* [CVE-2026-98550][MODERATE REGULAR] tipc: reject invalid and unexpected GRP_ACK_MSG to prevent bc_ackers underflow
@ 2026-10-19 2:03 21% AL-KERNEL
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From: AL-KERNEL @ 2026-10-19 2:03 UTC (permalink / raw)
To: kernel-cve
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 <[email protected]>
Cc: [email protected]
Signed-off-by: Eric Dumazet <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
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
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My contact email for such notifications is [email protected] (and both
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