[CVE-2026-89537][MODERATE 7.0] SUNRPC: Reject short RFC 4121 MIC tokens in gss_krb5_verify_mic_v2

[CVE-2026-89537][MODERATE 7.0] SUNRPC: Reject short RFC 4121 MIC tokens in gss_krb5_verify_mic_v2

[AL-KERNEL]

CVE: CVE-2026-89537
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: SUNRPC: Reject short RFC 4121 MIC tokens in gss_krb5_verify_mic_v2
Commit: 7a946b2e7207f968902f2147ab9b30726f82f7ab
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7a946b2e7207f968902f2147ab9b30726f82f7ab
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89537
Analysis date: Sat, 12 Sep 2026 04:24:54 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89537 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89537

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: MODERATE 7.0
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-2026-89537	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89537 CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:H';CWE-125;CWE-130;*CWE-20;Other CVSS 'AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '8.2';DESCR 'A short RFC 4121 MIC token can reach gss_krb5_verify_mic_v2 without the minimum length needed for the token header and Kerberos checksum. The verifier reads token header bytes and later builds checksum input from memory beyond the supplied token when read_token len is too small, which creates an out-of-bounds kernel read. A malicious RPCSEC_GSS peer can trigger this over the network, including a malicious NFS server replying to a client or a malicious client reaching server side GSS verification paths. For the CVSS the PR:N is used because the attacker does not need a local account or local code execution on the victim and only needs a reachable authenticated or negotiated RPCSEC_GSS protocol position. Impact is at least remote denial of service via kernel crash. In the paranoid score, limited confidentiality impact is kept because the primitive is an out-of-bounds read, but integrity impact is not raised because the patch does not indicate an overwrite or UAF write primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate (with actual score 5)	YES	REMOTE NETWORK KPANIC  DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH	NO	NO	checked

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

ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 4
* Conservative score: 4. No floor or cap changed the score.
* Raw paranoid score: 5
* Paranoid score: 5. No floor or cap changed the score.
* Final recommended bucket: **Actionable Moderate**::

2. Signal breakdown

Conservative signals:

* +2 Remote reachable / directly network-triggerable. The supplied context states that short MIC tokens can be accepted from the wire through RPCSEC_GSS paths, including malicious NFS server to client and client to server-side GSS verification paths.
* +1 Weak invalid-read primitive. The patch establishes an out-of-bounds kernel read from a too-short RFC 4121 MIC token before validating read_token->len.
* +1 Reliable kernel crash / strong DoS. Reading past a small slab allocation in kernel GSS verification can realistically cause an Oops or panic, and the supplied triage context treats the impact as remote DoS.

Paranoid additional signal:

* +1 Limited confidentiality concern. This is not a direct leak, but the primitive is an attacker-influenced OOB read in cryptographic MIC verification where out-of-object bytes may influence verification behavior observable by a network peer. This supports only a limited paranoid confidentiality concern, not integrity impact.

Non-awarded signals:

* RealUAF not awarded. No freed or stale lifetime object use is established.
* CLASS_A not awarded. No arbitrary write, controlled reclaim, callback control, authorization bypass, LPE, or protected-resource theft is shown.
* CLASS_B not awarded. The issue is an OOB read / crash primitive, not a lifetime or ownership corruption primitive.
* Integrity impact not awarded. The patch does not show an overwrite, state corruption primitive, or unauthorized modification.
* Strong exploitation leverage not awarded. No controlled memory corruption or useful post-read primitive is demonstrated.
* Extra Availability not awarded. The same possible kernel crash is already counted once.
* Race penalty not applied. This is not a timing race.
* Host-admin penalty not applied. The trigger is from a network RPCSEC_GSS peer and does not require local host-admin access on the victim.

3. Reachability analysis

The attacker is a network peer participating in RPCSEC_GSS Kerberos traffic. On the NFS client side, a malicious NFS server can return a short checksum opaque value that is allocated as a small mic.data buffer and then passed into gss_verify_mic. On the server side, malicious RPC clients can reach GSS verification paths with short checksum tokens according to the supplied caller analysis.

HostAdminRequired=NO because the victim-side vulnerable operation is reached by network protocol input, not by local host administrative control. NamespaceScopedPrivilege=NO because namespaces are not material to the described trigger. Prior setup may be needed in the operational sense because the system must use SUNRPC/NFS with RPCSEC_GSS Kerberos, but that is a deployment exposure condition rather than an attacker privilege requirement. No special hardware or race condition is involved.

4. Primitive and severity interpretation

The exact primitive is a read-only out-of-bounds kernel read caused by missing minimum length validation for RFC 4121 MIC tokens in gss_krb5_verify_mic_v2. The code reads fixed header bytes and later passes a checksum region derived from ptr + GSS_KRB5_TOK_HDR_LEN even when read_token->len is shorter than the required header plus checksum length.

This is not active structured UAF and not fixed stale storage. It is also not a cleanup/destructor lifetime issue and has no concrete repeated release or refcount mutation. The demonstrated consequence is remote kernel crash / denial of service. The speculative consequence is limited confidentiality impact through an OOB read influencing MIC verification behavior, but no direct disclosure path or arbitrary readback is established.

PrimitiveClass=CLASS_C because the established behavior is invalid OOB read and DoS without an independently useful corruption or boundary-bypass primitive. ImportantCandidate=NO because there is no evidence of LPE, write primitive, UAF, controlled reclaim, or authorization bypass. AutoDowngradeAllowed=NO because the issue is network reachable and involves an attacker-influenced kernel OOB read, so it should not be automatically closed as ordinary low-risk DoS without manual review.

5. One-sentence report phrase

A short RFC 4121 MIC token can reach SUNRPC Kerberos verification without the required minimum length, causing an attacker-influenced out-of-bounds kernel read and a network-triggerable denial of service, with limited paranoid confidentiality concern but no demonstrated write primitive or privilege escalation.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the bug is network reachable and involves an attacker-influenced kernel out-of-bounds read in RPCSEC_GSS Kerberos verification, even though the established impact is primarily DoS.

7. Call-site confidence

Call-site confidence: high

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

Patch: SUNRPC: Reject short RFC 4121 MIC tokens in gss_krb5_verify_mic_v2
Commit: 7a946b2e7207f968902f2147ab9b30726f82f7ab
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7a946b2e7207f968902f2147ab9b30726f82f7ab

Commit description:

gss_krb5_verify_mic_v2() reads the token ID at ptr[0..1], the flags
byte at ptr[2], and padding at ptr[3..7], then passes
ptr + GSS_KRB5_TOK_HDR_LEN and cksum_len to gss_krb5_mic_build_sg().
None of these accesses check read_token->len first.

The minimum safe token size is GSS_KRB5_TOK_HDR_LEN (16) plus
ctx->krb5e->cksum_len (12-24, depending on the enctype).  All callers
accept shorter tokens from the wire:

 - gss_unwrap_resp_integ() enforces only an upper bound
   (offset + len <= rcv_buf->len) before allocating
   mic.data = kmalloc(len) and passing it to gss_verify_mic().
   A malicious NFS server can therefore supply a short checksum
   opaque, producing a small slab allocation that the Kerberos MIC
   verifier reads past.

 - gss_validate() enforces only len <= RPC_MAX_AUTH_SIZE (400)
   before passing the wire-supplied length to
   gss_validate_seqno_mic(), which constructs a mic xdr_netobj
   and calls gss_verify_mic().

 - svcauth_gss_verify_header() enforces only
   checksum.len >= XDR_UNIT (4 bytes) before dispatching to
   gss_verify_mic().

 - svcauth_gss_unwrap_integ() checks only that the checksum fits
   in gsd->gsd_scratch.

Add a length guard at the top of gss_krb5_verify_mic_v2(), before any
ptr[] access or scatterlist construction.  Well-formed MIC tokens from
gss_krb5_get_mic_v2() already have exactly GSS_KRB5_TOK_HDR_LEN +
cksum_len bytes, so valid traffic is unaffected.

Reported-by: Chris Mason <clm@meta.com>
Fixes: de9c17e ("gss_krb5: add support for new token formats in rfc4121")
Cc: stable@vger.kernel.org
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260523165237.510204-1-cel@kernel.org
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/sunrpc/auth_gss/gss_krb5_unseal.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=7a946b2e7207f968902f2147ab9b30726f82f7ab

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

The original Linux kernel CVE announcement for CVE-2026-89537 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89537

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:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:H
  The Best / paranoid CVSS score: 8.2

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: 4
  Paranoid ActionableScore: 5

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: MODERATE
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): MODERATE 7.0

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).