[CVE-2026-100070][MODERATE REGULAR] netfilter: nf_nat_sip: rewind offset when NAT shrinks the packet [ Upstream

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

CVE: CVE-2026-100070
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: netfilter: nf_nat_sip: rewind offset when NAT shrinks the packet [ Upstream
Commit: 6828aca3d82717c2fda92af81c0dda642bc2b465
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6828aca3d82717c2fda92af81c0dda642bc2b465
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-100070
Analysis date: Fri, 25 Sep 2026 10:22:03 -0400
ActionableScore: 3
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-100070 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-100070

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 REGULAR
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-2026-100070	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-100070 CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N';*CWE-200;CWE-682;CWE-703;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N';BEST CVSS score: '5.3';DESCR 'A logic flaw in the SIP NAT helper can leave the Contact header parsing offset stale after NAT changes packet length. If address translation shortens a SIP packet, parsing of a subsequent Contact header can skip bytes and fail to translate that header. A remote SIP peer may receive an untranslated internal IP address or port when it can cause a NATed SIP message with multiple Contact headers to be processed. For the CVSS the PR:N value is used because no credentials or local access are required once the affected SIP NAT path is reachable. Exploitation requires the SIP conntrack NAT helper, a suitable NAT topology, and a packet where address translation shrinks the payload. The issue is network reachable in deployments exposing SIP traffic through this helper. Impact is limited to confidentiality through internal network address disclosure. No memory corruption, privilege escalation, or denial of service primitive is shown by the patch.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3)	YES	REMOTE SIMPLEFIX NETFILTER LEAK SKB PACKET  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4	NO	NO	checked

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

ActionableScore=3
PrimitiveClass=NONE
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=REQUIRED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 3
* Conservative score: 3
* Raw paranoid score: 3
* Paranoid score: 3
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

* +2 Remote reachable packet-processing path. An unauthenticated remote SIP peer can potentially reach the vulnerable helper through normal SIP traffic when the target actively uses the SIP conntrack NAT helper and the relevant NAT direction.
* +1 Confidentiality plausible. A subsequent Contact header can remain untranslated and disclose an internal SIP address or port to the remote peer.
* +0 Memory corruption. No UAF, OOB access, invalid lifetime transition, arbitrary write, or stale active object is established.
* +0 Integrity impact. Failure to rewrite a Contact header does not itself modify protected data or policy.
* +0 Availability impact. The patch shows parser offset misalignment and missed translation, not a crash, resource exhaustion, stall, or packet-processing failure affecting host availability.
* +0 Practical exploit-chain bonus. The patch plausibly establishes address disclosure, but no supplied evidence demonstrates a broader attack chain using the disclosed address.
* No negative niche-subsystem penalty. SIP NAT helper deployment is conditional, but an attacker can reach it through ordinary SIP traffic when it is enabled and exposed. The evidence does not establish that the relevant deployment is rare and unreachable.

3. Reachability analysis

The evaluated attacker is a remote SIP peer, not a local user or host administrator. The attacker does not need local credentials, CAP_NET_ADMIN, or host administrative access to transmit SIP packets to an already deployed SIP service behind NAT.

The affected path requires prior target-side deployment conditions. The nf_conntrack_sip and nf_nat_sip helper path must be enabled and associated with the relevant SIP traffic. The NAT translation must shorten a processed address representation, and the SIP message must contain a later Contact header whose parsing is affected by the stale offset. These conditions affect practical exposure and CVSS Attack Complexity, but do not require attacker-side privileges.

Namespace-scoped privilege is not relevant to the remote trigger and is unresolved for target-side helper configuration. The complete attack does not require special hardware, a race, boot configuration, or attacker-controlled local setup. SIP NAT helpers are more commonly deployed on enterprise voice gateways, SBC-adjacent systems, firewalls, and private VoIP infrastructure than on general Internet-facing hosts.

4. Primitive and severity interpretation

The exact primitive is a parser-position logic error. After map_addr changes skb length, coff remains relative to the old packet layout. If NAT shrinks the packet, the next Contact-header scan can start too far forward and silently skip a later Contact header.

This is not an active-object lifetime issue, UAF, stale pointer dereference, refcount mutation, buffer overflow, or semantic reinterpretation of active kernel memory. No repeated release or refcount operation exists. The patch does not establish kernel memory corruption, controlled data modification, privilege escalation, or denial of service.

The demonstrated practical consequence is limited disclosure of internal network addressing information when an untranslated Contact header is forwarded to a remote SIP peer. Such information can assist network reconnaissance, but the supplied patch does not establish protected-resource access, authentication bypass, integrity compromise, or a follow-on exploitation primitive.

PrimitiveClass is NONE because the flaw establishes an information-disclosure logic defect without a kernel corruption, ownership, lifetime, authorization, or boundary-bypass primitive. ImportantCandidate is NO because the patch-supported impact remains limited to address disclosure. AutoDowngradeAllowed is YES because there is no unresolved UAF, memory corruption, repeated lifetime mutation, authorization bypass, or stronger security boundary primitive. Manual review is still REQUIRED to determine whether the target actually runs and exposes the SIP NAT helper.

5. One-sentence report phrase

A remote SIP peer may cause the SIP NAT helper to skip translation of a later Contact header after packet length reduction, potentially disclosing an internal IP address or port when SIP NAT processing is enabled.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Verify whether nf_conntrack_sip and nf_nat_sip are enabled, whether SIP traffic is exposed to untrusted peers, and whether the affected NAT direction can rewrite a longer internal address into a shorter external address.

7. Call-site confidence

Call-site confidence: medium

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

Patch: netfilter: nf_nat_sip: rewind offset when NAT shrinks the packet [ Upstream
Commit: 6828aca3d82717c2fda92af81c0dda642bc2b465
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6828aca3d82717c2fda92af81c0dda642bc2b465

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=6828aca3d82717c2fda92af81c0dda642bc2b465

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

The original Linux kernel CVE announcement for CVE-2026-100070 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-100070

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:N/A:N
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
  The Best / paranoid CVSS score: 5.3

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: unknown
  Paranoid ActionableScore: 3

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 REGULAR

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).