same entry. This is CLASS_C because the supported terminal effect is stack-guard fault and host DoS. No useful write, stale-object mutation, disclosure, policy bypass, or leverage past the crash is shown. ImportantCandidate is therefore NO, and automatic Important-preservation is unnecessary. 5. One-sentence report phrase Recursive bridge multicast port-group cleanup can exhaust the kernel stack and crash the host when deleting an S,G entry with many STAR_EXCL ports. 6. Manual review advice MANUAL CHECK RECOMMENDED Confirm whether unprivileged multicast traffic can construct and delete enough affected groups across preconfigured bridge ports; this changes exposure, not the demonstrated crash-only primitive. 7. Call-site confidence Call-site confidence: medium ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: net: bridge: avoid recursive multicast port cleanup Commit: 695a5a4c69c922d9296b2d7ad5f1ce8c6858fc64 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=695a5a4c69c922d9296b2d7ad5f1ce8c6858fc64 Commit description: br_multicast_sg_del_exclude_ports() removes automatically added STAR_EXCL port groups by calling br_multicast_del_pg(). For S,G entries, that deletion calls back into br_multicast_sg_del_exclude_ports(), so a large number of STAR_EXCL ports consumes one kernel stack frame per port and can hit the stack guard page. Keep the complete port-group deletion path for cleanup, but suppress the S,G exclude-port cleanup while that helper is already walking the same entry. Normal deletion callers retain the existing behavior. Fixes: 8266a04 ("net: bridge: mcast: handle port group filter modes") Cc: stable@vger.kernel.org Reported-by: Vega Assisted-by: LLM Signed-off-by: Zixuan Chai Signed-off-by: Ren Wei Signed-off-by: David S. Miller Signed-off-by: Greg Kroah-Hartman Changed files: net/bridge/br_multicast.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=695a5a4c69c922d9296b2d7ad5f1ce8c6858fc64 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-99031 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99031 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:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H The Best / paranoid CVSS vector: AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H The Best / paranoid CVSS score: 5.3 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: 1 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: LOW KPANIC detected for this report: NO Published priority for this report (same as in Subject): LOW 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).