[CVE-2026-52993][IMPORTANT] tipc: fix double-free in tipc_buf_append() [ Upstream

[CVE-2026-52993][IMPORTANT] tipc: fix double-free in tipc_buf_append() [ Upstream

[AL-KERNEL]

CVE: CVE-2026-52993
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: NO
Patch: tipc: fix double-free in tipc_buf_append() [ Upstream
Commit: a438975a6dcdbd70865978c021650d1485586f0b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a438975a6dcdbd70865978c021650d1485586f0b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52993
Analysis date: Thu, 25 Jun 2026 17:10:22 -0400
ActionableScore: 7
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A network-reachable double-free in the TIPC reassembly path can occur when `tipc_msg_validate()` reallocates and frees an skb while `tipc_buf_append()` keeps a stale caller-side pointer, leading to kernel crash and possible memory-corruption impact.

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

The original Linux kernel CVE announcement for CVE-2026-52993 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52993

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.

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

CVE-2026-52993	IMPORTANT	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H';CWE-415;*CWE-416;*CWE-672;Other CVSS 'AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7.5';DESCR 'A double free in the TIPC reassembly path can occur because tipc_msg_validate can reallocate the reassembled skb and free the old skb while tipc_buf_append still keeps the caller side headbuf pointing to the freed object. A remote TIPC peer can potentially trigger this with fragmented TIPC traffic that reaches the LAST_FRAGMENT validation path and then fails validation after skb reallocation. For the CVSS the PR:N is used for the paranoid score because no local account on the victim is needed when the attacker can send traffic to the exposed TIPC bearer. The issue is network reachable in TIPC deployments, but TIPC is usually used on cluster or trusted internal networks rather than the public Internet. Impact is at least denial of service via kernel crash. In worst case it may allow confidentiality or integrity impact due to double free memory corruption and should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7) 	MAYBE	DANGER TIPC ERRORPATH UAF SKB LINUS 	-	-	checked

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

ActionableScore=7
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: 6
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2. TIPC message reassembly is reachable from a peer that can send TIPC traffic to an enabled bearer.
* Memory corruption, strong corruption primitive: +2. The patch fixes a double-free of an skb, which is a real kernel heap lifetime corruption class.
* Real lifetime corruption: +1. The old reassembled skb may be freed by `tipc_msg_validate()`, while `*headbuf` still points to that freed object.
* Reliable kernel crash / strong DoS: +1. Double-free in the skb path can realistically lead to kernel crash or allocator corruption.
* Weak LPE concern: +1. Double-free can sometimes become exploitable beyond DoS, but the patch does not show a controlled reclaim or overwrite primitive.
* Rare AND difficult-to-reach subsystem/configuration: -1. TIPC is not normally Internet-exposed and usually appears in cluster or internal deployments.

Paranoid adjustment:

* Stronger privilege-escalation concern: +1 over conservative. Because this is network-reachable double-free memory corruption in skb handling, manual triage should not auto-close it as DoS-only even though a practical exploit primitive is not demonstrated.

## 3. Reachability analysis

A remote TIPC peer can potentially trigger the bug by sending fragmented TIPC traffic that reaches the `LAST_FRAGMENT` path and causes `tipc_msg_validate()` to reallocate and then fail validation. No local account on the victim is required if the attacker can send traffic to the exposed TIPC bearer.

The path is not generally exposed on default Internet-facing systems. TIPC is more commonly used in clustered or trusted internal networks. Network namespaces may matter if TIPC is enabled inside containers or delegated network environments, but the main risk is a reachable TIPC endpoint rather than ordinary local syscall access.

## 4. Severity interpretation

This is stronger than an ordinary Moderate issue because it is not just a warning, leak, or NULL dereference. It is a double-free in a network protocol receive/reassembly path.

Realistic impact is likely kernel crash or allocator failure. Theoretical impact includes confidentiality or integrity compromise due to heap memory corruption, but the patch does not demonstrate a controlled object reclaim, arbitrary write, or reliable privilege escalation path. Therefore it is best treated as an Actionable Moderate at minimum and a Strong Important candidate for manual review.

## 5. One-sentence report phrase

A network-reachable double-free in the TIPC reassembly path can occur when `tipc_msg_validate()` reallocates and frees an skb while `tipc_buf_append()` keeps a stale caller-side pointer, leading to kernel crash and possible memory-corruption impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is network-triggerable double-free memory corruption in skb/TIPC handling. Even if the likely practical impact is DoS, the bug class and reachable protocol path justify manual review before downgrading or auto-closing.

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

Patch: tipc: fix double-free in tipc_buf_append() [ Upstream
Commit: a438975a6dcdbd70865978c021650d1485586f0b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a438975a6dcdbd70865978c021650d1485586f0b

Changed files:
  net/tipc/msg.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=a438975a6dcdbd70865978c021650d1485586f0b

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

The original Linux kernel CVE announcement for CVE-2026-52993 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52993

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:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:A/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.5

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: 6
  Paranoid ActionableScore: 7

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 alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).