[CVE-2026-64554][IMPORTANT] netfilter: bridge: fix stale prevhdr pointer in br_ip6_fragment()

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-64554
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: netfilter: bridge: fix stale prevhdr pointer in br_ip6_fragment()
Commit: 8c10778ec674b67a07ea042fcba64270f3f38a5a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8c10778ec674b67a07ea042fcba64270f3f38a5a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64554
Analysis date: Mon, 27 Jul 2026 22:27:20 -0400
ActionableScore: 9
ActionableScore lower bound: 7
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A stale prevhdr pointer in IPv6 bridge netfilter fragmentation can survive skb head reallocation in skb_checksum_help and later cause a use-after-free write in ip6_frag_next, allowing a network-adjacent attacker in affected bridge configurations to crash the kernel and potentially corrupt kernel memory.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-64554 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64554

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-64554	IMPORTANT	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-416;*CWE-787;*CWE-664;Other CVSS 'AV:A/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7.5';DESCR 'A use-after-free write can occur in the IPv6 bridge netfilter fragmentation path because br_ip6_fragment keeps prevhdr as a pointer into the skb head across skb_checksum_help. For a cloned skb, skb_checksum_help may reallocate the skb head through pskb_expand_head, leaving prevhdr stale and later used by ip6_frag_next for a one byte write into freed memory. For the CVSS the PR:N is used for the paranoid network-adjacent scenario because a host on the same bridged Layer 2 network may be able to send traffic into the vulnerable bridge forwarding path without local credentials on the target. The attack complexity is high because the trigger depends on bridge netfilter IPv6 fragmentation, checksum handling, and cloned skb state. Impact is at least denial of service via kernel panic and in the worst still defensible interpretation may include confidentiality and integrity impact due to kernel memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 9)	YES	REMOTE WRITE OOB DANGER NETFILTER UAF NETWORK SKB IPV6 KPANIC PACKET 	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=9
ActionableScoreLower=7

## 1. ActionableScore

* Conservative score: 7
* Paranoid score: 9
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2. The vulnerable path is in bridge IPv6 netfilter packet processing and can be reached by traffic entering a Linux bridge, realistically from an adjacent L2 or internal bridged network.
* Privilege escalation plausible, weak: +1. The primitive is a UAF write, but the patch does not demonstrate attacker-controlled reclaim, object replacement, callback control, or arbitrary write.
* Generic memory corruption, strong primitive: +2. KASAN confirms slab-use-after-free with a write in ip6_frag_next.
* Real lifetime corruption: +1. prevhdr becomes stale after skb_checksum_help can reallocate skb head via pskb_expand_head.
* Reliable kernel crash / strong DoS: +1. The report shows kernel panic from the UAF write path.
* Common packet-processing path: +1. This is netfilter bridge IPv6 forwarding/fragmentation logic.
* Hard trigger conditions: -1. Trigger requires cloned skb, checksum help, IPv6 bridge netfilter fragmentation, and suitable forwarding conditions.

Paranoid additions:

* Confidentiality impact plausible: +1. Kernel UAF write may enable memory corruption beyond DoS in a worst still-defensible interpretation.
* Integrity impact plausible: +1. A write-after-free in kernel networking code can potentially corrupt reused kernel objects, although no controlled exploit primitive is shown.

## 3. Reachability analysis

A host on the same bridged L2 or internal network may be able to trigger the vulnerable forwarding path without credentials on the target, so PR:N is defensible for the network-adjacent model. The local trace also shows packet_sendmsg, but AF_PACKET and bridge setup details may require capabilities depending on the system. The path is not general Internet remote AV:N, but it is realistic for bridge deployments using IPv6 netfilter and conntrack bridge support. Trigger reliability is reduced by the need for cloned skb state and checksum-driven head reallocation.

Call-site confidence: high, because the provided trace shows the exact path from bridge forwarding through nf_ct_bridge_post to br_ip6_fragment and ip6_frag_next.

## 4. Severity interpretation

This behaves above ordinary Moderate because it is confirmed memory corruption, not only a warning or NULL dereference. Realistic demonstrated impact is kernel crash and DoS. Theoretical impact includes possible confidentiality and integrity compromise because the primitive is a write-after-free in kernel packet processing, but no attacker-controlled reclaim or arbitrary write is demonstrated. Conservative handling is Strong Important candidate. At minimum it should be treated as Actionable Moderate and not auto-closed.

## 5. One-sentence report phrase

A stale prevhdr pointer in IPv6 bridge netfilter fragmentation can survive skb head reallocation in skb_checksum_help and later cause a use-after-free write in ip6_frag_next, allowing a network-adjacent attacker in affected bridge configurations to crash the kernel and potentially corrupt kernel memory.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a network-adjacent kernel UAF write in a packet-processing path with confirmed KASAN evidence and kernel panic, and the worst-case memory corruption impact is plausible enough to require analyst review.

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

Patch: netfilter: bridge: fix stale prevhdr pointer in br_ip6_fragment()
Commit: 8c10778ec674b67a07ea042fcba64270f3f38a5a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8c10778ec674b67a07ea042fcba64270f3f38a5a

Commit description:

br_ip6_fragment() gets prevhdr, a pointer into the skb head, from
ip6_find_1stfragopt(), then calls skb_checksum_help().  For a cloned skb
skb_checksum_help() reallocates the head via pskb_expand_head(), leaving
prevhdr dangling.  It is later dereferenced in ip6_frag_next(), causing a
use-after-free write.

Save prevhdr's offset before skb_checksum_help() and recompute it after,
like commit ef0efcd ("ipv6: Fix dangling pointer when ipv6
fragment").

  BUG: KASAN: slab-use-after-free in ip6_frag_next (net/ipv6/ip6_output.c:857)
  Write of size 1 at addr ffff888013ff5016 by task exploit/141
  Call Trace:
   ...
   kasan_report (mm/kasan/report.c:595)
   ip6_frag_next (net/ipv6/ip6_output.c:857)
   br_ip6_fragment (net/ipv6/netfilter.c:212)
   nf_ct_bridge_post (net/bridge/netfilter/nf_conntrack_bridge.c:407)
   nf_hook_slow (net/netfilter/core.c:619)
   br_forward_finish (net/bridge/br_forward.c:66)
   __br_forward (net/bridge/br_forward.c:115)
   maybe_deliver (net/bridge/br_forward.c:191)
   br_flood (net/bridge/br_forward.c:245)
   br_handle_frame_finish (net/bridge/br_input.c:229)
   br_handle_frame (net/bridge/br_input.c:442)
   ...
   packet_sendmsg (net/packet/af_packet.c:3114)
   ...
   do_syscall_64 (arch/x86/entry/syscall_64.c:94)
   entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)
  Kernel panic - not syncing: Fatal exception in interrupt

Fixes: 764dd16 ("netfilter: nf_conntrack_bridge: add support for IPv6")
Cc: stable@vger.kernel.org
Reported-by: AutonomousCodeSecurity@microsoft.com
Signed-off-by: Xiang Mei (Microsoft) <xmei5@asu.edu>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/ipv6/ip6_output.c
  mm/kasan/report.c
  net/ipv6/netfilter.c
  net/bridge/netfilter/nf_conntrack_bridge.c
  net/netfilter/core.c
  net/bridge/br_forward.c
  net/bridge/br_input.c
  net/packet/af_packet.c
  arch/x86/entry/syscall_64.c
  arch/x86/entry/entry_64.S

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=8c10778ec674b67a07ea042fcba64270f3f38a5a

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

The original Linux kernel CVE announcement for CVE-2026-64554 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64554

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:H/PR:N/UI:N/S:U/C:L/I:L/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: 7
  Paranoid ActionableScore: 9

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