[CVE-2026-72427][MODERATE 7.0] bpf: Fix effective prog array index with BPF_F_PREORDER [ Upstream

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

CVE: CVE-2026-72427
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: bpf: Fix effective prog array index with BPF_F_PREORDER [ Upstream
Commit: 525e408c27ae714e538b8c608c3a974df3ab6c92
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=525e408c27ae714e538b8c608c3a974df3ab6c92
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72427
Analysis date: Sat, 15 Aug 2026 21:08:13 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A cgroup BPF effective program array index mismatch with BPF_F_PREORDER can leave a detached bpf_prog referenced after bpf_prog_put(), creating a local privileged UAF that can crash the kernel and should be reviewed for possible privilege escalation.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-72427 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72427

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-2026-72427	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-682;*CWE-672;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:H/I:H/A:H';BEST CVSS score: '7';DESCR 'A use-after-free can occur in the BPF cgroup effective program array when BPF_F_PREORDER programs are present because replace and purge logic used a linear hlist position that does not match the computed effective array layout. During detach, purge_effective_progs can dummy out the wrong slot and leave the detached bpf_prog in the effective array after bpf_prog_put drops its reference. Later cgroup BPF hook execution can dereference a stale program pointer, causing a kernel crash and potentially enabling privilege escalation through kernel memory corruption. For the CVSS the PR:L is used for the paranoid score because reliable triggering requires local ability to attach, update, or detach cgroup BPF programs, but this capability may be delegated to container root, service accounts, or reduced capability administrators rather than full host root. The issue is not directly network reachable, although network traffic or other cgroup hook activity may exercise the stale effective program after local setup. Impact is at least local denial of service and in worst case may allow confidentiality and integrity impact due to UAF.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	INIT BPF UAF LINUS  KPANIC INCREASED_TO_MODERATE7_FROM_MODERATE_BASED_ON_GUESSCVSS KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN6 DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=6
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 6
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative scoring:

* +2 Generic memory corruption: explicit use-after-free is described. `purge_effective_progs()` may leave a detached `bpf_prog` in the effective array after `bpf_prog_put()` drops its reference.
* +1 Real lifetime corruption: stale pointer after refcount drop/free in a live cgroup BPF effective program array.
* +1 Reliable kernel crash / strong DoS: later cgroup BPF hook execution can dereference the stale program pointer.
* +1 Weak LPE concern: the stale object is a `bpf_prog` reachable from an execution path, but no controlled reclaim or object replacement is demonstrated.
* +1 Broad/security-sensitive subsystem: cgroup BPF hooks are commonly used in container, service, and networking policy environments.
* -2 Requires admin/root/CAP_* in typical deployments: attaching/updating/detaching cgroup BPF programs normally requires privileged BPF/cgroup capabilities.

Conservative total: 5.

Paranoid scoring:

* Same as conservative, but privilege penalty is reduced from -2 to -1 where cgroup BPF control is delegated to container root, service accounts, or reduced-capability administrators.
* +1 Confidentiality impact plausible in worst case due to UAF in a kernel execution path.
* +1 Integrity impact plausible in worst case due to possible corruption or stale executable object use.
* Capped at 6 by the BPF privilege gating rule because unprivileged BPF is typically disabled and no controlled reclaim, arbitrary write, or working LPE is shown.

Paranoid total: 6.

## 3. Reachability analysis

The bug is locally reachable through cgroup BPF attach, update, or detach operations involving `BPF_F_PREORDER`. In typical hardened product configurations this requires privileged capabilities such as `CAP_BPF`, `CAP_SYS_ADMIN`, `CAP_NET_ADMIN`, or equivalent root-controlled setup, so it is not a normal unprivileged local trigger.

Namespace or container delegation matters. If a container root, service account, or reduced-capability administrator can manage cgroup BPF programs, practical privilege level may be closer to PR:L than PR:H. The issue is not directly network reachable, but network or other cgroup hook activity can exercise the stale effective program after local setup.

Call-site confidence: high. The patch includes the affected replace and purge paths, the wrong effective array slot computation, and the `bpf_prog_put()` lifetime consequence.

## 4. Severity interpretation

This is not an ordinary Moderate cleanup issue. The patch explicitly fixes a stale `bpf_prog` lifetime bug that can become a UAF in a live cgroup BPF hook path. Realistic exploitation is constrained by privileged BPF access and the need for a specific preorder/cgroup hierarchy configuration, so the conservative result is Actionable Moderate.

Theoretical impact includes privilege escalation because the stale object is used from a kernel execution path. However, the patch does not show controlled reclaim, arbitrary write, or a demonstrated LPE chain, so the paranoid score is capped at 6 rather than treated as a full Important score automatically.

## 5. One-sentence report phrase

A cgroup BPF effective program array index mismatch with BPF_F_PREORDER can leave a detached bpf_prog referenced after bpf_prog_put(), creating a local privileged UAF that can crash the kernel and should be reviewed for possible privilege escalation.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED.
YES REQUIRES MANUAL CHECK.

Reason: explicit UAF in BPF/cgroup program lifetime handling, with potential execution-path reuse of a stale `bpf_prog` pointer, even though practical triggering is usually gated by privileged BPF access.

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

Patch: bpf: Fix effective prog array index with BPF_F_PREORDER [ Upstream
Commit: 525e408c27ae714e538b8c608c3a974df3ab6c92
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=525e408c27ae714e538b8c608c3a974df3ab6c92

Changed files:
  kernel/bpf/cgroup.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=525e408c27ae714e538b8c608c3a974df3ab6c92

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

The original Linux kernel CVE announcement for CVE-2026-72427 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72427

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:H/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7

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: 6

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).