[CVE-2026-97420][MODERATE REGULAR] bpf: NUL-terminate replaced sysctl value [ Upstream

[CVE-2026-97420][MODERATE REGULAR] bpf: NUL-terminate replaced sysctl value [ Upstream

[AL-KERNEL]

CVE: CVE-2026-97420
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: bpf: NUL-terminate replaced sysctl value [ Upstream
Commit: 76c14b10f50eb0afcc76e244562fa2d8c661e224
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=76c14b10f50eb0afcc76e244562fa2d8c661e224
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97420
Analysis date: Thu, 24 Sep 2026 21:16:24 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-97420 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97420

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: NO

A detailed explanation of the methodology and priority rules is included
at the end of this message.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-97420	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-97420 CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:H';CWE-170;CWE-125;*CWE-20;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '6.1';DESCR 'A missing NUL terminator in bpf_sysctl_set_new_value can cause a replacement sysctl value to be passed to a proc handler without the string termination expected by sysctl parsers. A local attacker able to attach a cgroup sysctl BPF program can provide a replacement value that causes a downstream parser to read beyond the allocated buffer. The reported KASAN trace shows a slab out of bounds read in strnchrnul through bitmap_parse and flow_limit_cpu_sysctl. For the CVSS the PR:L value is used for the paranoid score because delegated BPF or container environments can provide limited local BPF access without full host administration. Impact is at least a local denial of service if the invalid read faults in kernel context. Limited confidentiality impact is possible in the worst case due to the out of bounds read, but no information disclosure or write primitive is demonstrated by the patch. The issue is not network reachable.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2)	YES	READ KASAN OOB BPF HARDWARE LINUS MEMORY  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4	NO	NO	checked

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

ActionableScore=2
ActionableScoreLower=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 1
* Conservative score: 1
* Raw paranoid score: 2
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate**::

No floor or cap was applied.

2. Signal breakdown

* +1 Concrete invalid kernel memory access. The patch and KASAN report directly establish a one-byte slab out-of-bounds read in `strnchrnul()` after a cgroup/sysctl BPF program replaces a sysctl value without a NUL terminator.
* +1 Paranoid only. The invalid read occurs in kernel sysctl parsing and can plausibly produce a kernel fault or denial of service outside KASAN configurations if the read crosses into inaccessible memory. This is not a demonstrated reliable crash.

Important non-awarded signals:

* Controlled reclaim is not demonstrated.
* Arbitrary or directed write is not demonstrated.
* There is no UAF, double free, refcount corruption, or stale-object lifetime violation.
* Confidentiality impact is not awarded. The patch demonstrates an internal out-of-bounds read, but no bytes are returned to userspace, network peers, BPF programs, or another attacker-visible channel.
* Integrity impact is not awarded. The patch does not demonstrate an out-of-bounds write or a parser behavior that modifies attacker-unselected kernel state.
* Reliable kernel crash is not awarded. The supplied evidence shows a KASAN report, not a production-kernel panic or a deterministic fault.
* Local unprivileged trigger is not awarded. The complete privilege requirements for loading and attaching a cgroup/sysctl BPF program are not established as reachable by an ordinary local user.
* Local unprivileged high-control BPF API is not awarded. The BPF interface provides high control over the replacement string only after the attacker can load and attach the relevant BPF program, which is privilege-gated in common configurations.
* Remote reachability is not established.
* No privilege escalation or security-boundary bypass is established.

3. Reachability analysis

The immediate trigger requires a process that can load and attach a `BPF_PROG_TYPE_CGROUP_SYSCTL` program and invoke `bpf_sysctl_set_new_value()` while handling a write to a relevant sysctl. The supplied reproducer runs a cgroup/sysctl BPF program and writes to `/proc/sys/net/core/flow_limit_cpu_bitmap`.

The complete privilege boundary is unresolved from the supplied patch. In common hardened Linux configurations, BPF program loading and cgroup attachment are privilege-gated through capabilities such as `CAP_BPF`, `CAP_SYS_ADMIN`, `CAP_NET_ADMIN`, or distribution-specific BPF policy. However, the patch does not establish that full host-admin-equivalent privilege is always required, so `HostAdminRequired=UNKNOWN` is appropriate.

Namespace-scoped reachability is also unresolved. A cgroup or network namespace alone does not establish that an attacker can load the program, attach it to the required cgroup, write the target sysctl, and reach the vulnerable parser entirely below host administration. Therefore `NamespaceScopedPrivilege=UNKNOWN`.

There is no network trigger. The vulnerable operation is a local proc sysctl write processed through a BPF control-plane hook. No special hardware, race condition, firmware interaction, or prior device setup is required beyond the BPF and cgroup/sysctl configuration.

4. Primitive and severity interpretation

The exact primitive is a missing string terminator leading to a demonstrated slab out-of-bounds read. `bpf_sysctl_set_new_value()` copies `buf_len` bytes into `ctx->new_val` but previously did not write `ctx->new_val[buf_len] = '\0'`. A downstream sysctl parser can then scan beyond the allocated replacement buffer while searching for a delimiter or terminator.

This is not an active-object lifetime bug, fixed stale storage bug, UAF, refcount bug, or repeated release case. The parser performs an invalid read, and the supplied KASAN report demonstrates that the read reaches exactly one byte beyond a 4096-byte allocation.

The appropriate classification is `CLASS_C`: demonstrated invalid-state and out-of-bounds-read behavior with potential denial-of-service consequences, but without a useful corruption, disclosure, ownership, authorization, or control-flow primitive.

The demonstrated consequence is a KASAN slab-out-of-bounds report. A production denial of service is plausible but not directly demonstrated by the supplied evidence. Confidentiality, integrity, arbitrary kernel read, arbitrary kernel write, controlled data disclosure, and privilege escalation remain speculative and are not used for scoring.

`ImportantCandidate=NO` because this is a limited read-only OOB access in a privilege-gated BPF control-plane path, with no demonstrated attacker-visible disclosure, write primitive, controlled reuse, authorization bypass, or lifetime corruption. `AutoDowngradeAllowed=YES` because the remaining unresolved question is primarily practical reachability and crash behavior, not a concrete Important-preserving corruption or boundary-bypass primitive.

Likely operational severity is Low to ordinary Moderate. It should be reviewed for product-specific BPF delegation policy, but the patch evidence alone does not support Important classification.

5. One-sentence report phrase

A cgroup sysctl BPF program can provide a replacement sysctl value without a NUL terminator, causing downstream kernel sysctl parsers to read beyond the replacement buffer and potentially trigger a local denial of service.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Verify whether untrusted users, containers, or service accounts can load and attach cgroup/sysctl BPF programs and can write the affected sysctl paths in the evaluated product. If these capabilities remain host-admin-only, this issue can generally be handled as a low-priority local robustness defect.

7. Call-site confidence

Call-site confidence: medium

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

Patch: bpf: NUL-terminate replaced sysctl value [ Upstream
Commit: 76c14b10f50eb0afcc76e244562fa2d8c661e224
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=76c14b10f50eb0afcc76e244562fa2d8c661e224

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=76c14b10f50eb0afcc76e244562fa2d8c661e224

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

The original Linux kernel CVE announcement for CVE-2026-97420 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97420

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:L/PR:H/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:H
  The Best / paranoid CVSS score: 6.1

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: 1
  Paranoid ActionableScore: 2

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).