[CVE-2026-72253][MODERATE 7.0] netfilter: nf_conntrack_sip: validate skb_dst() before accessing it [ Upstream

[CVE-2026-72253][MODERATE 7.0] netfilter: nf_conntrack_sip: validate skb_dst() before accessing it [ Upstream

[AL-KERNEL]

CVE: CVE-2026-72253
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: netfilter: nf_conntrack_sip: validate skb_dst() before accessing it [ Upstream
Commit: e64a48c50a1ff565a98c6a98d82b5b942868e76e
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e64a48c50a1ff565a98c6a98d82b5b942868e76e
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72253
Analysis date: Sun, 16 Aug 2026 08:38:49 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: NO

Summary:
A NULL pointer dereference in `nf_conntrack_sip` can be triggered by network SIP traffic through tc ingress or Open vSwitch conntrack helper paths when `sip_external_media` is enabled, causing a kernel crash DoS without evidence of privilege escalation.

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

The original Linux kernel CVE announcement for CVE-2026-72253 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72253

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-2026-72253	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H';**CWE-476;*CWE-20;CWE-754;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7.5';DESCR 'A NULL pointer dereference in nf_conntrack_sip can occur because the SIP helper may access skb_dst(skb) even when tc ingress or openvswitch conntrack paths provide an skb without routing destination information. A remote sender may trigger the crash by sending SIP traffic through an affected tc or OVS conntrack helper path when sip_external_media is enabled. For the CVSS the PR:N value is used because the attacker does not need privileges on the target once the vulnerable helper configuration is exposed to network traffic. The base score uses AC:H because exploitation depends on a non-default module parameter and a specific packet processing path. The paranoid score uses AC:L for deployments where this exact helper path is already configured and reachable. Impact is denial of service via kernel crash. No concrete UAF, OOB write, information leak, or privilege escalation primitive is indicated by the code change.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	REMOTE DANGER SIMPLEFIX NETFILTER SKB KPANIC  KPANIC DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN6	NO	NO	checked

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

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

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

## 2. Signal breakdown

Triggered signals:

* Remote reachable / network-triggerable: +2
  A remote sender can supply SIP traffic that reaches the conntrack SIP helper through tc ingress or Open vSwitch ct helper paths.

* Reliable kernel crash / strong DoS: +1
  The old code dereferenced `skb_dst(skb)->dev` without validating that `skb_dst(skb)` exists. In the affected paths this can produce a NULL pointer dereference and kernel crash.

* Common networking core path or packet-processing path: +1
  The issue is in netfilter conntrack helper processing, reached from packet-processing paths such as OVS and tc action ct.

* Availability impact realistic: +1 in paranoid score only
  If the affected helper path is already configured and reachable, a crafted packet stream can plausibly cause host-wide kernel crash impact.

Negative signals:

* Non-default / configuration-dependent exposure: -1
  `sip_external_media` is disabled by default, and the crash requires a specific deployment using the SIP conntrack helper through tc ingress or Open vSwitch ct action.

Not triggered:

* No generic memory corruption score
  This is a NULL pointer dereference, not UAF, OOB write, double-free, type confusion, or refcount takeover.

* No privilege escalation score
  The patch does not show a reclaim primitive, writable stale object, callback control, arbitrary write, or other LPE-relevant primitive.

* No confidentiality or integrity score
  The observed effect is crash/DoS only.

## 3. Reachability analysis

The attacker can be remote if SIP traffic from the attacker is processed by an affected tc ingress or Open vSwitch conntrack helper path and `sip_external_media=1` is enabled. No privileges are needed on the target host once this network-facing configuration exists.

In typical deployments, this is not default exposure because `sip_external_media` is disabled by default and OVS/tc conntrack helper configuration is administrator-controlled. Namespaces or containers may matter if a tenant can influence OVS or tc rules, but normally this requires privileged network administration.

Realistic exploitation is therefore configuration-dependent but credible in exposed firewall, NAT, OVS, or service-provider networking setups using SIP helper functionality.

## 4. Severity interpretation

This behaves as an actionable Moderate network DoS issue, not an Important memory corruption vulnerability.

The theoretical impact is limited by the bug primitive: a NULL pointer dereference caused by missing validation of `skb_dst()`. There is no evidence of UAF, stale object reuse, overwrite, info leak, or privilege escalation. However, because the bug is network-triggerable in affected configurations and sits in packet-processing code, it should not be auto-closed as a trivial local-only crash.

## 5. One-sentence report phrase

A NULL pointer dereference in `nf_conntrack_sip` can be triggered by network SIP traffic through tc ingress or Open vSwitch conntrack helper paths when `sip_external_media` is enabled, causing a kernel crash DoS without evidence of privilege escalation.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended because the trigger is network reachable in affected configurations and the prior CVSS-style assessment reaches a high DoS score, but the primitive is still a NULL dereference with no demonstrated confidentiality, integrity, or LPE path.

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

Patch: netfilter: nf_conntrack_sip: validate skb_dst() before accessing it [ Upstream
Commit: e64a48c50a1ff565a98c6a98d82b5b942868e76e
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e64a48c50a1ff565a98c6a98d82b5b942868e76e

Changed files:
  net/netfilter/nf_conntrack_sip.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=e64a48c50a1ff565a98c6a98d82b5b942868e76e

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

The original Linux kernel CVE announcement for CVE-2026-72253 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72253

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