[CVE-2026-80976][MODERATE 7.0] seg6: reset IP6CB after IPv6 decapsulation

[CVE-2026-80976][MODERATE 7.0] seg6: reset IP6CB after IPv6 decapsulation

[AL-KERNEL]

CVE: CVE-2026-80976
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: seg6: reset IP6CB after IPv6 decapsulation
Commit: 2b154e96fcb3f01fd42765c64e0a56820fbc16eb
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2b154e96fcb3f01fd42765c64e0a56820fbc16eb
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80976
Analysis date: Fri, 11 Sep 2026 16:04:05 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate
Manual review required: NO

Summary:
A stale IP6CB nhoff value after SRv6 IPv6 decapsulation can cause ip6_protocol_deliver_rcu to read beyond the skb head and crash the kernel, with local unprivileged namespace reachability established and possible remote reachability when an SRv6 local endpoint is exposed.

======================================================================
ABOUT THIS REPORT
======================================================================

The original Linux kernel CVE announcement for CVE-2026-80976 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80976

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: NO

A detailed explanation of the methodology and priority rules is included
at the end of this message.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-80976	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-80976 CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H';CWE-125;CWE-665;CWE-823;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A stale IPv6 control block after SRv6 decapsulation can leave IP6CB nhoff and extension header state from the outer packet attached to the inner IPv6 packet. When End.DX6 or End.DT6 sends the inner packet into the IPv6 input path, ip6_protocol_deliver_rcu may use the stale offset and read beyond the skb head, which is reported as a KASAN slab out of bounds read. For the CVSS the PR:N is used in the paranoid network scenario because a preconfigured SRv6 endpoint may process a crafted packet without requiring an account on the victim. The base local scenario needs a local user able to use user and net namespaces to install a local SID and inject the crafted outer packet. The issue is potentially network reachable when SRv6 local processing is exposed, but SRv6 is commonly limited to controlled operator or data center networks. Impact is at least denial of service via kernel crash, with possible low confidentiality or integrity concern from an out of bounds read but no proven write primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate (with actual score 5)	YES	REMOTE WRITE KASAN OOB NETWORK SKB LINUS IPV6 KPANIC PACKET  KPANIC DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN6	NO	NO	checked

======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================

ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=YES
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

## 1. ActionableScore

* Raw conservative score: 4
* Conservative score: 4. No floor or cap applied.
* Raw paranoid score: 5
* Paranoid score: 5. No floor or cap applied.
* Final recommended bucket: **Actionable Moderate**::

## 2. Signal breakdown

Conservative signals:

* +1 Weak OOB read / invalid offset primitive. Stale IP6CB nhoff from the outer IPv6 packet is reused after SRv6 decapsulation and causes an out-of-bounds read in ip6_protocol_deliver_rcu.
* +1 Local unprivileged trigger. The supplied description states that an unprivileged user can reach End.DT6 from a user and network namespace by installing a local SID and injecting a crafted outer packet.
* +1 Reliable kernel crash / strong DoS. KASAN reports slab-out-of-bounds in the IPv6 input path, and the described effect is a kernel crash or fatal fault.
* +1 Networking packet-processing path. The bug is reached through SRv6 decapsulation and subsequent IPv6 input processing, not merely through passive configuration metadata.

Paranoid signals:

* +1 Weak OOB read / invalid offset primitive.
* +2 Potential remote packet reachability. If an SRv6 local endpoint using End.DX6 or End.DT6 is already configured and reachable, a crafted packet can reach the vulnerable decapsulation path without a local account on the victim.
* +1 Reliable kernel crash / strong DoS.
* +1 Networking packet-processing path.

Non-awarded signals:

* RealUAF not awarded. This is stale skb control-block metadata, not a freed object used after lifetime end.
* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Confidentiality not awarded. The OOB read is internal to kernel processing, and no user-visible or network-visible disclosure path is established.
* Integrity not awarded. No write primitive or security-state modification is established.
* Strong LPE or strong exploitation leverage not awarded. The patch supports invalid read and crash, not controlled memory corruption.
* Extra Availability not awarded. The same crash is already counted once.
* Race penalty not applied. This is not a timing race.
* Host-admin penalty not applied. The supplied evidence explicitly describes an unprivileged user and namespace-based local trigger.

## 3. Reachability analysis

The conservative attacker is a local unprivileged user. The supplied description states that the user can create or use a user namespace and network namespace, install a local SRv6 SID, and inject a crafted outer IPv6 packet with Hop-by-Hop and Destination Options headers followed by an SRH and a minimal inner IPv6 packet. Based on that supplied chain, host-admin-equivalent privilege is not required for the local trigger, and the relevant operation is treated as namespace-scoped.

The paranoid attacker is a remote packet sender against an already configured SRv6 local endpoint. This is technically plausible because End.DX6 and End.DT6 route the decapsulated inner IPv6 packet into the IPv6 input path. However, remote reachability depends on deployment: SRv6 local processing is usually configured intentionally in operator, lab, data-center, or routing environments and is not generally exposed on ordinary hosts.

No special hardware is required. No hard race is required. The main prerequisites are kernel support for SRv6 local actions and either attacker-controlled namespace setup for the local case or a preconfigured reachable SRv6 endpoint for the network case.

## 4. Primitive and severity interpretation

The exact primitive is stale control-block state after IPv6 decapsulation. decap_and_validate() makes the inner IPv6 packet the skb network header but, before the fix, did not clear IP6CB for the inner IPv6 case. As a result, nhoff and extension-header state collected while parsing the outer packet could be reused when ip6_protocol_deliver_rcu() handled the inner packet.

This is not a UAF and not a repeated release/refcount case. It is a stale metadata / invalid offset bug leading to a read beyond the skb head. The demonstrated consequence is slab-out-of-bounds read reported by KASAN and practical DoS via kernel crash. A confidentiality effect is not established because no path is shown where the out-of-bounds bytes are returned to userspace or sent over the network. An integrity effect is not established because no write or object-corruption primitive is shown.

PrimitiveClass is CLASS_C because the demonstrated behavior is primarily invalid-state / OOB-read / crash behavior without an independently useful exploitation primitive. ImportantCandidate is NO because there is no UAF, no arbitrary write, no controlled reclaim, no authorization bypass, and no demonstrated privilege escalation. AutoDowngradeAllowed is YES because no unresolved Important-grade primitive is established, although manual review is still recommended due to the packet-processing path and possible remote SRv6 exposure.

Operationally, this is an actionable Moderate issue: it should be reviewed for deployments that use SRv6 local processing, especially where SRv6 packets can arrive from untrusted networks.

## 5. One-sentence report phrase

A stale IP6CB nhoff value after SRv6 IPv6 decapsulation can cause ip6_protocol_deliver_rcu to read beyond the skb head and crash the kernel, with local unprivileged namespace reachability established and possible remote reachability when an SRv6 local endpoint is exposed.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended because the bug is in an IPv6 packet-processing path and may be remotely reachable in SRv6 deployments, but the established primitive is only an internal OOB read leading to DoS and does not currently show UAF, write corruption, disclosure, or privilege escalation.

## 7. Call-site confidence

Call-site confidence: high

======================================================================
UPSTREAM PATCH SUMMARY
======================================================================

Patch: seg6: reset IP6CB after IPv6 decapsulation
Commit: 2b154e96fcb3f01fd42765c64e0a56820fbc16eb
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2b154e96fcb3f01fd42765c64e0a56820fbc16eb

Commit description:

decap_and_validate() pulls the outer SRv6 headers and makes the inner
packet the skb network header. The IPv6 control block still contains
values collected while parsing the outer packet, including nhoff and
extension-header flags.

End.DX6 and End.DT6 route the inner IPv6 packet directly to the IPv6
input path. An unprivileged user can reach End.DT6 from a user and net
namespace by installing a local SID and injecting an outer packet with
Hop-by-Hop and Destination Options headers followed by an SRH and a
minimal inner IPv6 packet.

The outer extension headers leave a large nhoff in IP6CB. After
decapsulation, ip6_protocol_deliver_rcu() uses that stale offset on the
inner packet and reads beyond the skb head. KASAN reports:

  BUG: KASAN: slab-out-of-bounds in ip6_protocol_deliver_rcu
  ip6_protocol_deliver_rcu+0x1118/0x1450
  ip6_input_finish+0x11b/0x240
  seg6_local_input_core+0xed/0x2e0
  lwtunnel_input+0x1e9/0x4e0
  ipv6_rthdr_rcv+0x525f/0x6c50
  ip6_protocol_deliver_rcu+0xcb7/0x1450

Before clearing IP6CB for an inner IPv6 packet, save its incoming
interface index and L3 slave state. Restore both after the clear and set
nhoff to the inner IPv6 base-header nexthdr field.

Use IP6CB(skb)->iif rather than skb->skb_iif because VRF processing can
replace skb_iif with the L3 master while IP6CB keeps the receiving
interface. Preserve IP6SKB_L3SLAVE for the same reason.

Fixes: d7a669d ("ipv6: sr: add helper functions for seg6local")
Cc: stable@vger.kernel.org
Reported-by: Vega <vega@nebusec.ai>
Signed-off-by: Zhiling Zou <zhilinz@nebusec.ai>
Reviewed-by: Andrea Mayer <andrea.mayer@uniroma2.it>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/ipv6/seg6_local.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=2b154e96fcb3f01fd42765c64e0a56820fbc16eb

======================================================================
DETAILED REPORT METHODOLOGY
======================================================================

The original Linux kernel CVE announcement for CVE-2026-80976 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80976

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:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 7

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: 4
  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 alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).