From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-74748][MODERATE 7.0] netfilter: ipset: fix refcount race between list:set GC and swap [ Upstream Date: Wed, 26 Aug 2026 13:32:52 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-74748 X-AL-KERNEL-Priority: MODERATE 7.0 X-AL-KERNEL-Severity: MODERATE 7.0 X-AL-KERNEL-Base-Severity: MODERATE X-AL-KERNEL-KPANIC: NO X-AL-KERNEL-ActionableScore: 5 X-AL-KERNEL-ActionableScore-Lower: 3 X-AL-KERNEL-Commit: 97a01de0c6321b7210d30d0a4d60f10f561097c7 List-Id: CVE: CVE-2026-74748 Priority: MODERATE 7.0 AL-KERNEL base severity: MODERATE KPANIC flag: NO Patch: netfilter: ipset: fix refcount race between list:set GC and swap [ Upstream Commit: 97a01de0c6321b7210d30d0a4d60f10f561097c7 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=97a01de0c6321b7210d30d0a4d60f10f561097c7 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74748 Analysis date: Wed, 26 Aug 2026 13:32:52 -0400 ActionableScore: 5 ActionableScore lower bound: 3 Actionable bucket: Actionable Moderate at minimum Manual review required: YES Summary: A refcount race between ipset list:set garbage collection and IPSET_CMD_SWAP can decrement a stale set reference and trigger BUG_ON in interrupt context, causing a local CAP_NET_ADMIN reachable kernel panic. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-74748 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74748 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-2026-74748 MODERATE CHECK WITH IMPACT FROM ORIG NN LOW Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';CWE-362;CWE-667;*CWE-664;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.8';DESCR 'Manual check is recommended because this is a race in object lifetime and refcount handling. No live web search was available in this run, so the assessment is based on the supplied commit text and diff context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 4) YES REMOTE KASAN LOCK NETFILTER RACE DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5 NO NO checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=5 ActionableScoreLower=3 ## 1. ActionableScore * Conservative score: 3 * Paranoid score: 5 * Final recommended bucket: **Actionable Moderate at minimum**:: ## 2. Signal breakdown Triggered signals: * Real lifetime corruption: +1 The bug is a refcount lifetime race. `__ip_set_put_byindex()` resolves an index to a set pointer, while `ip_set_swap()` can move the relevant refcount to another index before the decrement. * Memory corruption, weak/indirect corruption candidate: +1 The primitive is not an arbitrary write or controlled UAF reclaim, but it is a stale pointer/refcount association bug in kernel object lifetime handling. * Reliable kernel crash / strong DoS: +1 The commit shows `BUG_ON(set->ref == 0)` followed by Oops in timer softirq and `Kernel panic - not syncing`. * Availability impact realistic: +1 The observed failure is a system-wide kernel panic in interrupt context, not only a warning. * Broad/default/common subsystem relevance: +1 Netfilter/ipset is a common firewall subsystem, although this is control-plane ipset management rather than packet-processing fast path. Subtracting signals: * Hard or unreliable race / special timing required: -1 The issue requires interleaving list:set GC from timer softirq with `IPSET_CMD_SWAP`. * Requires CAP_NET_ADMIN in typical deployments: -2 conservative Creating/swapping ipsets normally requires administrative network privileges. Paranoid adjustment: * Reduced privilege / delegated CAP_NET_ADMIN interpretation: penalty reduced from -2 to -1 In containers, network namespaces, firewall management services, or reduced-capability root contexts, CAP_NET_ADMIN may not be equivalent to full host root. * Weak LPE concern: +1 paranoid only Refcount/lifetime corruption in netfilter deserves manual review, but there is no demonstrated reclaim, arbitrary write, callback control, or stable privilege escalation primitive. ## 3. Reachability analysis The realistic trigger requires local ability to manage ipset objects and issue `IPSET_CMD_SWAP`, normally via CAP_NET_ADMIN. The racing side is `list_set_gc()` running from timer softirq when an expiring list:set member calls `list_set_del()` and then `ip_set_put_byindex()`. This is not directly network reachable and is not triggered by packet traffic alone. Namespaces and containers matter because CAP_NET_ADMIN may be delegated to a container root or service account, making the practical privilege level lower than full host root in some deployments. The path is not an ordinary unprivileged syscall path. Exploitation requires crafted ipset state plus timing with GC and swap. Call-site confidence: high. The commit includes the affected helper, the locking change, the race description, and the crash call trace. ## 4. Severity interpretation This behaves above an ordinary low-priority DoS because it is a real refcount/lifetime race in a common kernel networking subsystem and the demonstrated failure is a kernel panic. However, the conservative interpretation remains below Important because triggering normally requires CAP_NET_ADMIN and the patch does not show attacker-controlled reclaim, arbitrary write, type confusion, or a concrete LPE primitive. Theoretical memory corruption potential exists because the stale set pointer/refcount relationship is corrupted. Realistic exploitation evidence supports kernel crash/DoS, while privilege escalation remains only a weak concern requiring manual review. ## 5. One-sentence report phrase A refcount race between ipset list:set garbage collection and IPSET_CMD_SWAP can decrement a stale set reference and trigger BUG_ON in interrupt context, causing a local CAP_NET_ADMIN reachable kernel panic. ## 6. Manual review recommendation MANUAL CHECK REQUIRED Reason: this is not just a validation bug or ordinary resource leak. It is a race in object lifetime and refcount handling inside netfilter, with a demonstrated kernel panic and a plausible but unproven corruption concern under delegated CAP_NET_ADMIN scenarios. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: netfilter: ipset: fix refcount race between list:set GC and swap [ Upstream Commit: 97a01de0c6321b7210d30d0a4d60f10f561097c7 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=97a01de0c6321b7210d30d0a4d60f10f561097c7 Changed files: net/netfilter/ipset/ip_set_core.c net/netfilter/ipset/ip_set_list_set.c kernel/time/timer.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=97a01de0c6321b7210d30d0a4d60f10f561097c7 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-74748 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74748 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:H/PR:H/UI:N/S:U/C:N/I:N/A:H The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H The Best / paranoid CVSS score: 5.8 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: 3 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: NO 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).