* [CVE-2026-99138][IMPORTANT] tcp: fix use-after-free in do_tcp_getsockopt(TCP_CC_INFO)
@ 2026-10-29 2:41 21% AL-KERNEL
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From: AL-KERNEL @ 2026-10-29 2:41 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-99138
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: NO
Patch: tcp: fix use-after-free in do_tcp_getsockopt(TCP_CC_INFO)
Commit: 86814e5370068ec2bf0caa6e6d55c3e5bfc6c324
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=86814e5370068ec2bf0caa6e6d55c3e5bfc6c324
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99138
Analysis date: Wed, 28 Oct 2026 22:41:59 -0400
ActionableScore: 4
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES
Summary:
A concurrent BPF congestion-control unregister can leave `TCP_CC_INFO` processing calling through a stale `icsk_ca_ops` object, causing a real kernel UAF with unresolved callback-reuse leverage.
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ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99138 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99138
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: IMPORTANT
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-2026-99138 IMPORTANT https://www.kernelcve.org/list/?q=CVE-2026-99138 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT IMPORTANT AEGIS_AS=6 LEGACY=IMPORTANT RESULT=SAME INDEPENDENT_LEGACY=MODERATE_KPANIC INDEPENDENT_RESULT=DIFF TEMP_KPANIC_MERGE=IMPORTANT_HARD_VETO PRE_MERGE_LEGACY=MODERATE_KPANIC (final reconciliation agrees; DECISION: both IMPORTANT) 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-362;*CWE-672;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A race in do_tcp_getsockopt for TCP_CC_INFO can read a BPF struct_ops congestion control object after it has been freed and can then dereference or call its stale get_info function pointer. A local process can repeatedly query TCP_CC_INFO on a socket, but the lifetime transition requires the BPF congestion control object to be replaced or unregistered concurrently and reliable control may depend on delegated BPF or network administration capabilities. For the CVSS the PR:L value reflects required local code execution and practical influence over the affected socket or congestion control lifecycle. AC:H reflects the need to hit a concurrent free window. Impact is at least a local denial of service and the stale indirect function call provides a plausible but unproven path to kernel code execution or privilege escalation if freed memory reuse can be controlled. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 7 and the production ActionableScore returned 4 with recommendation Actionable Moderate / Strong Important candidate requiring manual review. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage LOW to MODERATE_KPANIC because the manual review requirement was applied. final reconciliation stage MODERATE_KPANIC to IMPORTANT because a specific evidence based protection or manual review veto was applied and KP specialist did not establish a qualifying panic. The legacy KP specialist produced a review artifact and its decision was processed by the applicable KP rules. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK and TEMP_KPANIC_MERGE_IMPORTANT_VETO and KPANIC_UNSUPPORTED_REMOVED. The independent weaker Aegis shadow evaluated CVSS 2.5 and ActionableScore 6 with primitive class CLASS_B and manual review REQUIRED. Its independently reconciled impact was IMPORTANT. Cross model aggregation changed the production result under an enabled evidence gated merge rule. The final reported impact is IMPORTANT.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 4) MAYBE REMOTE READ LOCK BPF LEAK UAF NETWORK INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK TEMP_KPANIC_MERGE_IMPORTANT_VETO KPANIC_UNSUPPORTED_REMOVED - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO
1. ActionableScore
* Raw base score: 4.
* Conservative score: 4. The real-UAF floor of 4 applies but does not change the score.
* Raw paranoid score: 4.
* Paranoid score: 4; no additional controlled-reclaim or privilege assumptions are justified.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
2. Signal breakdown
* +2 — Real active-object UAF: `icsk_ca_ops` can be freed while `do_tcp_getsockopt()` retains and uses the stale pointer.
* +1 — Specific active structured use: the stale object’s `get_info` function pointer is read and potentially called. This is materially stronger than a shallow cleanup dereference, but is not controlled callback hijacking.
* +1 — Plausible kernel crash/DoS: KASAN observed an eight-byte post-free read, and stale indirect-call processing can fault.
* +0 — No race penalty: concurrency is required, but the evidence does not establish a narrow, one-shot, or externally uncontrollable race.
* +0 — No local-unprivileged bonus: ordinary code can invoke `getsockopt(TCP_CC_INFO)`, but the complete trigger also needs concurrent BPF congestion-control replacement or unregister, whose privilege boundary is unresolved.
* No controlled reclaim, attacker-shaped replacement, arbitrary write, disclosure, repeated refcount mutation, or proven LPE is shown. Confidentiality and integrity points are therefore withheld.
* Extra availability is not counted separately from the same crash.
* B4-PENDING does not apply; no authorization decision is changed from DENY to ALLOW.
3. Reachability analysis
A local process can repeatedly query TCP congestion-control information on an affected socket. The freeing transition requires concurrent replacement or unregister of BPF `struct_ops` congestion control. The evidence does not establish whether one below-host-admin principal can perform both stages, whether delegated BPF/network capabilities suffice, or whether all required checks are namespace-scoped. The complete trigger is therefore potentially compound, with `HostAdminRequired` and namespace reachability unknown. No special hardware is required.
4. Primitive and severity reading
This is an active structured-object UAF, not fixed stale storage or cleanup-only use. The stale path reads an ops member and may make an indirect call through `get_info`. That supports CLASS_B with meaningful unknown leverage past a simple crash, but not CLASS_A because reclaim, callback-target control, or another directed corruption primitive is absent.
ImportantCandidate is YES because a concrete active-object UAF reaches an indirect function-pointer consumer and the BPF/delegated privilege boundary remains unresolved. Auto-downgrade is disallowed pending analysis of who controls unregister/replacement and whether freed ops storage or associated trampolines can be predictably reused.
5. One-sentence report phrase
A concurrent BPF congestion-control unregister can leave `TCP_CC_INFO` processing calling through a stale `icsk_ca_ops` object, causing a real kernel UAF with unresolved callback-reuse leverage.
6. Manual review advice
MANUAL CHECK REQUIRED
Determine the complete BPF privilege model, race controllability, object/trampoline reclamation behavior, and whether one below-host-admin actor can drive both lifetime transition and stale callback invocation.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: tcp: fix use-after-free in do_tcp_getsockopt(TCP_CC_INFO)
Commit: 86814e5370068ec2bf0caa6e6d55c3e5bfc6c324
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=86814e5370068ec2bf0caa6e6d55c3e5bfc6c324
Commit description:
do_tcp_getsockopt() reads icsk->icsk_ca_ops and dereferences the
get_info function pointer without rcu_read_lock(). With BPF struct_ops
congestion control, ca_ops can point to dynamically allocated memory
that is freed concurrently, resulting in a use-after-free when the
kernel dereferences or calls through the stale pointer.
BUG: KASAN: slab-use-after-free in do_tcp_getsockopt+0x2037/0x23e0
Read of size 8 at addr ffff888013701258 by task exploit/149
do_tcp_getsockopt+0x2037/0x23e0 (net/ipv4/tcp.c:4564)
tcp_getsockopt+0x91/0xf0
__sys_getsockopt+0xf7/0x170
Fix this by wrapping the ca_ops load and get_info call within
rcu_read_lock()/rcu_read_unlock(), and using READ_ONCE() to load
the icsk_ca_ops pointer.
Fixes: 0baf26b ("bpf: tcp: Support tcp_congestion_ops in bpf")
Suggested-by: Eric Dumazet <[email protected]>
Cc: [email protected]
Cc: [email protected]
Reviewed-by: Eric Dumazet <[email protected]>
Signed-off-by: Cen Zhang (Microsoft Security FORGE Labs) <[email protected]>
Reviewed-by: Jiayuan Chen <[email protected]>
Reviewed-by: Matthieu Baerts (NGI0) <[email protected]>
Link: https://patch.msgid.link/65fd3816ed5d541d9edd4bf4fcf97104a2cf907a.1787870710.git.blbllhy@gmail.com
Signed-off-by: Jakub Kicinski <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
net/ipv4/tcp.c
net/ipv4/tcp_dctcp.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=86814e5370068ec2bf0caa6e6d55c3e5bfc6c324
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99138 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99138
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:L/UI:N/S:U/C:L/I:L/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: unknown
Paranoid ActionableScore: 4
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: IMPORTANT
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): IMPORTANT
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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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