[CVE-2026-89538][MODERATE 7.0] SUNRPC: Reject krb5 v2 wrap tokens with oversized ec field

[CVE-2026-89538][MODERATE 7.0] SUNRPC: Reject krb5 v2 wrap tokens with oversized ec field

[AL-KERNEL]

CVE: CVE-2026-89538
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: SUNRPC: Reject krb5 v2 wrap tokens with oversized ec field
Commit: dfcd81ab45613d45996fbf483d4ef54b0ec90ab9
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=dfcd81ab45613d45996fbf483d4ef54b0ec90ab9
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89538
Analysis date: Fri, 11 Sep 2026 22:13:05 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89538 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89538

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-2026-89538	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89538 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:L/UI:N/S:U/C:N/I:N/A:H';*CWE-20;CWE-1284;CWE-130;Other CVSS 'AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '6.5';DESCR 'SUNRPC GSS krb5 v2 unwrap accepts authenticated wrap tokens where the encrypted ec field is larger than the remaining wrapped payload segment. A peer that already holds a valid GSS context can send such a malformed token and force xdr_buf_trim to trim past the wrapped blob boundary, leaving the xdr_buf in an invalid logical state with zero length or inconsistent iov lengths. For the CVSS the PR:L is used because the attacker needs a valid RPCSEC_GSS authentication context but does not need local privileges on the victim. The issue is network reachable through SUNRPC services that use Kerberos privacy or integrity wrapping, such as NFS with krb5i or krb5p. Impact is primarily denial of service or request processing disruption. A direct privilege escalation primitive is not evident from the patch, but the network reachable parser state issue should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4)	YES	REMOTE OOB NETWORK TEST KPANIC 	NO	NO	checked

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

ActionableScore=4
ActionableScoreLower=3
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 3
* Conservative score: 3
* Raw paranoid score: 4
* Paranoid score: 4
* Final recommended bucket: **Borderline / manual review recommended**::

No floor or cap changed the score.

2. Signal breakdown

Conservative positive signals:

* +2 Remote reachable / directly network-triggerable: a peer with a valid RPCSEC_GSS Kerberos context can send a malformed authenticated wrap token to a SUNRPC service.
* +1 Memory layout / buffer invariant restoration: the patch enforces consistency between ec, header, tail, offset, and the remaining wrapped segment before xdr_buf_trim() mutates xdr_buf lengths.

Paranoid additional signal:

* +1 Availability concern: an oversized authenticated ec can leave xdr_buf in a semantically invalid state with zero length or inconsistent iov lengths, which can plausibly disrupt RPC request handling.

Non-awarded signals:

* Real UAF not awarded: no freed or post-lifetime object use is established.
* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Confidentiality not awarded: no disclosure path is shown.
* Integrity not awarded: no unauthorized modification of protected state is shown.
* Strong DoS / reliable kernel crash not awarded conservatively: the patch establishes invalid xdr_buf state and malformed-token rejection, but does not show a panic or fatal fault.
* Strong semantic reinterpretation not awarded: the issue is an invalid length/trim boundary in an active buffer descriptor, not a demonstrated reinterpretation of attacker bytes as active credentials, pointers, ownership, or executable control state.
* Concrete repeated release/refcount not applicable.
* Extra availability not double-counted.

3. Reachability analysis

The attacker is a network peer that already holds a valid GSS context. This is below host-admin on the victim, so HostAdminRequired=NO. The issue is not unauthenticated because random mutation of ec is rejected by the authenticated header comparison.

The vulnerable path is reachable through SUNRPC services using Kerberos wrapping, for example NFS configurations using krb5i or krb5p. Prior administrative setup of such a service is required for exposure, but that is normal service deployment rather than a per-attack host-admin requirement.

Namespace-scoped privilege is not relevant to the described network RPCSEC_GSS path. No special hardware, boot option, or race timing is required. The trigger is protocol input after a valid authenticated context exists.

4. Primitive and severity interpretation

The exact primitive is malformed authenticated input causing xdr_buf_trim() to operate with an oversized trim length relative to the remaining wrapped payload segment. This can leave the xdr_buf descriptor in an internally inconsistent logical state.

This is not an active-object UAF, not fixed stale storage, and not a cleanup/destructor lifetime bug. No concrete repeated release/refcount operation exists. The first lifetime operation does not exist, and no second lifetime mutation exists.

PrimitiveClass=CLASS_C because the demonstrated behavior is invalid-state / request-processing disruption without an independently useful corruption primitive. The demonstrated consequence is malformed token handling leading to invalid xdr_buf state. The speculative consequence is broader RPC service disruption if later code consumes the inconsistent descriptor.

ImportantCandidate=NO because no Class A primitive, real UAF, controlled corruption, authorization bypass, protected-resource access, or plausible LPE primitive is established. AutoDowngradeAllowed=YES because the issue remains a network-authenticated Class C parser/state-consistency bug without unresolved kernel corruption leverage.

Likely operational severity is Moderate-like but worth manual review because it is network reachable in Kerberos-protected SUNRPC deployments.

5. One-sentence report phrase

SUNRPC GSS krb5 v2 unwrap can accept an authenticated token with an oversized ec field, causing xdr_buf_trim() to cut past the wrapped payload boundary and leave xdr_buf in an invalid state, allowing an authenticated network peer to disrupt RPC request handling.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: the issue is network reachable after RPCSEC_GSS authentication and affects kernel SUNRPC buffer parsing, but the supplied evidence does not establish UAF, arbitrary write, disclosure, privilege escalation, or a reliable host-wide crash.

7. Call-site confidence

Call-site confidence: medium

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

Patch: SUNRPC: Reject krb5 v2 wrap tokens with oversized ec field
Commit: dfcd81ab45613d45996fbf483d4ef54b0ec90ab9
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=dfcd81ab45613d45996fbf483d4ef54b0ec90ab9

Commit description:

gss_krb5_unwrap_v2() sets buf->len to a logical
length, which can be much smaller than head[0].iov_len
(the allocated receive-page capacity).  It then calls
xdr_buf_trim() with a trim length derived from the 16-bit
"extra count" (ec) field in the Kerberos v2 token header.

The ec field is authenticated by the post-decrypt memcmp()
against the encrypted header copy, so a randomly-mutated
value is rejected.  However, any peer holding a valid GSS
context can legitimately encrypt a token whose ec exceeds
the plaintext length.  Per RFC 4121, such a token is
structurally malformed.

Although xdr_buf_trim() now clamps the buf->len subtraction
to avoid unsigned underflow, the buffer is still left in a
semantically invalid state (zero length, inconsistent iov
lengths) when ec is oversized.

Reject these tokens before calling xdr_buf_trim(), giving
callers a well-defined GSS_S_DEFECTIVE_TOKEN error and
keeping the xdr_buf internally consistent.  The wrapped blob
begins at a nonzero offset -- both callers pass len as
offset + opaque_len -- so buf->len still counts the offset
bytes that precede the blob.  Compare the trim length
against the remaining wrapped segment, buf->len - offset,
rather than the whole buffer; comparing against buf->len
alone leaves an offset-wide window in which an oversized ec
passes the test and xdr_buf_trim() cuts into the bytes ahead
of the blob.

Fixes: cf4c024 ("sunrpc: trim off EC bytes in GSSAPI v2 unwrap")
Cc: stable@vger.kernel.org
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260528-tier2-v1-1-d026a1415e0b@oracle.com
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_wrap.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=dfcd81ab45613d45996fbf483d4ef54b0ec90ab9

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

The original Linux kernel CVE announcement for CVE-2026-89538 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89538

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:L/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 6.5

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: 3
  Paranoid ActionableScore: 4

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).