From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-72256][MODERATE REGULAR] netfilter: xt_cluster: reject template conntracks in hash match Date: Sun, 16 Aug 2026 08:30:02 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-72256 X-AL-KERNEL-Priority: MODERATE REGULAR X-AL-KERNEL-Severity: MODERATE REGULAR X-AL-KERNEL-Base-Severity: MODERATE X-AL-KERNEL-KPANIC: NO X-AL-KERNEL-ActionableScore: 4 X-AL-KERNEL-ActionableScore-Lower: 1 X-AL-KERNEL-Commit: 4558bd7b47c7be82dffd837f27be8ea3ecee557d List-Id: CVE: CVE-2026-72256 Priority: MODERATE REGULAR AL-KERNEL base severity: MODERATE KPANIC flag: NO Patch: netfilter: xt_cluster: reject template conntracks in hash match Commit: 4558bd7b47c7be82dffd837f27be8ea3ecee557d Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4558bd7b47c7be82dffd837f27be8ea3ecee557d Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72256 Analysis date: Sun, 16 Aug 2026 08:30:02 -0400 ActionableScore: 4 ActionableScore lower bound: 1 Actionable bucket: Borderline, manual review recommended Manual review required: YES Summary: `xt_cluster_mt() can hash an incomplete template conntrack attached by the raw table CT target, allowing packets that hit a specific netfilter ruleset to trigger a WARN_ON path and possible DoS, but no concrete memory corruption or privilege escalation primitive is shown.` ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-72256 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72256 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 REGULAR 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-2026-72256 MODERATE 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:N/I:N/A:H';CWE-843;CWE-754;*CWE-20;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.9';DESCR 'xt_cluster_mt can treat a template conntrack attached by the raw table CT target as a fully initialized conntrack object. Template conntracks carry IPS_TEMPLATE state and may not have a valid tuple for xt_cluster_hash(), which can reach a WARN_ON path when l3num is zero. For the CVSS the PR:N value is used in the paranoid network-trigger interpretation because a packet sender may not need local privileges if a vulnerable ruleset is already installed. AC:H is used for that interpretation because the issue depends on a specific non-default netfilter ruleset combining CT templates and the xt_cluster match. Impact is denial of service via warning or kernel crash under panic_on_warn, with no concrete evidence of information disclosure or privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 3) YES REMOTE SIMPLEFIX NETFILTER DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5 NO NO checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=4 ActionableScoreLower=1 ## 1. ActionableScore * Conservative score: 1 * Paranoid score: 4 * Final recommended bucket: **Borderline, manual review recommended** ## 2. Signal breakdown Conservative signals: * Common networking packet-processing path: +1. `xt_cluster` is evaluated from the netfilter packet path once the rule exists. * Requires CAP_NET_ADMIN in typical deployments: -2. Creating the vulnerable ruleset normally requires privileged firewall control. * Rare/non-default rule combination: -1. Trigger requires a specific combination of raw table CT template handling and `xt_cluster` match. * WARN_ON continuation only: +0. The patch shows a WARN_ON path due to incomplete conntrack state, but no UAF, OOB write, arbitrary write, or direct crash primitive is shown. Paranoid signals: * Remote/network-triggerable after vulnerable ruleset is installed: +2. A packet sender may be able to trigger the affected match path without local privileges if the firewall is already configured this way. * Common networking packet-processing path: +1. The affected code executes during packet classification. * Broad/common subsystem exposure: +1. Netfilter is a common kernel firewall subsystem. * Availability impact plausible: +1. The main concrete impact is WARN_ON and possible crash if `panic_on_warn` or similar hardening policy is enabled. * Rare/non-default rule combination: -1. The trigger depends on a specific firewall configuration. * No memory corruption/LPE bonus: +0. The patch rejects template conntracks before hashing, but does not show UAF, OOB access, stale pointer reuse, write primitive, callback control, or refcount corruption. ## 3. Reachability analysis Reliable setup of the vulnerable state normally requires CAP_NET_ADMIN because an attacker must configure netfilter rules involving the raw table CT target and the `xt_cluster` match. However, if such a ruleset is already installed by an administrator or orchestration system, an external packet sender may be able to reach the vulnerable packet path. Network namespaces can lower practical exposure if untrusted users have delegated firewall control, but in ordinary host deployments this remains privileged configuration. The path is not default-enabled as a complete exploit condition because it needs a specific non-default ruleset. Call-site confidence: high. The patch directly changes the packet match function `xt_cluster_mt()` and the commit explains the raw CT template path. ## 4. Severity interpretation This behaves more like a borderline Moderate issue than an Important-class kernel vulnerability. The realistic demonstrated impact is state confusion leading to a WARN_ON path, with possible DoS in hardened configurations that panic on warnings. There is no supported evidence of confidentiality impact, integrity impact, privilege escalation, or generic memory corruption. The reason it should not be auto-dismissed is the packet-path reachability after a vulnerable ruleset is installed. ## 5. One-sentence report phrase `xt_cluster_mt() can hash an incomplete template conntrack attached by the raw table CT target, allowing packets that hit a specific netfilter ruleset to trigger a WARN_ON path and possible DoS, but no concrete memory corruption or privilege escalation primitive is shown.` ## 6. Manual review recommendation MANUAL CHECK RECOMMENDED Reason: the bug is packet-path reachable under a specific vulnerable firewall configuration, but the concrete primitive is only WARN_ON/state confusion rather than demonstrated memory corruption or LPE. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: netfilter: xt_cluster: reject template conntracks in hash match Commit: 4558bd7b47c7be82dffd837f27be8ea3ecee557d Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4558bd7b47c7be82dffd837f27be8ea3ecee557d Commit description: xt_cluster_mt() treats any non-NULL nf_ct_get() result as a fully initialized conntrack and passes it to xt_cluster_hash(). This causes a state confusion bug when the raw table CT target attaches a template conntrack to skb->_nfct before normal conntrack processing. Templates carry IPS_TEMPLATE status but do not have a valid tuple for hashing yet, so xt_cluster_hash() can hit its WARN_ON() path on the zeroed l3num field. Reject template conntracks before hashing them. This matches existing netfilter handling for template objects and avoids hashing incomplete conntrack state. Fixes: 0269ea4 ("netfilter: xtables: add cluster match") Cc: stable@vger.kernel.org Reported-by: Yuan Tan Reported-by: Yifan Wu Reported-by: Juefei Pu Reported-by: Zhengchuan Liang Reported-by: Xin Liu Assisted-by: Codex:GPT-5.4 Signed-off-by: Wyatt Feng Signed-off-by: Ren Wei Signed-off-by: Pablo Neira Ayuso Signed-off-by: Greg Kroah-Hartman Changed files: net/netfilter/xt_cluster.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=4558bd7b47c7be82dffd837f27be8ea3ecee557d ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-72256 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72256 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:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:L The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H The Best / paranoid CVSS score: 5.9 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: 1 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: NO Published priority for this report (same as in Subject): MODERATE REGULAR 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).