From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-98550][MODERATE REGULAR] tipc: reject invalid and unexpected GRP_ACK_MSG to prevent bc_ackers underflow Date: Sun, 18 Oct 2026 22:03:58 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-98550 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: edc6593c442278fceff0120e8cc58bc592168c35 List-Id: CVE: CVE-2026-98550 Priority: MODERATE REGULAR AL-KERNEL base severity: MODERATE KPANIC flag: NO Patch: tipc: reject invalid and unexpected GRP_ACK_MSG to prevent bc_ackers underflow Commit: edc6593c442278fceff0120e8cc58bc592168c35 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=edc6593c442278fceff0120e8cc58bc592168c35 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98550 Analysis date: Sun, 18 Oct 2026 22:03:58 -0400 ActionableScore: 3 ActionableScore lower bound: unknown Actionable bucket: Borderline / manual review required Manual review required: YES Summary: A network-adjacent TIPC group member can send an invalid GRP_ACK_MSG that underflows bc_ackers and permanently blocks group broadcasts on the affected socket, causing a persistent service-level denial of service without demonstrated kernel memory corruption. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-98550 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98550 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-2026-98550 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-98550 CHECK WITH IMPACT FROM ORIG NN IMPORTANT | AEGIS IMPACT MODERATE AEGIS_AS=1 LEGACY=MODERATE RESULT=SAME (final reconciliation agrees; DECISION: both MODERATE) Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H';CWE-191;*CWE-20;**CWE-400;Other CVSS 'AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '6.5';DESCR 'TIPC group ACK processing accepts invalid or unexpected GRP_ACK_MSG values that do not correspond to the current broadcast round. An ACK received when bc_ackers is zero can underflow the u16 counter to 65535, while an incorrect sequence during an active round can decrement the counter more than once for the same member. This can make tipc_group_bc_cong permanently report congestion and block future group broadcasts on the affected socket. For the CVSS the PR:N value is used because no privileges on the target are required when an attacker can transmit crafted TIPC messages as a recognized group member. The attack is network adjacent rather than generally Internet reachable because it requires access to the TIPC bearer or cluster network. The demonstrated impact is availability loss for the affected group socket, with no supported UAF, OOB access, information disclosure, or privilege escalation primitive. Automated impact decision path. The original SimpleNet classifier selected IMPORTANT. The primary LLM estimated paranoid CVSS 6.5 and the production ActionableScore returned 3 with recommendation Borderline / manual review required. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were ActionableScore thresholds stage IMPORTANT to MODERATE because the ActionableScore threshold 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 DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN5 and DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4. The independent weaker Aegis shadow evaluated CVSS 4.3 and ActionableScore 1 with primitive class CLASS_C and manual review RECOMMENDED. Its independently reconciled impact was MODERATE. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is MODERATE.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review required (with actual score 3) MAYBE OOB LOCK TIPC IMPROVEONLY LINUS PACKET DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN5 DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 - - checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=3 PrimitiveClass=CLASS_C RealUAF=NO HostAdminRequired=NO NamespaceScopedPrivilege=UNKNOWN ImportantCandidate=NO ManualReview=REQUIRED AutoDowngradeAllowed=YES RequiresHostUID0=NO AuthorizationDecisionBypass=NO ProtectedResourceImpact=NONE B4PendingReview=NO B4PendingConfirmed=NO 1. ActionableScore * Raw conservative score: 3 * Conservative score: 3 * Raw paranoid score: 3 * Paranoid score: 3 * No floor or cap was applied. * Final recommended bucket: **Borderline / manual review required**:: 2. Signal breakdown * +2 Direct network-adjacent trigger. A TIPC group member can submit an unexpected or stale GRP_ACK_MSG that reaches the vulnerable counter update. This is limited to a reachable TIPC bearer or cluster network rather than arbitrary Internet traffic. * +1 Persistent service-level DoS. Underflow changes the u16 bc_ackers counter to 65535, making tipc_group_bc_cong report permanent congestion and blocking future broadcasts on the affected group socket. * No additional availability point was awarded. Blocking the group socket is the same demonstrated consequence, and no kernel panic or separate host-wide failure is established. * No race penalty applies. The invalid ACK sequences cause a deterministic state transition rather than requiring a narrow timing window. * No UAF, OOB access, heap overwrite, arbitrary write, disclosure, or privilege-escalation primitive is established. * V32, V33, B3-PENDING, B4-PENDING, and B5-R do not apply. The modified value is ordinary TIPC accounting state, not checksum metadata, shared backing, authorization state, or retry-accumulated heap storage. * No broad-subsystem bonus applies because TIPC group communication is not shown to be broadly or commonly exposed. 3. Reachability analysis The evaluated attacker is a remote or network-adjacent TIPC group member capable of transmitting a crafted GRP_ACK_MSG. The attacker does not require UID 0 or administrative privileges on the target host. The target must already have a configured and reachable TIPC bearer and an affected group socket. The supplied evidence does not establish whether an ordinary local user can create the complete relevant TIPC topology inside an attacker-created namespace, so NamespaceScopedPrivilege remains UNKNOWN. This uncertainty does not negate the documented peer-triggered path. The trigger is deterministic. An ACK when bc_ackers is zero causes immediate underflow, while two accepted sequence values during one broadcast round can decrement the counter twice. No special hardware or race timing is required. 4. Primitive and severity interpretation The exact primitive is an attacker-influenced u16 accounting underflow in active TIPC group state. It is not an out-of-bounds integer calculation and does not write outside the bc_ackers field. The corrupted value is later consumed by tipc_group_bc_cong, but the demonstrated result is protocol congestion state and loss of group-broadcast availability. This is CLASS_C because it is an invalid-state and DoS primitive without independent kernel memory corruption. The practical consequence is persistent broadcast blockage on the affected socket. Host-wide panic, confidentiality loss, integrity compromise, and privilege escalation are not supported. ImportantCandidate is NO because no Important-preserving corruption or security-boundary primitive exists. AutoDowngradeAllowed is YES because manual review concerns practical TIPC exposure and service criticality, not unresolved memory-corruption leverage. 5. One-sentence report phrase A network-adjacent TIPC group member can send an invalid GRP_ACK_MSG that underflows bc_ackers and permanently blocks group broadcasts on the affected socket, causing a persistent service-level denial of service without demonstrated kernel memory corruption. 6. Manual review recommendation MANUAL CHECK REQUIRED Confirm whether untrusted nodes can access the deployed TIPC bearer, become or impersonate recognized group members, and whether loss of the affected group socket causes broader clustered-service disruption. 7. Call-site confidence Call-site confidence: high ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: tipc: reject invalid and unexpected GRP_ACK_MSG to prevent bc_ackers underflow Commit: edc6593c442278fceff0120e8cc58bc592168c35 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=edc6593c442278fceff0120e8cc58bc592168c35 Commit description: Commit 48a5fe3 ("tipc: fix bc_ackers underflow on duplicate GRP_ACK_MSG") rejected duplicate/stale ACKs in tipc_group_proto_rcv() by returning early when less_eq(acked, m->bc_acked). However, that check remains incomplete in two ways: 1. When grp->bc_ackers is zero (e.g. on a quiet group, when replicast ACKs were not requested, or after all expected members have already acknowledged), an unexpected GRP_ACK_MSG with acked > m->bc_acked passes less_eq() and unconditionally decrements grp->bc_ackers. Because bc_ackers is a u16, this wraps to 65535, causing tipc_group_bc_cong() to permanently report congestion and blocking all future group broadcasts on the socket. 2. During an active broadcast round (grp->bc_ackers > 0), the sender transmits packet S and advances grp->bc_snd_nxt to S + 1. Receivers increment their expected counter to S + 1 upon consuming packet S, so the only valid ACK value for the current round is strictly acked == grp->bc_snd_nxt. However, tipc_group_update_bc_members() initializes each member's m->bc_acked to prev = grp->bc_snd_nxt - 1 (S - 1 before increment). This leaves a 2-sequence gap (S - 1 to S + 1) in sequence space. An incoming ACK is therefore neither rejected as duplicate nor prevented from decrementing grp->bc_ackers if an unexpected or stale value (such as S) is received. A member sending acked = S followed by acked = S + 1 could decrement grp->bc_ackers twice in the same round, prematurely clearing bc_ackers or underflowing it. Fix this by: - Dropping GRP_ACK_MSG immediately if grp->bc_ackers is zero. - Requiring acked == grp->bc_snd_nxt and rejecting duplicates where m->bc_acked == acked. Because replicast broadcast rounds are strictly sequential, only grp->bc_snd_nxt can be acknowledged, and each member can acknowledge at most once per round. Note that a related pre-existing issue in tipc_group_delete_member() (where grp->bc_ackers decrementing to zero upon member departure does not restore *grp->open or trigger a socket wakeup) will be addressed in a separate patch. Fixes: 48a5fe3 ("tipc: fix bc_ackers underflow on duplicate GRP_ACK_MSG") Fixes: 2f48771 ("tipc: guarantee that group broadcast doesn't bypass group unicast") Reported-by: James Burton Cc: stable@vger.kernel.org Signed-off-by: Eric Dumazet Link: https://patch.msgid.link/20260913044233.193927-1-edumazet@google.com Signed-off-by: Jakub Kicinski Signed-off-by: Greg Kroah-Hartman Changed files: net/tipc/group.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=edc6593c442278fceff0120e8cc58bc592168c35 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-98550 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98550 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:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L The Best / paranoid CVSS vector: AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H The Best / paranoid CVSS score: 6.5 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).