[CVE-2026-98299][MODERATE REGULAR] tcp: do not let tcp_rmem be set below 4096 [ Upstream

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

CVE: CVE-2026-98299
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: tcp: do not let tcp_rmem be set below 4096 [ Upstream
Commit: 879907631b9e70eedb62d76461b29afa106ebe7a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=879907631b9e70eedb62d76461b29afa106ebe7a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98299
Analysis date: Sat, 10 Oct 2026 23:22:04 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
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-2026-98299 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98299

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-2026-98299	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98299 CHECK WITH IMPACT FROM ORIG NN IMPORTANT | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 LEGACY=MODERATE RESULT=DIFF (independent Legacy and Aegis differ; FINAL: MODERATE, Aegis: MODERATE_KPANIC)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';CWE-369;*CWE-20;CWE-1284;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.9';DESCR 'A divide by zero can occur in tcp_rcvbuf_grow or tcp_rcv_space_adjust when tcp_rmem is configured below a safe minimum and tp rcvq_space space is initialized to zero. After this unsafe configuration is present, incoming TCP data can reach dynamic receive buffer adjustment and cause a kernel exception or panic. For the CVSS the PR:N value is used for the paranoid scenario because a remote peer needs no account or privileges on the target after an administrator or deployment has already configured tcp_rmem below 4096. The paranoid attack vector is AV:N with AC:H because successful triggering depends on this uncommon preexisting configuration. Impact is denial of service through a kernel crash. No UAF, out of bounds access, information disclosure, or write primitive is supported by the analyzed code path, so confidentiality and integrity remain unaffected.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2)	MAYBE	REMOTE DANGER UAF NETWORK KERNEL_PANIC_PLUS_UAF HARDWARE  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4	-	-	checked

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

ActionableScore=2
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw conservative score: 2
* Conservative score: 2
* Raw paranoid score: 2
* Paranoid score: 2
* No floor or cap changed either score.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

Positive signals:

* +1 Reliable kernel crash / strong DoS. A zero value in `tp->rcvq_space.space` is used as the divisor in `tcp_rcvbuf_grow()` or `tcp_rcv_space_adjust()`, producing a divide exception and potentially a kernel panic.
* +1 Common networking core path. The terminal operation occurs in ordinary TCP receive-buffer adjustment after data arrives.

No negative points were applied:

* Host-admin privilege was not subtracted because the complete privilege boundary for writing this per-network-namespace sysctl is not established by the supplied context.
* No rare-subsystem penalty applies. TCP is common, although the unsafe `tcp_rmem` configuration is non-default.
* No race penalty applies. The failure follows a deterministic invalid configuration and data-receive sequence.

Material non-awarded signals:

* Direct remote reachability +2 was not awarded because network traffic alone cannot create the unsafe state. A separate actor must first configure `tcp_rmem` below 4096.
* Local unprivileged trigger +1 was not awarded because ordinary-user ability to perform the complete sysctl configuration sequence is not established.
* No corruption bonus applies. The primitive is division by zero, not an OOB write, UAF, stale-object use, or arbitrary write.
* No extra Availability point was awarded because it would duplicate the same kernel crash.
* No Confidentiality, Integrity, authorization-bypass, or privilege-escalation signal is established.
* B5-R does not apply because there is no retry-accumulated heap OOB write or later consumption of corrupted descriptor state.
* B3-PENDING and B4-PENDING do not apply.

3. Reachability analysis

The unsafe state is created by writing a very small value, such as 1, to `net.ipv4.tcp_rmem`. Once that configuration exists, an unauthenticated remote TCP peer may supply data that reaches dynamic receive-buffer adjustment and triggers the divide exception.

This is not direct remote reachability under the scoring rules because privileged or otherwise authorized local configuration is a material prerequisite. The supplied patch shows a mode `0644` network sysctl but does not fully establish whether an ordinary user can create a user and network namespace, obtain the relevant authority, expose a TCP endpoint, and complete the vulnerable sequence. Therefore both effective host-admin requirement and complete namespace-scoped reachability remain unknown.

There is no hardware prerequisite and no timing race. The configuration is unusual and below the documented minimum, but no rarity penalty is applied because attacker access to the relevant namespace and sysctl has not been conclusively resolved.

4. Primitive and severity interpretation

The exact primitive is a zero denominator produced by insufficient minimum-value validation for `tcp_rmem`. The resulting zero propagates into TCP receive-window state and is later consumed by a division operation.

This is neither fixed stale storage nor an active structured-object UAF. There is no release, post-lifetime access, repeated refcount mutation, heap overwrite, or shared-backing write.

PrimitiveClass is CLASS_C because the demonstrated security consequence is a kernel exception and denial of service without an independently useful corruption primitive. Stronger consequences such as memory corruption, information disclosure, privilege escalation, or arbitrary kernel access are unsupported.

ImportantCandidate is NO because no Important-preserving primitive is established. AutoDowngradeAllowed is YES because the issue remains a configuration-dependent crash and no unresolved corruption, ownership, or authorization boundary exists.

Operational severity is ordinary Moderate or Low-like. A manual check is still recommended to resolve whether an unprivileged user can configure the affected sysctl inside an attacker-created network namespace and thereby turn the sequence into a reliable host-wide local DoS.

5. One-sentence report phrase

An unsafe tcp_rmem value below 4096 can initialize TCP receive-buffer state with a zero divisor, allowing later TCP data processing to trigger a divide exception and kernel crash after the required local configuration has been established.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Confirm the exact user-namespace and network-namespace permissions for writing `net.ipv4.tcp_rmem`; this affects reachability but does not change the established crash-only primitive.

7. Call-site confidence

Call-site confidence: high

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

Patch: tcp: do not let tcp_rmem be set below 4096 [ Upstream
Commit: 879907631b9e70eedb62d76461b29afa106ebe7a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=879907631b9e70eedb62d76461b29afa106ebe7a

Changed files:
  net/ipv4/tcp_input.c
  net/ipv4/sysctl_net_ipv4.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=879907631b9e70eedb62d76461b29afa106ebe7a

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

The original Linux kernel CVE announcement for CVE-2026-98299 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98299

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:H
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.9

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: 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).