[CVE-2026-63920][MODERATE 7.0] ipv6: validate extension header length before copying to cmsg

[CVE-2026-63920][MODERATE 7.0] ipv6: validate extension header length before copying to cmsg

[AL-KERNEL]

CVE: CVE-2026-63920
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: ipv6: validate extension header length before copying to cmsg
Commit: 81394827dfb72772c50d0ae3bdfa094428a5d76d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=81394827dfb72772c50d0ae3bdfa094428a5d76d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63920
Analysis date: Sun, 19 Jul 2026 19:23:25 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
An unprivileged local user may use nftables in a user namespace to rewrite an IPv6 extension header length after parsing, causing recvmsg cmsg construction to copy out-of-bounds slab memory to userspace and potentially crash hardened kernels.

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

The original Linux kernel CVE announcement for CVE-2026-63920 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63920

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-2026-63920	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:H';CWE-125;*CWE-200;*CWE-20;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:L';BEST CVSS score: '7.1';DESCR 'An IPv6 extension header length can be trusted again during cmsg construction after it was already parsed, allowing nftables payload write rules to corrupt the hdrlen byte before recvmsg processing. When ip6_datagram_recv_specific_ctl calls put_cmsg with this modified length, a small extension header can be copied as a much larger buffer and disclose adjacent slab memory to userspace. For the CVSS the PR:L is selected for the paranoid score because a local unprivileged user can reach nftables through user namespaces on affected systems and then trigger the receive path from a local process. The issue is not directly network reachable by packet traffic alone because the important step is the local control plane rewrite after IPv6 parsing. Impact is kernel memory disclosure and possible denial of service via out of bounds read. There is no direct write primitive shown, but the kernel memory disclosure potential makes this suitable for manual review.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 6)	YES	REMOTE READ KASAN OOB NETWORK SKB IPV6 KPANIC 	NO	NO	checked

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

ActionableScore=6
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 6
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1
  The commit states that nftables payload-write is reachable from an unprivileged user namespace.

* Local unprivileged high-control kernel data-plane API refinement: +1
  nftables lets the attacker modify packet header bytes after IPv6 parsing and before recvmsg cmsg construction.

* Confidentiality impact plausible: +1
  put_cmsg() can copy up to 2040 bytes from an 8-byte extension header area, producing a slab out-of-bounds kernel memory disclosure to userspace.

* Common networking core path or packet-processing path: +1
  The affected code is in IPv6 datagram recvmsg ancillary-data handling, with nftables involved in the trigger path.

* Broad/default/common subsystem exposure: +1
  IPv6 sockets and netfilter/nftables are common kernel subsystems. Practical reachability depends on unprivileged user namespace policy.

Paranoid additional signal:

* Reliable kernel crash / strong DoS: +1
  KASAN reports a slab-out-of-bounds read in put_cmsg(). On hardened/debug systems or panic-on-oops configurations this can become a reliable local DoS, although the primary primitive is information disclosure.

Not counted:

* Remote reachable: +0
  Network packets alone do not appear sufficient. The important step is local nftables payload rewriting after parsing.

* Privilege escalation plausible: +0
  No write primitive, UAF reclaim, object replacement, refcount abuse, or callback control is shown.

* Generic strong memory corruption: +0
  This is an out-of-bounds read copied to userspace, not an OOB write or UAF.

Call-site confidence: high. The patch shows the vulnerable put_cmsg() call sites and the new bounds checks.

## 3. Reachability analysis

A local unprivileged user can potentially trigger this when unprivileged user namespaces allow nftables access in a network namespace. The attacker needs to create or control the nftables payload-write rule, arrange an IPv6 packet with extension headers, enable the relevant IPv6 socket options, and receive the packet through recvmsg() so that cmsg construction copies the corrupted length.

This is not directly remotely triggerable in the normal sense. A remote peer may provide packets, but the vulnerability depends on local post-parse header rewriting through nftables. If unprivileged user namespaces or unprivileged nftables use are disabled, practical triggering likely requires CAP_NET_ADMIN, which lowers operational risk.

## 4. Severity interpretation

This is more than an ordinary Moderate issue because it provides a concrete unprivileged kernel slab out-of-bounds read with user-visible disclosure. It is not a demonstrated privilege escalation bug by itself, since the patch does not show an overwrite, UAF, type confusion, or arbitrary memory manipulation primitive.

Realistic impact is kernel memory disclosure and possible local DoS on hardened/debug configurations. Theoretical exploitation value is higher because kernel memory disclosure can assist other exploit chains, but the standalone primitive remains read-only.

## 5. One-sentence report phrase

An unprivileged local user may use nftables in a user namespace to rewrite an IPv6 extension header length after parsing, causing recvmsg cmsg construction to copy out-of-bounds slab memory to userspace and potentially crash hardened kernels.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This should not be auto-closed because the issue is an unprivileged slab out-of-bounds read with direct kernel memory disclosure potential in a common networking path.

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

Patch: ipv6: validate extension header length before copying to cmsg
Commit: 81394827dfb72772c50d0ae3bdfa094428a5d76d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=81394827dfb72772c50d0ae3bdfa094428a5d76d

Commit description:

ip6_datagram_recv_specific_ctl() builds IPV6_{HOPOPTS,DSTOPTS,RTHDR}
cmsgs (and their IPV6_2292* legacy counterparts) by trusting the
on-wire hdrlen byte (ptr[1]) when computing the put_cmsg() length.
The length was validated only at parse time (ipv6_parse_hopopts(),
etc.).  An nftables payload-write expression can rewrite hdrlen after
parsing and before the skb reaches recvmsg; the write itself is
in-bounds but put_cmsg() then reads up to ((hdrlen+1) << 3) = 2040
bytes from an 8-byte header.  nftables is reachable from an
unprivileged user namespace, so this is an unprivileged
slab-out-of-bounds read:

  BUG: KASAN: slab-out-of-bounds in put_cmsg+0x3ac/0x540
   put_cmsg+0x3ac/0x540
   udpv6_recvmsg+0xca0/0x1250
   sock_recvmsg+0xdf/0x190
   ____sys_recvmsg+0x1b1/0x620

Add ipv6_get_exthdr_len() which validates that at least two bytes
are accessible before reading the hdrlen field, then checks the
computed length against skb_tail_pointer(skb), returning 0 on
failure.  Extension headers are kept in the linear skb area by
pskb_may_pull() during input, so skb_tail_pointer() is the correct
bound.

Use ipv6_get_exthdr_len() at all non-AH call sites: the five
standalone cmsg blocks (HbH, 2292HbH, 2292DSTOPTS x2, 2292RTHDR)
and the three standard cases in the extension-header walk loop
(DSTOPTS, ROUTING, default).  AH retains an inline bounds check
because its length formula differs ((ptr[1]+2)<<2).

The walk loop also gets a pre-read bounds check at the top to
validate ptr before any case accesses ptr[0] or ptr[1].

When the walk loop detects a corrupted header, return from the
function instead of continuing to process later socket options.

Cc: stable@vger.kernel.org
Fixes: 1da177e ("Linux-2.6.12-rc2")
Signed-off-by: Qi Tang <tpluszz77@gmail.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20260523143245.2281415-1-tpluszz77@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/ipv6/datagram.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=81394827dfb72772c50d0ae3bdfa094428a5d76d

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

The original Linux kernel CVE announcement for CVE-2026-63920 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63920

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:H/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:H
  The Best / paranoid CVSS score: 7.1

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: 5
  Paranoid ActionableScore: 6

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).