[CVE-2026-63886][IMPORTANT] scsi: target: iscsi: Validate CHAP_R length before base64 decode

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

CVE: CVE-2026-63886
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: NO
Patch: scsi: target: iscsi: Validate CHAP_R length before base64 decode
Commit: 82454e6f21e56ea9a0a9de7d0ff7e1dfb83e34d6
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=82454e6f21e56ea9a0a9de7d0ff7e1dfb83e34d6
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63886
Analysis date: Sun, 19 Jul 2026 22:30:21 -0400
ActionableScore: 9
ActionableScore lower bound: unknown
Actionable bucket: High-end Important / possible Critical candidate
Manual review required: YES

Summary:
A pre-authentication heap buffer overflow in the Linux iSCSI target CHAP BASE64 response handling allows a remote initiator with network access to the target portal to overwrite kernel heap memory, causing DoS and potentially enabling broader kernel compromise.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-63886 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63886

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-2026-63886	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN LOW	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 'A pre-authentication heap buffer overflow exists in the Linux iSCSI target CHAP authentication path. The CHAP_R value can be BASE64 decoded into client_digest before validating that the decoded output fits the allocated digest buffer. A remote initiator can provide an oversized BASE64 CHAP_R response and overwrite kernel heap memory during authentication. For the CVSS the PR:N value is appropriate because exploitation requires only network reachability to the iSCSI target and does not require successful authentication or a local user account. The issue is network reachable when the iSCSI target portal is exposed to the attacker, although iSCSI is commonly deployed on isolated storage or data center networks rather than the public Internet. Impact includes denial of service via kernel crash and may include confidentiality and integrity compromise because this is an attacker controlled out-of-bounds write in kernel heap memory.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is High-end Important / possible Critical candidate (with actual score 9)	YES	INIT IMPROVEONLY  INCREASED_TO_HIGH_BASED_ON_GUESSCVSS8.5PLUS	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=9

## 1. ActionableScore

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

## 2. Signal breakdown

* Remote reachable / network-triggerable: +2
  The vulnerable path is in the iSCSI target CHAP authentication flow and can be reached by a remote iSCSI initiator during login/authentication.

* Local unprivileged trigger: +0
  Not needed for scoring because the relevant trigger is remote network reachability to the target portal.

* Privilege escalation plausible: +2
  This is a pre-authentication attacker-influenced kernel heap overflow. Even if no exploit chain is demonstrated, a remote OOB write in kernel heap memory is a strong LPE/RCE concern.

* Generic memory corruption, strong corruption primitive: +2
  The patch and commit explicitly describe an out-of-bounds write into `client_digest`, allocated as `chap->digest_size`, with up to 63 or 79 bytes of overflow depending on SHA-256 or MD5.

* Reliable kernel crash / strong DoS: +1
  A kernel heap overflow in an authentication parser is realistically crashable and can likely be triggered repeatedly while the target is exposed.

* Confidentiality impact plausible: +1
  Plausible due to kernel heap corruption, though no direct info leak primitive is shown.

* Integrity impact plausible: +1
  Plausible due to attacker-influenced OOB write into kernel heap memory.

* Important filesystem/storage path: +1
  The affected code is in the SCSI target iSCSI storage target stack.

* Rare/configuration-dependent exposure: -1
  iSCSI targets are commonly deployed in storage/data-center networks, but they are not normally exposed on general-purpose hosts or the public Internet.

## 3. Reachability analysis

A remote iSCSI initiator can trigger the bug by reaching an exposed Linux iSCSI target portal and sending an oversized BASE64-encoded `CHAP_R` response during CHAP authentication. Successful authentication is not required because the overflow occurs while validating the response.

Namespaces and containers are not central to the trigger. The attack is network-facing against the kernel target service. The main practical condition is that the host must run the Linux iSCSI target with CHAP support and expose the target portal to the attacker. In real deployments this is more likely inside storage or data-center networks than on the public Internet, but within such networks the pre-auth nature makes the issue highly actionable.

Call-site confidence: high. The patch directly shows the vulnerable decode call, the destination allocation size is described, and the commit explains the exact overflow sizes.

## 4. Severity interpretation

This behaves closer to an Important or possible Critical-class kernel vulnerability than an ordinary Moderate issue. The realistic demonstrated impact is at least remote kernel crash/DoS. The theoretical but technically defensible worst case includes confidentiality and integrity compromise because this is an attacker-influenced kernel heap OOB write in a pre-authentication network path.

This is not merely an input-validation cleanup. The validation is added before decoding specifically because the previous post-decode length check occurred after the kernel heap overflow had already happened.

## 5. One-sentence report phrase

A pre-authentication heap buffer overflow in the Linux iSCSI target CHAP BASE64 response handling allows a remote initiator with network access to the target portal to overwrite kernel heap memory, causing DoS and potentially enabling broader kernel compromise.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: remote pre-authentication kernel heap OOB write in a storage target authentication path. This should not be auto-closed because it combines network reachability, strong memory corruption, and plausible confidentiality/integrity impact.

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

Patch: scsi: target: iscsi: Validate CHAP_R length before base64 decode
Commit: 82454e6f21e56ea9a0a9de7d0ff7e1dfb83e34d6
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=82454e6f21e56ea9a0a9de7d0ff7e1dfb83e34d6

Commit description:

chap_server_compute_hash() allocates client_digest as
kzalloc(chap->digest_size) and then, for BASE64-encoded responses,
passes chap_r directly to chap_base64_decode() without checking whether
the input length could produce more than digest_size bytes of output.

chap_base64_decode() writes to the destination unconditionally as long
as there is input to consume. With MAX_RESPONSE_LENGTH set to 128 and
the "0b" prefix stripped by extract_param(), up to 127 base64 characters
can reach the decoder. 127 characters decode to 95 bytes. For SHA-256
(digest_size=32) this overflows client_digest by 63 bytes; for MD5
(digest_size=16) the overflow is 79 bytes.

The length check at line 344 fires after the write has already happened.

The HEX branch in the same switch statement already validates the length
up front. Apply the same approach to the BASE64 branch: strip trailing
base64 padding characters, then reject any input whose data length
exceeds DIV_ROUND_UP(digest_size * 4, 3) before calling the decoder.

Stripping trailing '=' before the comparison handles both padded and
unpadded encodings. chap_base64_decode() already returns early on '=',
so the full original string is still passed to the decoder unchanged.

The mutual CHAP path decodes CHAP_C into initiatorchg_binhex, which is
kzalloc(CHAP_CHALLENGE_STR_LEN). extract_param() caps initiatorchg at
CHAP_CHALLENGE_STR_LEN characters, so at most CHAP_CHALLENGE_STR_LEN-1
base64 characters reach the decoder. The maximum decoded size,
DIV_ROUND_UP((CHAP_CHALLENGE_STR_LEN-1) * 3, 4), is less than
CHAP_CHALLENGE_STR_LEN, so no overflow is possible there. A comment is
added at the call site to document this.

Fixes: 1e57338 ("scsi: target: iscsi: Support base64 in CHAP")
Cc: stable@vger.kernel.org
Signed-off-by: Alexandru Hossu <hossu.alexandru@gmail.com>
Reviewed-by: David Disseldorp <ddiss@suse.de>
Link: https://patch.msgid.link/20260521151121.808477-1-hossu.alexandru@gmail.com
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/target/iscsi/iscsi_target_auth.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=82454e6f21e56ea9a0a9de7d0ff7e1dfb83e34d6

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

The original Linux kernel CVE announcement for CVE-2026-63886 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63886

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: unknown
  Paranoid ActionableScore: 9

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).