[CVE-2026-89550][MODERATE 7.0] SUNRPC: svcauth_gss: enforce krb5 token minimum length

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

CVE: CVE-2026-89550
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: SUNRPC: svcauth_gss: enforce krb5 token minimum length
Commit: dd6afc6cab8c5d387d1ed2f069562ef7bdadd651
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=dd6afc6cab8c5d387d1ed2f069562ef7bdadd651
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89550
Analysis date: Sat, 12 Sep 2026 05:54:49 -0400
ActionableScore: 4
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89550 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89550

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-89550	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89550 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:L/UI:N/S:U/C:L/I:N/A:H';*CWE-20;CWE-125;CWE-369;Other CVSS 'AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7.1';DESCR 'A minimum length validation bug in SUNRPC svcauth_gss_unwrap_priv can pass a too short krb5 privacy token into gss_unwrap() and gss_krb5_unwrap_v2(). The krb5 unwrap path expects the RFC 4121 token header and may read past the supplied token before integrity verification, then rotate_left() can reach a zero length buffer and divide by zero. For the CVSS the PR:L is used because practical triggering normally requires a usable RPCSEC_GSS krb5 context or authenticated access to the NFS/RPC security flavor, not local privileges on the victim host. The issue is network reachable when an NFS server exposes RPCSEC_GSS privacy to untrusted clients. Impact is at least remote denial of service via kernel crash. The paranoid score adds C:L because the patch context shows an invalid read past the supplied token, but there is no clear arbitrary write or UAF primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4)	YES	REMOTE OOB UAF NETWORK KPANIC  DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH	NO	NO	checked

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

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

1. ActionableScore

* Raw conservative score: 4
* Conservative score: 4
* Raw paranoid score: 4
* Paranoid score: 4
* No floor or cap changed the score.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

Positive signals:

* +2 Remote reachable / directly network-triggerable: attacker-supplied RPCSEC_GSS krb5 privacy token length reaches the vulnerable server-side unwrap path over SUNRPC/NFS when such service is exposed.
* +1 Reliable kernel crash / strong DoS: truncated or header-only token can drive rotate_left() to a zero-length buffer and cause divide by zero.
* +1 Weak / indirect corruption candidate: the bug includes an invalid OOB-style token header read / malformed structured interpretation before integrity verification, but no disclosure or write primitive is established.

Non-awarded signals:

* +0 Real UAF: no stale lifetime object or use-after-free is established.
* +0 Controlled reclaim / arbitrary write: not demonstrated.
* +0 Confidentiality: internal read past supplied token is shown, but no user-visible or network-visible disclosure path is established.
* +0 Integrity: no unauthorized modification of kernel or protected state is established.
* +0 Practical exploit chain: trigger-to-crash is already counted as DoS and is not an independent higher-impact exploit chain.
* +0 Extra availability: same crash is not double-counted.
* +0 Strong LPE / strong exploitation leverage: no UAF, OOB write, refcount takeover, controlled object replacement, callback control, or privilege-escalation primitive is shown.
* +0 Race penalty: not a race.
* +0 Rare subsystem penalty: RPCSEC_GSS privacy is deployment-dependent, but supplied evidence does not justify a concrete rare-and-difficult reachability penalty.

Negative signals:

* No negative privilege penalty applied. The attacker is not host-admin on the victim, but likely needs an authenticated or usable RPCSEC_GSS krb5 context.

3. Reachability analysis

The vulnerable path is server-side SUNRPC/NFS RPCSEC_GSS privacy handling. A remote client that can send crafted RPCSEC_GSS privacy traffic to the exposed NFS/RPC service can influence the opaque token length passed into gss_unwrap(). Practical triggering appears to require the service to be configured for krb5 privacy and to accept the attacker as a usable RPCSEC_GSS peer, so this is not treated as unauthenticated PR:N. It is still below host-admin on the victim, so HostAdminRequired=NO.

Namespace-scoped privilege is not relevant to the supplied trigger. No local namespace capability chain is needed or established. No special hardware is required. No race timing is involved. Prior service configuration is required in the operational sense, but the scoring does not subtract for it because exposed NFS/RPCSEC_GSS service is a deployment condition rather than proof of host-admin-only exploitation.

4. Primitive and severity interpretation

The exact primitive is missing minimum-length validation for a krb5 sealed token before the krb5 unwrap core parses RFC 4121 header fields and performs rotation. This produces invalid structured input handling, possible read past the supplied token boundary, and a divide-by-zero crash path.

This is not an active structured UAF and not fixed stale storage. There is no concrete repeated release/refcount operation. The demonstrated consequence is remote denial of service against the kernel NFS server. The theoretical confidentiality concern is limited because an internal OOB read is mentioned, but there is no evidence that bytes are returned to the attacker. The theoretical integrity or privilege-escalation concern is not supported by the patch mechanics.

PrimitiveClass=CLASS_C because the established behavior is malformed-input parsing leading to crash/DoS, with only weak invalid-read concern and no independently useful corruption primitive. ImportantCandidate=NO because Important-grade exploitation leverage is not established. AutoDowngradeAllowed=YES because this is a Class C network DoS without UAF, write corruption, ownership violation, authorization bypass, or unresolved strong Class B primitive. Operational severity is borderline/actionable for exposed NFS RPCSEC_GSS privacy deployments, but not a strong Important candidate from the supplied evidence.

5. One-sentence report phrase

A malformed short krb5 RPCSEC_GSS privacy token can reach the SUNRPC server unwrap path, trigger invalid token-header processing and a divide-by-zero crash, causing network-reachable denial of service for exposed NFS servers using krb5 privacy.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: network-reachable kernel crash in an authentication/encryption protocol path should be reviewed, but the supplied evidence supports DoS and weak invalid-read concern only, not UAF, arbitrary write, disclosure, or privilege escalation.

7. Call-site confidence

Call-site confidence: high

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

Patch: SUNRPC: svcauth_gss: enforce krb5 token minimum length
Commit: dd6afc6cab8c5d387d1ed2f069562ef7bdadd651
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=dd6afc6cab8c5d387d1ed2f069562ef7bdadd651

Commit description:

svcauth_gss_unwrap_priv() validates only an upper bound on the
wire-supplied opaque length before handing the buffer to
gss_unwrap():

    if (len > xdr_stream_remaining(xdr))
            goto unwrap_failed;
    offset = xdr_stream_pos(xdr);
    ...
    maj_stat = gss_unwrap(ctx, offset, offset + len, buf);

The wire value `len` flows unchanged as the upper bound into the
krb5 unwrap path, so a len in [0, 16] passes this check and is
handed to gss_unwrap(). For a krb5 v2 context that lands in
gss_krb5_unwrap_v2(), which reads the 16-byte RFC 4121 token
header fields at ptr+4 and ptr+6 and then calls rotate_left()
before any integrity check. With a sub-header length the header
reads run past the token, and _rotate_left()'s `shift %= buf->len`
path can divide by zero when buf->len has been driven to zero by
the truncated token. A header-only token (len == 16) is equally
invalid: with a non-zero RRC field and the opaque blob ending at
the XDR buffer boundary, rotate_left() builds a zero-length
subbuffer, reaching the same division.

Reject the token at the server entry point before it reaches the
krb5 unwrap core. A valid sealed RFC 4121 token must contain
the 16-byte header plus at least some encrypted payload.

Fix by adding a minimum-length check immediately after the
existing upper-bound check:

    if (len <= GSS_KRB5_TOK_HDR_LEN)
            goto unwrap_failed;

Fixes: 7c9fdcf ("[PATCH] knfsd: svcrpc: gss: server-side implementation of rpcsec_gss privacy")
Cc: stable@vger.kernel.org
Assisted-by: kres (claude-opus-4-7)
Signed-off-by: Chris Mason <clm@meta.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260524010213.557424-2-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/svcauth_gss.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=dd6afc6cab8c5d387d1ed2f069562ef7bdadd651

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

The original Linux kernel CVE announcement for CVE-2026-89550 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89550

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:H
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:H
  The Best / paranoid CVSS score: 7.1

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: 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).