[CVE-2026-72251][IMPORTANT] netfilter: nf_nat_sip: reload possible stale data pointer

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

CVE: CVE-2026-72251
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: netfilter: nf_nat_sip: reload possible stale data pointer
Commit: bded21a4bf9bf86a79148be735723a97ca9a7532
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bded21a4bf9bf86a79148be735723a97ca9a7532
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72251
Analysis date: Sun, 16 Aug 2026 07:57:22 -0400
ActionableScore: 8
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A stale data pointer in the netfilter SIP NAT helper can remain after `skb_ensure_writable()` reallocates cloned SKB data, allowing crafted SIP traffic to potentially trigger use-after-free-like access, packet header corruption, or kernel DoS when the SIP NAT helper path is enabled.

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

The original Linux kernel CVE announcement for CVE-2026-72251 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72251

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-2026-72251	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H';*CWE-416;*CWE-664;CWE-703;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '8.1';DESCR 'A stale data pointer bug in the netfilter SIP NAT helper can occur when skb_ensure_writable reallocates or replaces skb data for a cloned packet while nf_nat_sip keeps using the old dptr value. The old buffer may be freed and later SIP parsing or mangling may access stale packet data, making this a use after free candidate rather than a simple warning. A remote attacker can potentially trigger the path with crafted SIP traffic when the SIP conntrack and NAT helper is enabled and the traffic reaches the forced_dport handling path. For the CVSS the PR:N because no local account is required if the affected firewall or NAT system processes attacker controlled SIP packets. AC:H is used because exploitation depends on non default helper configuration, SIP specific state, and skb clone or rewrite conditions. Impact is at least remote denial of service and packet header corruption, and in the paranoid case may include confidentiality or integrity impact due to stale pointer memory access.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	REMOTE DANGER NETFILTER SKB LINUS PACKET  KPANIC INCREASED_TO_MODERATE_FROM_LOW_BASED_ON_GUESSCVSS KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN6	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=8
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 8
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2. The affected path processes SIP traffic in the netfilter SIP NAT helper when the helper is enabled and traffic reaches this NAT handling path.
* Memory corruption, strong corruption primitive: +2. The patch context explicitly describes a stale data pointer after `skb_ensure_writable()` may reallocate cloned SKB data and free the old buffer.
* Real lifetime corruption: +1. This is a stale pointer after possible free/reallocation of SKB data.
* Availability impact realistic: +1. A stale pointer in packet mangling/parsing can plausibly crash the kernel or corrupt packet processing state.
* Common networking / packet-processing path: +1. This is in netfilter packet processing, though specifically in the SIP helper.
* Hard or conditional trigger: -1. Triggering depends on `forced_dport`, SIP helper state, cloned SKB behavior, and the relevant packet direction.
* Rare / non-default helper path: -1. SIP conntrack/NAT helpers are not universally enabled by default and often require explicit firewall/NAT configuration.

Paranoid additional signals:

* Weak LPE concern: +1. The stale pointer is in kernel networking memory and could theoretically become more than DoS if reuse is attacker-influenced, but no controlled reclaim or write primitive is demonstrated.
* Confidentiality impact plausible: +1. Stale freed SKB data access could theoretically expose or mix stale kernel/network data, but this is not proven.
* Integrity impact plausible: +1. The commit explicitly mentions possible header corruption, and stale data use in NAT mangling can affect packet integrity.

## 3. Reachability analysis

A remote peer can potentially trigger this if it can send SIP traffic through an affected firewall/NAT system where `nf_conntrack_sip` and `nf_nat_sip` are enabled and the traffic reaches the `forced_dport` handling path. No local account on the target is required in that network-facing deployment, so PR is effectively none for remote packet processing. However, realistic triggering is configuration-dependent because SIP helpers are not always enabled by default, and the vulnerable branch appears to require specific SIP state plus cloned SKB behavior. Namespaces and containers are not the primary issue unless they control netfilter helper configuration or can route crafted SIP traffic through the host NAT path.

Call-site confidence: medium-high. The patch directly shows the stale `dptr` repair after `skb_ensure_writable()`, but full downstream use of `dptr` is not included in the supplied snippet.

## 4. Severity interpretation

This is stronger than an ordinary Moderate because it is a network packet-processing stale pointer bug in netfilter with explicit old-buffer-free semantics. The conservative interpretation is Actionable Moderate because exploitation depends on a non-default helper path and specific SKB conditions. The paranoid interpretation is a Strong Important candidate because the primitive is a UAF-like stale pointer in a remotely influenced packet path, with plausible crash and packet corruption impact, even though no reliable privilege escalation primitive is demonstrated.

Theoretical memory corruption potential is real. Realistic exploitation evidence is limited, so the score should be used to force manual review rather than to assume reliable remote code execution.

## 5. One-sentence report phrase

A stale data pointer in the netfilter SIP NAT helper can remain after `skb_ensure_writable()` reallocates cloned SKB data, allowing crafted SIP traffic to potentially trigger use-after-free-like access, packet header corruption, or kernel DoS when the SIP NAT helper path is enabled.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED.

Reason: network-reachable netfilter packet path plus stale freed SKB data pointer is a kernel lifetime-corruption class that should not be auto-closed, even though the practical trigger depends on SIP helper configuration and specific SKB conditions.

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

Patch: netfilter: nf_nat_sip: reload possible stale data pointer
Commit: bded21a4bf9bf86a79148be735723a97ca9a7532
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bded21a4bf9bf86a79148be735723a97ca9a7532

Commit description:

quoting sashiko:
 ------------------------------------------------------------------------
 [..] noticed a potential memory bug and header corruption involving the
 SIP NAT helper.

 In net/netfilter/nf_nat_sip.c:nf_nat_sip():
	if (skb_ensure_writable(skb, skb->len)) {
		nf_ct_helper_log(skb, ct, "cannot mangle packet");
		return NF_DROP;
	}
	uh = (void *)skb->data + protoff;
	uh->dest = ct_sip_info->forced_dport;
	if (!nf_nat_mangle_udp_packet(skb, ct, ctinfo, protoff,
				      0, 0, NULL, 0)) {

 If a cloned or fragmented SKB is reallocated by skb_ensure_writable(), the
 old data buffer is freed. However, nf_nat_sip() fails to update *dptr to
 point to the new buffer.

 It also appears to use nf_nat_mangle_udp_packet() on what could be a TCP
 packet, which would overwrite the sequence number with a checksum update.
 ------------------------------------------------------------------------

nf_conntrack_sip linerizes skbs, hence no fragmented skb can be seen.
But clones are possible, so rebuild dptr.

Disable nf_nat_mangle_udp_packet() branch for TCP streams.
It doesn't look like this can ever happen, else we should have received
bug reports about this, so just check the conntrack is UDP and drop
otherwise.

The calling conntrack_sip set ->forced_dport for SIP_HDR_VIA_UDP messages,
so I don't think this is ever expected to be true for a TCP stream.

Fixes: 7266507 ("netfilter: nf_ct_sip: support Cisco 7941/7945 IP phones")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-sonnet-4-6
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/netfilter/nf_nat_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=bded21a4bf9bf86a79148be735723a97ca9a7532

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

The original Linux kernel CVE announcement for CVE-2026-72251 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72251

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:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 8.1

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: 8

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).