[CVE-2026-74667][MODERATE 7.0] net/packet: reset the MAC header on the packet-socket transmit path

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

CVE: CVE-2026-74667
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: net/packet: reset the MAC header on the packet-socket transmit path
Commit: 1e43a1d66615f411d427f9df1f46dd049d9e3681
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1e43a1d66615f411d427f9df1f46dd049d9e3681
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74667
Analysis date: Sat, 22 Aug 2026 16:51:54 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
An unset MAC header in the packet socket transmit bypass path can reach driver transmit code and cause an out-of-bounds kernel read when `eth_hdr(skb)` is used, leading primarily to local denial of service with possible limited memory-safety impact.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-74667 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74667

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-74667	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';CWE-125;CWE-665;CWE-754;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.8';DESCR 'An unset MAC header in the packet socket transmit path can reach ndo_start_xmit when PACKET_QDISC_BYPASS sends frames through dev_direct_xmit. A driver that reads eth_hdr(skb) on transmit may then read from an invalid offset around 64 KiB past skb head, causing an out-of-bounds kernel read and possible crash. For the CVSS the PR:L is used because reliable triggering requires local code execution with AF_PACKET transmit capability, typically CAP_NET_RAW or a delegated equivalent rather than full administrator privileges. The issue is not directly network reachable because the trigger is a local packet socket control and transmit path, not received packet traffic. Impact is at least local denial of service via kernel crash. For the paranoid score, limited confidentiality or integrity impact is kept due to the kernel out-of-bounds read candidate, but no arbitrary write or UAF primitive is shown.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	REMOTE KASAN OOB SKB QDISC PACKET  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**

## 2. Signal breakdown

Conservative signals:

* Weak or indirect corruption candidate: +1
  The bug produces an invalid MAC header offset and an out-of-bounds kernel read candidate in driver transmit handling, but no write primitive, UAF, double-free, or controlled object replacement is shown.

* Reliable kernel crash / strong DoS: +1
  A driver that reads `eth_hdr(skb)` on this path can dereference an invalid address around 64 KiB past `skb->head`, matching a KASAN-style OOB access class and plausibly causing a kernel crash.

* Broad/default/common subsystem: +1
  `AF_PACKET` and the packet socket transmit path are common Linux networking interfaces, though gated by capabilities in many deployments.

* Common networking core / packet-processing path: +1
  The issue is in `net/packet/af_packet.c` and affects packet socket TX behavior reaching `ndo_start_xmit()` through `dev_direct_xmit()`.

Paranoid additional signal:

* Local unprivileged or delegated capability trigger: +1
  In deployments with user namespaces, containers, or delegated `CAP_NET_RAW`, a non-host-root actor may be able to create packet sockets and exercise this path. This justifies the higher paranoid score.

Not awarded:

* No strong memory corruption +2
  The primitive is read-only OOB from an unset MAC header, not an OOB write, UAF, double-free, arbitrary write, or type confusion.

* No strong LPE plausibility +2
  The patch does not show attacker-controlled reclaim, callback control, write-after-free, object replacement, or another LPE-enabling primitive.

* No remote reachable +2
  The trigger is local packet socket transmission, not received network traffic.

## 3. Reachability analysis

The bug is triggered locally through packet sockets, especially when `PACKET_QDISC_BYPASS` sends frames through `dev_direct_xmit()` instead of the normal `__dev_queue_xmit()` path. In typical host deployments, creating the relevant packet socket requires `CAP_NET_RAW`, so this is best treated as PR:L rather than PR:N or PR:H. Namespaces and containers matter because `CAP_NET_RAW` may be delegated inside a network namespace, making the practical trigger available to reduced-privilege or containerized workloads. Exploitation also depends on a transmit driver or path that reads `eth_hdr(skb)` after the unset MAC header reaches `ndo_start_xmit()`.

## 4. Severity interpretation

This behaves like an actionable Moderate issue rather than an ordinary Low/Moderate cleanup. The realistic impact is local kernel crash or KASAN-detected OOB read through a networking transmit path. Theoretical confidentiality impact is limited because this is an OOB read candidate, but there is no demonstrated disclosure channel. Integrity and privilege escalation are not strongly supported because the patch does not show a write primitive or lifetime corruption. Manual review is still justified because this is a kernel memory-safety issue in a common networking path with namespace-sensitive reachability.

## 5. One-sentence report phrase

An unset MAC header in the packet socket transmit bypass path can reach driver transmit code and cause an out-of-bounds kernel read when `eth_hdr(skb)` is used, leading primarily to local denial of service with possible limited memory-safety impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is not a pure resource leak or validation-only fix. It is a kernel OOB read candidate in a networking transmit path, with practical reachability depending on `CAP_NET_RAW`, namespaces, `PACKET_QDISC_BYPASS`, and driver behavior.

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

Patch: net/packet: reset the MAC header on the packet-socket transmit path
Commit: 1e43a1d66615f411d427f9df1f46dd049d9e3681
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1e43a1d66615f411d427f9df1f46dd049d9e3681

Commit description:

packet_parse_headers() resets the MAC header only for a SOCK_RAW frame
whose socket did not bind a protocol. A protocol-bound SOCK_RAW socket,
any SOCK_DGRAM frame, and the legacy SOCK_PACKET path therefore leave
skb->mac_header unset here.

For frames sent via __dev_queue_xmit() this is harmless: it resets the
MAC header unconditionally. But the packet-socket PACKET_QDISC_BYPASS
path uses dev_direct_xmit(), which does not, so the frame reaches
ndo_start_xmit() with the MAC header unset. A driver that reads
eth_hdr(skb) on transmit then dereferences skb->head + (u16)~0, an
out-of-bounds access ~64 KiB past the head -- the same class fixed for
one consumer in commit f508900 ("macsec: do not read an unset MAC
header in macsec_encrypt()").

packet_parse_headers() runs only on the transmit path, where skb->data
points at the start of the L2 header for every packet-socket type
regardless of its length: SOCK_RAW and SOCK_PACKET carry a user-supplied
header and SOCK_DGRAM has one built by dev_hard_header(). Reset the MAC
header unconditionally, mirroring __dev_queue_xmit(), so the frame is
anchored on the bypass path too.

Found by 0sec ( https://0sec.ai ) using automated source analysis;
verified against source and matched to the macsec KASAN report in
f508900 . Compile-tested.

Fixes: 75c6577 ("net/packet: Ask driver for protocol if not provided by user")
Cc: stable@vger.kernel.org
Signed-off-by: Doruk Tan Ozturk <doruk@0sec.ai>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20260724144015.63219-1-doruk@0sec.ai
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/packet/af_packet.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=1e43a1d66615f411d427f9df1f46dd049d9e3681

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

The original Linux kernel CVE announcement for CVE-2026-74667 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74667

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:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 5.8

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: NO
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).