From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-46101][LOW] netfilter: reject zero shift in nft_bitwise Date: Wed, 27 May 2026 11:14:06 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-46101 X-AL-KERNEL-Priority: LOW X-AL-KERNEL-Severity: LOW X-AL-KERNEL-Base-Severity: LOW X-AL-KERNEL-KPANIC: NO X-AL-KERNEL-ActionableScore: 3 X-AL-KERNEL-ActionableScore-Lower: 0 X-AL-KERNEL-Commit: bffef0acec9c3b837a785248a893137fb7f26c95 List-Id: CVE: CVE-2026-46101 Priority: LOW AL-KERNEL base severity: LOW KPANIC flag: NO Patch: netfilter: reject zero shift in nft_bitwise Commit: bffef0acec9c3b837a785248a893137fb7f26c95 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bffef0acec9c3b837a785248a893137fb7f26c95 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46101 Analysis date: Wed, 27 May 2026 11:14:06 -0400 ActionableScore: 3 ActionableScore lower bound: 0 Actionable bucket: Borderline, manual review recommended Manual review required: YES Summary: A malformed nftables bitwise shift rule with shift value 0 can reach the packet path and trigger undefined 32-bit shift behavior, potentially causing incorrect rule evaluation or sanitizer DoS, but rule installation normally requires CAP_NET_ADMIN. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-46101 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46101 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: LOW 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-2026-46101 LOW CHECK Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:H';CWE-758;CWE-682;*CWE-20;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:L/A:N';BEST CVSS score: '6.1';DESCR 'A malformed nft_bitwise shift expression could allow a zero shift operand to reach the packet path. The carry propagation logic then computes a 32 bit shift on a u32 value, which is undefined behavior and may cause incorrect rule evaluation or sanitizer triggered denial of service. For the CVSS the PR:L is used only in the paranoid score because delegated network administration or privileged container setups may let a less trusted local actor install nftables rules. The typical host case requires CAP_NET_ADMIN and is closer to PR:H. The issue is not directly network reachable because the malformed rule must be installed through the local nftables control plane. Impact is mainly integrity of packet filtering logic or availability on affected builds, with no UAF, out of bounds write, arbitrary write, information disclosure, or privilege escalation primitive shown by the patch.';NO MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 2) YES REMOTE SIMPLEFIX NETFILTER PACKET DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3 YES NO checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=3 ActionableScoreLower=0 ## 1. ActionableScore * Conservative score: 0 * Paranoid score: 3 * Final recommended bucket: **Borderline, manual review recommended**:: ## 2. Signal breakdown Conservative signals: * Broad/default/common subsystem exposure: +1 nftables/netfilter is a common packet filtering subsystem. * Integrity impact weakly plausible: +1 Undefined behavior in a bitwise expression may cause incorrect rule evaluation, but only for a malformed rule installed through the control plane. * Availability impact weakly plausible: +1 On sanitizer or debug builds, the undefined shift may trigger warnings or panic. Subtracting signals: * Requires CAP_NET_ADMIN in typical deployments: -2 Installing nftables rules normally requires administrative network privileges. * Constrained primitive / no memory corruption: -1 The issue is a zero-shift undefined behavior case, not UAF, OOB write, arbitrary write, or object lifetime corruption. Paranoid signals: * Local delegated trigger concern: +1 In containers, namespaces, or delegated network administration setups, a less trusted local actor may be able to install nftables rules. * Packet filtering policy impact concern: +1 Once installed, the malformed expression can affect packet path evaluation. No signals apply for remote reachability, strong memory corruption, real lifetime corruption, privilege escalation, information disclosure, or sensitive-object access. ## 3. Reachability analysis The bug is reachable through the nftables control plane by installing a malformed `nft_bitwise` shift expression with shift value 0. It is not directly network-triggerable because packets only exercise the expression after a local privileged or delegated actor installs the rule. In normal host deployments this requires CAP_NET_ADMIN, so exploitation by an ordinary unprivileged user is unlikely. In delegated network namespaces or privileged containers, the effective privilege requirement may be lower, but impact is usually limited to that network namespace unless broader host firewall control is delegated. ## 4. Severity interpretation This behaves like a low-to-borderline netfilter validation bug rather than an Important vulnerability. The patch prevents undefined behavior from reaching the packet path, but it does not show memory corruption or privilege escalation. Realistic impact is malformed-rule rejection, incorrect rule behavior, or sanitizer-triggered DoS. Broader security impact depends on whether less trusted actors can install nftables rules that affect meaningful traffic policy. ## 5. One-sentence report phrase A malformed nftables bitwise shift rule with shift value 0 can reach the packet path and trigger undefined 32-bit shift behavior, potentially causing incorrect rule evaluation or sanitizer DoS, but rule installation normally requires CAP_NET_ADMIN. ## 6. Manual review recommendation MANUAL CHECK RECOMMENDED Reason: no memory corruption is shown, but this is netfilter packet-path logic with possible policy impact in delegated network administration environments. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: netfilter: reject zero shift in nft_bitwise Commit: bffef0acec9c3b837a785248a893137fb7f26c95 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bffef0acec9c3b837a785248a893137fb7f26c95 Commit description: Reject zero shift operands for nft_bitwise left and right shift expressions during initialization. The carry propagation logic computes the carry from the adjacent 32-bit word using BITS_PER_TYPE(u32) - shift. A zero shift operand turns this into a 32-bit shift, which is undefined behaviour. Reject zero shift operands in the control plane, alongside the existing check for values greater than or equal to 32, so malformed rules never reach the packet path. Fixes: 567d746 ("netfilter: bitwise: add support for shifts.") Cc: stable@kernel.org Reported-by: Yuan Tan Reported-by: Yifan Wu Reported-by: Juefei Pu Reported-by: Xin Liu Signed-off-by: Kai Ma Signed-off-by: Ren Wei Reviewed-by: Fernando Fernandez Mancera Signed-off-by: Pablo Neira Ayuso Signed-off-by: Greg Kroah-Hartman Changed files: net/netfilter/nft_bitwise.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=bffef0acec9c3b837a785248a893137fb7f26c95 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-46101 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46101 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:L/A:N The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:H The Best / paranoid CVSS score: 6.1 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: 0 Paranoid ActionableScore: 3 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: LOW KPANIC detected for this report: NO Published priority for this report (same as in Subject): LOW 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).