[CVE-2026-63887][IMPORTANT] scsi: target: iscsi: Bound iscsi_encode_text_output() appends to rsp_buf [ Upstream

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

CVE: CVE-2026-63887
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: scsi: target: iscsi: Bound iscsi_encode_text_output() appends to rsp_buf [ Upstream
Commit: cb84e974fb172bc71386289f37b78ea679410b39
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=cb84e974fb172bc71386289f37b78ea679410b39
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63887
Analysis date: Sun, 19 Jul 2026 12:34:39 -0400
ActionableScore: 10
ActionableScore lower bound: 9
Actionable bucket: High-end Important / possible Critical candidate
Manual review required: YES

Summary:
A remote pre-authentication heap out-of-bounds write in the Linux iSCSI target login response encoder can overflow an 8192 byte kernel buffer with attacker-influenced text records, causing denial of service and a defensible worst case of kernel memory corruption with confidentiality and integrity impact.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-63887 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63887

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
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CVE-2026-63887	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H';*CWE-787;CWE-122;*CWE-20;BEST CVSS score: '9.8';DESCR 'An out of bounds write in the iSCSI target login negotiation path occurs because iscsi_encode_text_output appends text response records into a fixed 8192 byte rsp_buf without checking the remaining capacity. A remote initiator can trigger the overflow by sending a crafted Login PDU with many small or unknown keys, causing the target to generate a much larger response such as NotUnderstood records. For the CVSS the PR:N is selected because no local account or authenticated session is required when the iSCSI target portal is reachable by the attacker. The issue is network reachable over the iSCSI target TCP service, although real deployments commonly place iSCSI on isolated storage or data center networks rather than the public Internet. Impact is kernel heap memory corruption with potential denial of service and a defensible worst case of confidentiality and integrity compromise.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is High-end Important / possible Critical candidate (with actual score 10)	YES	OOB INIT LOG IMPROVEONLY KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=10
ActionableScoreLower=9

## 1. ActionableScore

* Conservative score: 9
* Paranoid score: 10
* Final recommended bucket: **High-end Important / possible Critical candidate**

## 2. Signal breakdown

* Remote reachable / network-triggerable: +2. The vulnerable path is in iSCSI target login negotiation over TCP and can be reached by a remote initiator if the target portal is exposed.
* Memory corruption, Strong corruption primitive: +2. The old code used unbounded `sprintf()` into a fixed 8192 byte `rsp_buf`, and the commit describes a possible roughly 24 KiB heap overrun in kmalloc-8k.
* Privilege escalation plausible: +2. This is not merely a NULL dereference or warning. It is a remote kernel heap out-of-bounds write with attacker-influenced text records, so worst-case confidentiality and integrity compromise is defensible even if no exploit is demonstrated.
* Reliable kernel crash / strong DoS: +1. A large heap overwrite is very likely to crash or destabilize the kernel.
* Confidentiality impact plausible: +1. Heap corruption can plausibly expose or corrupt adjacent kernel objects, though no direct info leak is shown.
* Integrity impact plausible: +1. Kernel heap overwrite can plausibly corrupt control or data objects.
* Important filesystem/storage path: +1. This is in the SCSI target iSCSI storage service login path.
* Rare / non-default exposed service: -1 for the conservative score. Linux iSCSI targets are commonly restricted to storage or data-center networks and are not normally public Internet services.

Paranoid score removes the exposure penalty because many enterprise/storage deployments intentionally expose iSCSI target portals to multiple remote hosts on an internal network.

## 3. Reachability analysis

A remote unauthenticated iSCSI initiator can trigger the vulnerable code if it can connect to the Linux iSCSI target portal and send crafted Login PDUs. No local user account, namespace access, or container escape is required for the network-facing case. The vulnerable service is not enabled by default on ordinary Linux systems, and realistic exposure is usually limited to storage networks, virtualization clusters, or data-center internal networks. Call-site confidence: high, because both affected callers in `iscsi_target_handle_csg_zero()` and `iscsi_target_handle_csg_one()` are visible and pass the fixed `MAX_KEY_VALUE_PAIRS` buffer size after the patch.

## 4. Severity interpretation

This behaves like an Important-class kernel vulnerability, with possible Critical handling in exposed environments. The realistic minimum impact is remote denial of service through kernel heap corruption. The theoretical worst case is stronger than ordinary Moderate because the patch fixes an actual out-of-bounds heap write, not just validation cleanup or a constrained warning path. Practical code execution or privilege escalation is not demonstrated, but the primitive is serious enough that auto-closing as ordinary Moderate would be unsafe.

## 5. One-sentence report phrase

A remote pre-authentication heap out-of-bounds write in the Linux iSCSI target login response encoder can overflow an 8192 byte kernel buffer with attacker-influenced text records, causing denial of service and a defensible worst case of kernel memory corruption with confidentiality and integrity impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED. YES REQUIRES MANUAL CHECK.

This is a remote, pre-authentication, attacker-influenced kernel heap overwrite in a storage protocol parser, so it should receive manual security review even if the affected service is usually restricted to internal storage networks.

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UPSTREAM PATCH SUMMARY
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Patch: scsi: target: iscsi: Bound iscsi_encode_text_output() appends to rsp_buf [ Upstream
Commit: cb84e974fb172bc71386289f37b78ea679410b39
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=cb84e974fb172bc71386289f37b78ea679410b39

Changed files:
  drivers/target/iscsi/iscsi_target_nego.c
  drivers/target/iscsi/iscsi_target_parameters.c
  drivers/target/iscsi/iscsi_target_parameters.h

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=cb84e974fb172bc71386289f37b78ea679410b39

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

The original Linux kernel CVE announcement for CVE-2026-63887 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63887

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: Not available
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 9.8

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: 9
  Paranoid ActionableScore: 10

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).