* [CVE-2026-89541][MODERATE 7.0] SUNRPC: harden gss_unwrap_resp_priv length checks
@ 2026-09-11 23:30 21% AL-KERNEL
0 siblings, 0 replies; 1+ results
From: AL-KERNEL @ 2026-09-11 23:30 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-89541
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: SUNRPC: harden gss_unwrap_resp_priv length checks
Commit: 89a15a50f84d32d4b99db86f957427fcbe20a99a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=89a15a50f84d32d4b99db86f957427fcbe20a99a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89541
Analysis date: Fri, 11 Sep 2026 19:30:41 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate
Manual review required: YES
Summary:
A crafted RPCSEC_GSS privacy reply from a malicious or compromised NFS server can exploit broken length checks in `gss_unwrap_resp_priv()` to trigger kernel out-of-bounds reads on the client, causing DoS and requiring manual review for limited confidentiality impact.
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ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89541 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89541
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-2026-89541 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-89541 CHECK WITH IMPACT FROM ORIG NN MODERATE Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H';CWE-190;CWE-125;*CWE-20;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:H';BEST CVSS score: '7';DESCR 'A length validation flaw in the SUNRPC RPCSEC_GSS client privacy unwrap path can allow a crafted server reply to bypass bounds checks. The vulnerable check used offset plus opaque_len with u32 arithmetic, so a very large opaque_len can wrap the computed end offset and make gss_unwrap operate on an invalid range. A too short opaque_len can also pass into the Kerberos unwrap code and cause out of bounds reads while parsing the token header or during later buffer rotation. For the CVSS the PR:N value is used because the attacker does not need an account or code execution on the client and can act through a malicious or compromised NFS server endpoint. The issue is network reachable in deployments where an NFS client uses RPCSEC_GSS privacy mode such as krb5p against an attacker controlled or compromised server. Impact is at least denial of service via kernel crash and may include limited confidentiality exposure from out of bounds reads. Integrity impact is kept limited in the paranoid score because no clear arbitrary write primitive is shown.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate (with actual score 5) YES REMOTE OOB NETWORK KPANIC 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 applied.
* Raw paranoid score: 5
* Paranoid score: 5. No floor or cap applied.
* Final recommended bucket: **Actionable Moderate**::
## 2. Signal breakdown
Conservative signals:
* +2 Remote reachable / directly network-triggerable: a malicious or compromised krb5p NFS server can return a crafted RPCSEC_GSS reply to the client.
* +1 Concrete bounds/integer-validation defect: `offset + opaque_len` is computed in `u32` and can wrap, allowing an invalid `end < begin` range to reach `gss_unwrap()`.
* +1 Reliable kernel crash / strong DoS: supplied context states that crafted replies can drive kernel out-of-bounds reads in `gss_krb5_unwrap_v2()` and later processing, plausibly causing client kernel fault or crash.
Paranoid additional signal:
* +1 Limited confidentiality concern: OOB reads occur in kernel parsing of attacker-supplied network response data. A direct disclosure path is not proven, so this is limited and paranoid-only.
Non-awarded signals:
* RealUAF not awarded: no stale lifetime object or use-after-free is established.
* CLASS_A not awarded: no arbitrary write, controlled corruption, controlled reclaim, callback hijack, or demonstrated LPE.
* Integrity not awarded conservatively: no write primitive or unauthorized modification is shown.
* Extra availability not awarded: the same crash/DoS is not double-counted.
* Race/timing penalty not applied: this is not a timing race.
* Rare subsystem penalty not applied: NFS with RPCSEC_GSS privacy is configuration-dependent, but not enough evidence shows a rare or hard-to-reach subsystem.
## 3. Reachability analysis
The attacker model is a malicious or compromised NFS server endpoint responding to an NFS client using RPCSEC_GSS privacy mode such as `krb5p`. The attacker does not need local code execution or an account on the victim client, so host-admin-equivalent privilege is not required.
This is not a namespace-scoped privilege issue. No local namespace capability is needed for the attacker side. The practical prerequisite is that the victim client uses krb5p RPCSEC_GSS against an attacker-controlled or compromised server, or that the attacker can otherwise influence that server response path.
No special hardware is required. No race is required. Exposure is limited by deployment: NFS krb5p is typically used in managed internal environments rather than arbitrary public Internet services, but once the client talks to a malicious server, the vulnerable parser path is reachable.
## 4. Primitive and severity interpretation
The exact primitive is an integer wraparound and missing lower-bound validation in `gss_unwrap_resp_priv()`, leading to out-of-bounds reads in the SUNRPC RPCSEC_GSS client privacy unwrap path.
This is not active structured UAF and not fixed stale storage. It is a network-driven invalid range / OOB-read condition. The demonstrated consequence is client-side kernel crash or DoS. A limited confidentiality concern is defensible in paranoid scoring because kernel OOB reads occur during processing of a server-controlled reply, but no concrete leak-back path or arbitrary read primitive is established.
PrimitiveClass is CLASS_C because the evidence shows invalid-state / OOB-read / crash behavior without an independently useful corruption primitive. ImportantCandidate is NO because there is no demonstrated memory corruption write, privilege escalation, ownership violation, or authorization bypass. AutoDowngradeAllowed is NO because the issue is network-reachable from a malicious server and has a concrete kernel OOB-read primitive, so it should not be automatically closed as ordinary low-risk DoS.
Likely operational severity: actionable Moderate, with manual review required due to network reachability and kernel OOB-read behavior.
## 5. One-sentence report phrase
A crafted RPCSEC_GSS privacy reply from a malicious or compromised NFS server can exploit broken length checks in `gss_unwrap_resp_priv()` to trigger kernel out-of-bounds reads on the client, causing DoS and requiring manual review for limited confidentiality impact.
## 6. Manual review recommendation
MANUAL CHECK REQUIRED
Reason: the issue is network-reachable from a malicious or compromised NFS server and includes a concrete kernel OOB-read primitive, even though arbitrary write, UAF, and LPE are not established.
## 7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: SUNRPC: harden gss_unwrap_resp_priv length checks
Commit: 89a15a50f84d32d4b99db86f957427fcbe20a99a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=89a15a50f84d32d4b99db86f957427fcbe20a99a
Commit description:
gss_unwrap_resp_priv() validates the RPCSEC_GSS opaque length with
offset = (u8 *)(p) - (u8 *)head->iov_base;
if (offset + opaque_len > rcv_buf->len)
goto unwrap_failed;
maj_stat = gss_unwrap(ctx->gc_gss_ctx, offset,
offset + opaque_len, rcv_buf);
Both operands are u32 and the sum is computed in u32. A reply with
opaque_len near 0xffffffff makes offset + opaque_len wrap to a small
value that is below rcv_buf->len, so the bound check passes and
gss_unwrap() is called with end < begin. The check also lacks a
lower bound, so any opaque_len in [0, GSS_KRB5_TOK_HDR_LEN) is
accepted and forwarded to gss_krb5_unwrap_v2(), whose pre-decrypt
header reads at ptr+4 and ptr+6 then run past the token.
A krb5p NFS server returning a crafted RPCSEC_GSS reply can drive
the client into out-of-bounds reads in gss_krb5_unwrap_v2() and the
rotate_left() loop that follows.
Fix by replacing the single combined check with three guards that
are safe in u32 arithmetic and that enforce the RFC 4121 minimum
outer token length:
if (offset > rcv_buf->len)
goto unwrap_failed;
if (opaque_len > rcv_buf->len - offset)
goto unwrap_failed;
if (opaque_len < GSS_KRB5_TOK_HDR_LEN)
goto unwrap_failed;
The first guard makes the subtraction in the second guard
unconditionally safe; offset is derived from a successful
xdr_inline_decode() in the head kvec, so in practice it already
satisfies the bound. The floor mirrors the server-side check added
in commit 5b757c2e57a5 ("SUNRPC: svcauth_gss: enforce krb5 token
minimum length").
Fixes: 2d2da60 ("RPCSEC_GSS: client-side privacy support")
Cc: [email protected]
Assisted-by: kres (claude-opus-4-7)
Signed-off-by: Chris Mason <[email protected]>
Reviewed-by: Jeff Layton <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Chuck Lever <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
net/sunrpc/auth_gss/auth_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=89a15a50f84d32d4b99db86f957427fcbe20a99a
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89541 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89541
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:H/PR:N/UI:N/S:U/C:L/I:N/A:H
The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H
The Best / paranoid CVSS score: 7
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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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