[CVE-2026-74581][MODERATE 7.0] net: ipv6: clear suppressed fib6 rule result

[CVE-2026-74581][MODERATE 7.0] net: ipv6: clear suppressed fib6 rule result

[AL-KERNEL]

CVE: CVE-2026-74581
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: net: ipv6: clear suppressed fib6 rule result
Commit: 90c57310e266eb94e4a80d6b15a9ca131d2e82cb
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=90c57310e266eb94e4a80d6b15a9ca131d2e82cb
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74581
Analysis date: Fri, 21 Aug 2026 13:03:09 -0400
ActionableScore: 8
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A stale pointer in IPv6 FIB rule suppression can return a released `rt6_info` / `dst` after `ip6_rt_put_flags()`, allowing network-triggered route lookups under suppress-rule configurations to reach `dst_release()` on an expired object and cause at least a kernel DoS, with UAF-class impact requiring manual review.

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

The original Linux kernel CVE announcement for CVE-2026-74581 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74581

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-2026-74581	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	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-672;CWE-825;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '8.1';DESCR 'A stale pointer bug in IPv6 fib rule suppression can leave res rt6 pointing to a released rt6_info after ip6_rt_put_flags. If no later rule replaces the result, fib6_rule_lookup can return the released dst to rt6_lookup and later dst_release may operate on an expired object. For the CVSS the PR:N is used because a remote attacker may only need network reachability to trigger IPv6 route lookups when a suppressing IPv6 rule is already configured on the target. AC:H is used because reliable triggering depends on non default policy routing with suppress rules and a lookup path where no replacement route is selected. Impact is at least denial of service via kernel crash. In the paranoid interpretation, the stale dst pointer is a use-after-free candidate in the network stack and may allow confidentiality or integrity impact if freed memory is reused in an attacker influenced way.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 8)	YES	REMOTE SIMPLEFIX LEAK NETWORK IPV6 KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH	YES	NO	checked

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

ActionableScore=8
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: **6**
* Paranoid score: **8**
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* **Remote reachable / network-triggerable: +2**
  IPv6 route lookups can be triggered by network traffic if the target already has a suppressing IPv6 FIB rule configuration.

* **Memory corruption, strong corruption primitive: +2**
  The patch describes a released `rt6_info` / `dst` object being left in `res->rt6` and returned to callers. This is a stale pointer / UAF-class lifetime bug, not just a warning or validation issue.

* **Real lifetime corruption: +1**
  `ip6_rt_put_flags()` releases the route, but the result structure still retains the pointer.

* **Reliable kernel crash / strong DoS: +1**
  The commit explicitly says the next put reaches `rcuref_put_slowpath()` from `dst_release()`, indicating a concrete crash/refcount failure path.

* **Common networking core path: +1**
  IPv6 FIB/rule lookup is a core networking routing path.

* **Special configuration / triggering constraints: -1**
  Reliable triggering depends on a non-default IPv6 policy routing setup with suppressing rules and no later rule replacing the result.

Conservative total: **6**

Paranoid additions / adjustments:

* **Weak LPE concern: +1**
  The stale released route object is returned into a central network object lifetime path. No attacker-controlled reclaim is demonstrated, so this is weak, but manual review is justified.

* **Do not apply the special-configuration subtraction in worst case: +1 effective**
  In environments where such suppressing IPv6 rules are already deployed, a remote attacker may only need to cause lookups.

Paranoid total: **8**

## 3. Reachability analysis

The vulnerable path is in IPv6 FIB rule suppression. Configuring the relevant suppressing rule normally requires `CAP_NET_ADMIN`, so an attacker usually cannot create the vulnerable configuration from scratch. However, once such policy routing is present, IPv6 route lookups can be exercised by traffic, making the bug potentially network-triggerable in affected deployments.

Namespaces can matter: a container or network namespace with delegated `CAP_NET_ADMIN` may be able to create similar rules inside that namespace, but host-wide impact depends on which namespace and routing context are affected. The path is not a public protocol parser by itself, but it is in a core IPv6 routing lookup path.

Realistic exploitation requires a specific routing policy state, so this is not a universal remote bug. Still, the affected object is a released `rt6_info` / `dst` returned to later code, which makes this more serious than ordinary DoS-only routing misbehavior.

## 4. Severity interpretation

This should not be treated as an ordinary Moderate auto-close issue. The demonstrated consequence is at least a kernel crash through a stale released route object. The realistic impact is likely DoS in many deployments, but the primitive is a real lifetime/UAF-class bug in networking core code.

Theoretical privilege escalation is not proven: there is no demonstrated attacker-controlled reclaim, object replacement, type confusion, callback control, or arbitrary write. However, because the stale pointer is returned to callers in a central network lifetime path, the paranoid score keeps it in **Strong Important candidate** territory for manual review.

## 5. One-sentence report phrase

A stale pointer in IPv6 FIB rule suppression can return a released `rt6_info` / `dst` after `ip6_rt_put_flags()`, allowing network-triggered route lookups under suppress-rule configurations to reach `dst_release()` on an expired object and cause at least a kernel DoS, with UAF-class impact requiring manual review.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**

Reason: this is a real stale pointer / UAF-class lifetime bug in a common IPv6 routing path, with potential network-triggerability under existing suppress-rule configurations. The demonstrated impact is DoS, but the object lifetime primitive is strong enough that it should not be auto-closed without manual kernel security review.

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

Patch: net: ipv6: clear suppressed fib6 rule result
Commit: 90c57310e266eb94e4a80d6b15a9ca131d2e82cb
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=90c57310e266eb94e4a80d6b15a9ca131d2e82cb

Commit description:

fib6_rule_suppress() drops a suppressed route with ip6_rt_put_flags(),
but leaves res->rt6 pointing at the released rt6_info.

If no later rule supplies a replacement, fib6_rule_lookup() still sees
res.rt6 and returns that stale dst to its caller. A suppressing rule can
therefore leak a released route back to rt6_lookup(), and the next put
hits rcuref_put_slowpath() from dst_release().

Clear res->rt6 when suppressing the route so suppressed lookups fall
through to the null dst instead of reusing the released one.

Fixes: cdef485 ("ipv6: fix memory leak in fib6_rule_suppress")
Cc: stable@vger.kernel.org
Reported-by: Vega <vega@nebusec.ai>
Signed-off-by: Zhiling Zou <zhilinz@nebusec.ai>
Signed-off-by: Ren Wei <enjou1224z@gmail.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/4b8acb7787d54e440155585dd32ebdf0bef7d122.1784710966.git.zhilinz@nebusec.ai
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/ipv6/fib6_rules.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=90c57310e266eb94e4a80d6b15a9ca131d2e82cb

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

The original Linux kernel CVE announcement for CVE-2026-74581 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74581

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: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: 6
  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: 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).