* [CVE-2026-92489][MODERATE 7.0] xfrm: Fix skb double-free in xfrm_dev_direct_output() [ Upstream
@ 2026-09-18 9:37 21% AL-KERNEL
0 siblings, 0 replies; 1+ results
From: AL-KERNEL @ 2026-09-18 9:37 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-92489
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: xfrm: Fix skb double-free in xfrm_dev_direct_output() [ Upstream
Commit: 621871b696b108026bf4b44ed4085ffa2102f417
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=621871b696b108026bf4b44ed4085ffa2102f417
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-92489
Analysis date: Fri, 18 Sep 2026 05:37:53 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES
Summary:
No one-sentence report phrase was found.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-92489 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-92489
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 7.0
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-2026-92489 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-92489 CHECK WITH IMPACT FROM ORIG NN IMPORTANT Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H';CWE-415;*CWE-416;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7.5';DESCR 'A double free in xfrm_dev_direct_output occurs when local_out consumes an skb or transfers its ownership, for example when Netfilter drops the packet, but the XFRM output path calls kfree_skb on the same skb again. This is a kernel memory corruption condition which can cause a crash and may allow confidentiality or integrity impact if an attacker can reliably influence allocator reuse after the double free. For the CVSS the PR:N in the paranoid score reflects an attacker that only needs to send network adjacent traffic through an already configured vulnerable XFRM device offload and Netfilter path. The issue is not generally reachable from the public Internet and depends on a specific IPsec XFRM forwarding configuration. Impact is at least denial of service through kernel memory corruption. Worst case confidentiality integrity and privilege escalation impact requires manual review of the affected configuration and allocator behavior.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5) SKIP CVE-2026-92489 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES NO NO unknown MAYBE DANGER UAF SKB LINUS KPANIC PACKET DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
1. ActionableScore
* Raw conservative score: 4
* Conservative score: 5
* Raw paranoid score: 5
* Paranoid score: 5
* Applied conservative floor: 4 -> 5. FLOOR 5 applies because the patch establishes a concrete repeated lifetime mutation on the same potentially freed skb reference.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
2. Signal breakdown
* +2 Real UAF and lifetime violation. local_out may consume and free the skb, while xfrm_dev_direct_output retains and later passes the same skb pointer to kfree_skb.
* +1 Concrete repeated release operation. The first local_out path may end skb lifetime when Netfilter drops the packet. The exact skb reference remains reachable in xfrm_dev_direct_output and the later kfree_skb performs a second lifetime-affecting operation.
* +1 Kernel crash or strong DoS plausibility. A second skb free can cause allocator corruption, refcount faults, memory safety checks, or a kernel crash.
* +1 Paranoid repeated lifetime corruption leverage. The second kfree_skb actively mutates skb lifetime state through storage whose original skb lifetime may already have ended. This is not based only on theoretical heap reuse.
Important non-awarded signals:
* Controlled skb reclaim or attacker-controlled replacement object is not demonstrated.
* Arbitrary or directed kernel write is not demonstrated.
* Confidentiality and integrity impact are not awarded because no disclosure, controlled write, or sensitive-object corruption primitive is established.
* Direct remote reachability is not awarded. The path requires an existing XFRM device direct-output and Netfilter configuration.
* Local unprivileged reachability is not awarded because the supplied evidence does not establish that an ordinary user can drive the complete XFRM offload path.
* Structured stale-object leverage is not separately awarded. The meaningful stale operation is already counted as the concrete second lifetime mutation.
* Weak LPE concern is not separately awarded for the same repeated-free fact.
* No race penalty applies. This is an ordered ownership-handling failure, not a narrow timing race.
* No subsystem rarity penalty applies. XFRM offload configuration is a prerequisite, but the attacker privilege and deployment accessibility are unresolved rather than positively restricted.
Repeated-release assessment:
* First release can end lifetime: Yes. local_out returning a value other than 1 means skb ownership was consumed or transferred. The commit explicitly identifies the Netfilter drop case, where skb is freed.
* Exact reference survives: Yes. The local skb variable remains in xfrm_dev_direct_output after local_out returns.
* Second operation mutates lifetime state: Yes. kfree_skb is a real skb release operation and can free or refcount-release the skb.
* Concrete repeated release signal: Awarded, +1.
* Paranoid repeated-refcount leverage: Awarded, +1, because the second release actively acts on potentially post-lifetime skb storage.
* FLOOR 5: Applied.
3. Reachability analysis
The vulnerable operation requires traffic to enter xfrm_dev_direct_output and for the destination local_out callback to consume the skb, such as through a Netfilter drop decision. The affected path appears to require an existing XFRM device packet-offload deployment, relevant XFRM policy or routing, and a local_out path that consumes the packet.
A network-adjacent attacker may be able to provide traffic if a host administrator has already deployed the affected IPsec XFRM offload configuration and the traffic is routed through it. However, the supplied patch does not establish that an external attacker can reach the complete vulnerable path without such prior configuration. Direct remote reachability is therefore not established.
HostAdminRequired is UNKNOWN. Configuring XFRM devices and offload policies often involves CAP_NET_ADMIN, but the evidence does not establish whether an attacker can use an already configured path without host administrative privilege. NamespaceScopedPrivilege is also UNKNOWN because no complete namespace-scoped setup and trigger sequence is supplied. No special physical hardware access is established, although compatible XFRM device offload support is a practical feature prerequisite.
4. Primitive and severity interpretation
The exact primitive is a concrete skb double-free and therefore a real kernel lifetime violation. This is not merely an invalid dereference. The first operation can transfer or consume skb ownership and may free the skb. The later kfree_skb performs a second lifetime mutation through the same pointer.
This is Class B, not Class A. The patch establishes real post-lifetime memory-management corruption, but does not establish controlled skb reuse, attacker-shaped replacement contents, arbitrary kernel read or write, callback control, credential manipulation, or privilege escalation.
The demonstrated operational consequence is kernel memory corruption with realistic kernel crash or denial-of-service potential. A stronger confidentiality, integrity, or privilege-escalation consequence remains unresolved rather than established. ImportantCandidate is YES because a concrete repeated release through potentially freed skb storage has more unresolved exploitation leverage than an ordinary crash-only stale-pointer dereference. AutoDowngradeAllowed is NO because automatically treating this only as a low-priority Netfilter crash would discard the concrete second lifetime mutation.
5. One-sentence report phrase
A double free in xfrm_dev_direct_output can occur when local_out consumes an skb, for example after a Netfilter drop, and the XFRM path subsequently calls kfree_skb on the same skb. This can cause kernel memory corruption and denial of service, while exploitation beyond a crash requires manual review of reachable XFRM offload and Netfilter configurations.
6. Manual review recommendation
MANUAL CHECK REQUIRED
A concrete double-free performs a second lifetime mutation through a potentially dangling skb reference. Review whether untrusted local or network-adjacent traffic can traverse an already configured XFRM device direct-output path and trigger a consuming local_out handler.
7. Call-site confidence
Call-site confidence: medium
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: xfrm: Fix skb double-free in xfrm_dev_direct_output() [ Upstream
Commit: 621871b696b108026bf4b44ed4085ffa2102f417
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=621871b696b108026bf4b44ed4085ffa2102f417
Changed files:
net/xfrm/xfrm_output.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=621871b696b108026bf4b44ed4085ffa2102f417
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-92489 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-92489
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:H/I:H/A:H
The Best / paranoid CVSS score: 7.5
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: 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: MODERATE
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): MODERATE 7.0
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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