* [CVE-2026-99286][IMPORTANT] xen-netback: Fix XenBus connection failure cleanup
@ 2026-11-11 2:10 21% AL-KERNEL
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From: AL-KERNEL @ 2026-11-11 2:10 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-99286
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: NO
Patch: xen-netback: Fix XenBus connection failure cleanup
Commit: ade3082dcdee17c5767069883936d64ff83f6c9f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ade3082dcdee17c5767069883936d64ff83f6c9f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99286
Analysis date: Tue, 10 Nov 2026 22:10:53 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES
Summary:
A guest-influenced Xen netback connection-failure race can leave a freed queue array published to host statistics readers, causing a host-side active-object UAF and plausible crash.
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ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99286 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99286
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-2026-99286 IMPORTANT https://www.kernelcve.org/list/?q=CVE-2026-99286 CHECK WITH IMPACT FROM ORIG NN IMPORTANT | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=6 LEGACY=IMPORTANT RESULT=DIFF INDEPENDENT_LEGACY=IMPORTANT INDEPENDENT_RESULT=DIFF TEMP_KPANIC_MERGE=IMPORTANT_HARD_VETO PRE_MERGE_LEGACY=IMPORTANT (final Legacy and Aegis differ; FINAL: IMPORTANT, Aegis: MODERATE_KPANIC) Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:C/C:L/I:N/A:H';*CWE-416;CWE-362;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:C/C:N/I:N/A:H';BEST CVSS score: '6.4';DESCR 'A race in xen netback XenBus connection failure cleanup can leave vif num_queues visible as nonzero after synchronize_net begins. A later xenvif_get_stats or xenvif_get_ethtool_stats reader can capture that value and access queue objects while they are being deinitialized or after their allocation is freed, creating a UAF read or dereference with a plausible host backend crash. Triggering requires a frontend controlled connection failure together with concurrent host statistics collection, so AC:H is appropriate and the issue is not directly network reachable. For the CVSS the PR:L value reflects control of an unprivileged guest frontend rather than host administrator privileges. Availability impact is high. Limited confidentiality impact is retained only for the paranoid score because stale queue memory can be read into host statistics, while no attacker visible disclosure channel or write primitive is established. Automated impact decision path. The original SimpleNet classifier selected IMPORTANT. The primary LLM estimated paranoid CVSS 6.4 and the production ActionableScore returned 5 with recommendation Actionable Moderate / Strong Important candidate requiring manual review. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage IMPORTANT to IMPORTANT because the CVSS score, vector or bug class threshold rule was applied. final reconciliation stage IMPORTANT to IMPORTANT because a specific evidence based protection or manual review veto was applied and KP specialist did not establish a qualifying panic. The legacy KP specialist produced a review artifact and its decision was processed by the applicable KP rules. The legacy FD or DH downgrade specialist produced a review artifact and its decision was processed by the applicable downgrade and preservation rules. Exact triggered rule markers were LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS and INCREASED_TO_HIGH_BASED_ON_GUESSCVSS and TEMP_KPANIC_MERGE_IMPORTANT_VETO and KPANIC_UNSUPPORTED_REMOVED. The independent weaker Aegis shadow evaluated CVSS 3.0 and ActionableScore 6 with primitive class CLASS_B and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. Cross model aggregation changed the production result under an enabled evidence gated merge rule. The final reported impact is IMPORTANT.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5) MAYBE RACE LEAK UAF NETWORK LINUS LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS INCREASED_TO_HIGH_BASED_ON_GUESSCVSS TEMP_KPANIC_MERGE_IMPORTANT_VETO KPANIC_UNSUPPORTED_REMOVED - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO
1. ActionableScore
* Raw base score: 3.
* Conservative score: 5. REAL UAF FLOOR 5 applies because a below-host-admin guest can influence the connection-failure lifecycle and a host reader consumes active queue state after its lifetime may end.
* Raw paranoid score: 4.
* Paranoid score: 5 after the same floor; no additional unsupported disclosure or corruption impact is counted.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
2. Signal breakdown
* +2 — Real UAF/lifetime violation: `num_queues` remains nonzero while `queues[]` entries are deinitialized and the allocation is freed, allowing statistics readers to use stale queue objects.
* +1 — Concrete RCU/synchronization lifetime mismatch: new readers can capture nonzero `num_queues` after `synchronize_net()` starts and therefore are not covered by that grace period. The fix unpublishes the count before synchronization.
* +1 — Plausible host-kernel crash from stale queue dereference, counted once.
* -1 — The trigger requires overlap between frontend-controlled connection failure and host statistics collection; the guest does not directly control the latter timing.
* Paranoid +1 — The stale object is active structured queue state consumed by host statistics code, and the exact post-free field behavior is unresolved.
* Controlled reclaim, attacker-shaped replacement contents, arbitrary write, callback control, and attacker-visible disclosure are not shown. No extra availability or confidentiality point is awarded.
3. Reachability analysis
The supported attacker is an unprivileged Xen guest frontend, not a host user or administrator. It can influence connection setup failure, while host-side statistics or ethtool collection supplies the concurrent reader. Thus the trigger is partly environmental but does not require the attacker to possess host-admin powers. No Linux namespace capability is involved, and no unusual physical hardware prerequisite is shown. The race is timing-sensitive, although connection attempts and periodic statistics collection may permit retries.
4. Primitive and severity reading
This is an active structured-object UAF, not fixed stale storage. The error cleanup frees `vif->queues` while a published nonzero queue count can direct later privileged host readers into those queues. There is no repeated release/refcount mutation and no independently proven Class A exploit primitive.
The shown consequence is stale host-kernel queue access with plausible backend/host crash. Disclosure through returned statistics and stronger guest-to-host corruption remain unproven. Nevertheless, the concrete UAF, later privileged consumer, and guest/host trust boundary make this an Important review candidate under the active-state boundary rule. Automatic downgrade would discard unresolved cross-boundary post-lifetime behavior.
B4 authorization-state preservation does not apply: no authorization guard changes DENY to ALLOW.
5. One-sentence report phrase
A guest-influenced Xen netback connection-failure race can leave a freed queue array published to host statistics readers, causing a host-side active-object UAF and plausible crash.
6. Manual review advice
MANUAL CHECK REQUIRED
Review the exact statistics fields read after queue deinitialization, guest retry control, and whether stale data can escape through host-visible statistics or affect other host state.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: xen-netback: Fix XenBus connection failure cleanup
Commit: ade3082dcdee17c5767069883936d64ff83f6c9f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ade3082dcdee17c5767069883936d64ff83f6c9f
Commit description:
xenvif_get_stats and xenvif_get_ethtool_stats capture vif->num_queues
under rcu_read_lock. We need to unpublish vif->num_queues before
synchronize_net to ensure that readers that come afterwards will not
read a non-zero vif->num_queues and attempt to access invalid queues.
This is the same cleanup algorithm as used in backend_disconnect and
added in commit b17075d ("xen-netback: fix race condition on XenBus
disconnect").
This is XSA-531
Fixes: 5fb756f ("xen-netback: Fix UAF when reading statistics data")
Signed-off-by: Tu Dinh <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
drivers/net/xen-netback/xenbus.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=ade3082dcdee17c5767069883936d64ff83f6c9f
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99286 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99286
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:C/C:N/I:N/A:H
The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:C/C:L/I:N/A:H
The Best / paranoid CVSS score: 6.4
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: 5
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: NO
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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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