[CVE-2026-80849][IMPORTANT] net/tcp-ao: fix use-after-free of current_key on reconnect to another peer

[CVE-2026-80849][IMPORTANT] net/tcp-ao: fix use-after-free of current_key on reconnect to another peer

[AL-KERNEL]

CVE: CVE-2026-80849
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: net/tcp-ao: fix use-after-free of current_key on reconnect to another peer
Commit: 84a93b4e012587d0a4a84ffb23ec6da18e9d85f9
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=84a93b4e012587d0a4a84ffb23ec6da18e9d85f9
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80849
Analysis date: Fri, 04 Sep 2026 16:09:20 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-80849 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80849

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-2026-80849	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-80849 CHECK WITH IMPACT FROM ORIG NN 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-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'TCP-AO can leave current_key pointing to a freed key object during reconnect to another peer. tcp_inbound_ao_hash runs under RCU and may update current_key on the established fast path while tcp_ao_connect_init later removes AO keys that do not match the new peer and frees them with call_rcu. A later transmit path can pick up the stale current_key in tcp_get_current_key and tcp_ao_transmit_skb can read traffic key material from freed memory, causing at least a kernel crash and possible integrity or confidentiality impact from use-after-free behavior. For the CVSS the PR:L is used because reliable triggering generally requires a local user or process able to create or control a TCP-AO socket and drive reconnect behavior. The issue is not a standalone unauthenticated network attack, although packets from a TCP-AO peer can participate in the race by changing rnext_keyid. Impact is at least local denial of service and the UAF nature makes privilege escalation or broader memory corruption impact plausible enough for manual review.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6)  SKIP CVE-2026-80849 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	REMOTE DANGER UAF NETWORK KPANIC 	-	-	checked

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

ActionableScore=6
ActionableScoreLower=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 5
* Conservative score: 5
* Raw paranoid score: 6
* Paranoid score: 6
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

No floor or cap changed the score.

2. Signal breakdown

Positive conservative signals:

* +2 Real UAF / lifetime violation. `current_key` can remain pointing to a TCP-AO key freed via `call_rcu()`, and later transmit code consumes it through `tcp_get_current_key()` and `tcp_ao_transmit_skb()`.
* +1 Reliable kernel crash / strong DoS. The patch-supported stale dereference of a freed key object can plausibly cause a kernel fault or panic.
* +1 Local below-host-admin trigger. The scenario is driven by a local process controlling a TCP-AO socket lifecycle and reconnect behavior, with PR:L rather than host-admin-only reachability.
* +1 Attacker-influenced active-object lifetime manipulation. The local process can drive disconnect/reconnect and key selection/lifetime transitions, while peer packets with `rnext_keyid` participate in updating `current_key`.

Additional paranoid signal:

* +1 Structured stale-object exploitation leverage. The stale object is an active TCP-AO key, and the later stale use consumes security-relevant traffic key material in the transmit authentication path. This is more meaningful than an immediate cleanup-only stale dereference, but controlled reclaim or arbitrary write is not demonstrated.

Non-awarded signals:

* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Demonstrated privilege escalation not established.
* Direct unauthenticated remote reachability not established.
* Concrete repeated release/refcount signal not awarded. The bug is stale pointer dereference/use after RCU free, not a second `put` or `release` on the exact same dangling reference.
* Extra Availability not awarded because the same crash is already counted.
* Confidentiality and Integrity are not awarded as separate ActionableScore points conservatively. The paranoid point covers unresolved structured UAF leverage, but no concrete disclosure or unauthorized write primitive is shown.
* Race penalty not applied. Although this is a race, the supplied evidence shows a repeatable socket lifecycle and peer-packet interaction rather than a proven one-shot externally uncontrollable timing window.

Repeated release/refcount checklist:

* First release can end lifetime: yes, the key is unlinked and freed via `call_rcu()`.
* Exact same reference survives: yes, `current_key` may still point to the freed key.
* Second operation is a real lifetime/refcount mutation: no, later code reads/uses the stale key rather than performing a second release/refcount mutation.
* Concrete repeated release/refcount +1 awarded: no.
* Paranoid repeated-refcount leverage +1 awarded: no.
* FLOOR 5 applied: no explicit floor needed because raw conservative score is already 5.

3. Reachability analysis

The evaluated attacker is a local process that can create or control a TCP-AO socket, drive disconnect and reconnect to another peer, and influence the timing of peer traffic that carries `rnext_keyid`. This supports PR:L style reachability rather than a pure remote unauthenticated network attack.

The network peer participates in the race by sending packets that can cause `tcp_inbound_ao_hash()` to update `current_key`, but the complete reliable trigger also depends on local socket lifecycle control. Therefore this is not scored as directly remote reachable.

Host-admin-equivalent privilege is not positively established from the supplied evidence. Namespace-scoped privilege is unresolved because the supplied patch does not establish that the complete TCP-AO setup and reconnect sequence is fully namespace-scoped.

No special hardware is required. Prior system support for TCP-AO is required, but the patch does not support treating this as rare hardware or physical-only. The race involves RCU, softirq timing, and reconnect behavior, but repeated attempts appear feasible for a process controlling the socket lifecycle.

4. Primitive and severity interpretation

The exact primitive is a real RCU/lifetime UAF of an active structured `tcp_ao_key` object. The stale pointer is not limited to fixed stale storage or a cleanup-only destructor path. It survives into the transmit path, where TCP-AO authentication key material is read from a freed object.

This is PrimitiveClass CLASS_B. It is not CLASS_A because the supplied evidence does not establish controlled reclaim, arbitrary read/write, callback control, refcount takeover, credential corruption, or demonstrated LPE.

Demonstrated consequence is at least local denial of service through kernel crash. Speculative consequence is stronger memory corruption or privilege escalation if stale key-object reuse can be shaped in a useful way, but that is not proven by the patch. The active structured UAF in TCP authentication state creates enough unresolved exploitation leverage that manual review is required and automatic downgrade is not appropriate.

ImportantCandidate is YES because this is a non-cleanup active-object UAF in a security-relevant networking authentication path with attacker-influenced lifecycle and state. AutoDowngradeAllowed is NO because the unresolved structured UAF leverage would be lost by treating it as ordinary crash-only Moderate.

Likely operational severity is Actionable Moderate at minimum, with Strong Important candidacy depending on whether manual analysis can establish controlled reuse, useful disclosure, unauthorized authentication-state manipulation, or LPE.

5. One-sentence report phrase

TCP-AO can leave current_key pointing to a freed AO key during reconnect, allowing a later transmit path to read traffic key material from freed memory and causing at least local DoS with unresolved UAF exploitation risk.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a real active-object UAF in TCP-AO key handling, not a simple resource leak or validation-only bug, and exploitation leverage beyond crash remains unresolved.

7. Call-site confidence

Call-site confidence: high

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

Patch: net/tcp-ao: fix use-after-free of current_key on reconnect to another peer
Commit: 84a93b4e012587d0a4a84ffb23ec6da18e9d85f9
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=84a93b4e012587d0a4a84ffb23ec6da18e9d85f9

Commit description:

tcp_inbound_ao_hash() is called before bh_lock_sock_nested() is taken,
with only rcu_read_lock() held. On the fast path for established
sockets, if the rnext_keyid sent by the peer differs from
current_key->sndid, the key the peer asked for is looked up and stored
in current_key. The lookup is inside the RCU read side, but current_key
outlives it.

When the socket is disconnected and connect() is called again for
another peer, tcp_ao_connect_init() unlinks every key that does not
match the new peer and frees it with call_rcu(). If current_key points
at such a key, it is cleared to NULL.

The fast path reads sk_state only once on entry, so a softirq that got
into it while the socket was still established can update current_key
after that loop has already run. The update is inside the RCU read side,
so it comes before the call_rcu() callback, and once the callback frees
the key, current_key is left pointing at freed memory.

The next transmission picks that pointer up in tcp_get_current_key().
tcp_ao_transmit_skb() then reads the traffic key from the freed object,
which is the use-after-free.

Wait for one grace period before unlinking, and only if a key is going
to be removed. By the time tcp_connect() runs the socket is already in
TCP_SYN_SENT, and TCP_AO_ESTABLISHED does not contain TCPF_SYN_SENT, so
a softirq entering after the wait cannot reach the fast path, and the
ones already in it have finished. The existing NULL handling in the loop
is then enough.

Fixes: 0a3a809 ("net/tcp: Verify inbound TCP-AO signed segments")
Cc: stable@vger.kernel.org
Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Link: https://patch.msgid.link/aoIriv3pHDgII2YR@v4bel
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/ipv4/tcp_ao.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=84a93b4e012587d0a4a84ffb23ec6da18e9d85f9

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

The original Linux kernel CVE announcement for CVE-2026-80849 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80849

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: 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: IMPORTANT
KPANIC detected for this report: YES
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 alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).