From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-89537][MODERATE 7.0] SUNRPC: Reject short RFC 4121 MIC tokens in gss_krb5_verify_mic_v2 Date: Sat, 12 Sep 2026 04:24:54 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-89537 X-AL-KERNEL-Priority: MODERATE 7.0 X-AL-KERNEL-Severity: MODERATE 7.0 X-AL-KERNEL-Base-Severity: MODERATE X-AL-KERNEL-KPANIC: YES X-AL-KERNEL-ActionableScore: 5 X-AL-KERNEL-ActionableScore-Lower: 4 X-AL-KERNEL-Commit: 7a946b2e7207f968902f2147ab9b30726f82f7ab List-Id: 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 Fixes: de9c17e ("gss_krb5: add support for new token formats in rfc4121") Cc: stable@vger.kernel.org Reviewed-by: Jeff Layton Link: https://patch.msgid.link/20260523165237.510204-1-cel@kernel.org Signed-off-by: Chuck Lever Signed-off-by: Greg Kroah-Hartman 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).