From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-72250][MODERATE REGULAR] netfilter: nf_conntrack_reasm: guard mac_header adjustment after IPv6 defrag Date: Sat, 15 Aug 2026 19:46:00 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-72250 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: 3 X-AL-KERNEL-Commit: 6e8cd710ca35c576f5f2e5a396047c9ac61f75e5 List-Id: CVE: CVE-2026-72250 Priority: MODERATE REGULAR AL-KERNEL base severity: MODERATE KPANIC flag: NO Patch: netfilter: nf_conntrack_reasm: guard mac_header adjustment after IPv6 defrag Commit: 6e8cd710ca35c576f5f2e5a396047c9ac61f75e5 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6e8cd710ca35c576f5f2e5a396047c9ac61f75e5 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72250 Analysis date: Sat, 15 Aug 2026 19:46:00 -0400 ActionableScore: 4 ActionableScore lower bound: 3 Actionable bucket: Borderline manual review recommended / below Important Manual review required: YES Summary: An IPv6 netfilter defragmentation bug can incorrectly convert an unset skb MAC header sentinel into a valid-looking offset on the LOCAL_OUT path, causing later packet-processing code to interpret headroom as a MAC header and potentially leading to local DoS. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-72250 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72250 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-2026-72250 MODERATE CHECK WITH IMPACT FROM ORIG NN MODERATE 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-190;CWE-682;CWE-125;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:L';BEST CVSS score: '5.8';DESCR 'An IPv6 netfilter defragmentation metadata bug can corrupt the logical state of skb mac_header on the NF_INET_LOCAL_OUT path. The skb has no link layer header yet, so mac_header may still contain the unset sentinel value, and adding the fragment header size can wrap it into a small offset that later appears valid. Later code may treat bytes in skb headroom as a real MAC header, which can cause incorrect packet metadata interpretation and possible availability impact in dependent paths. For the CVSS the PR:L is used because reliable triggering likely requires local code or a reduced capability process able to generate suitable IPv6 fragmented output on the victim, but not necessarily full administrator privileges in all deployments. The issue is not directly reachable from remote inbound packets because the described problematic path is LOCAL_OUT rather than normal receive processing. Impact is at least low confidentiality risk from incorrect header reads and availability risk, while the paranoid interpretation keeps limited integrity impact for manual review without converting the bug into a proven memory corruption primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review recommended / below Important (with actual score 4) YES REMOTE SIMPLEFIX NETFILTER NETWORK SKB LINUS TEST IPV6 PACKET DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5 NO NO checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=4 ActionableScoreLower=3 ## 1. ActionableScore * Conservative score: 3 * Paranoid score: 4 * Final recommended bucket: **Borderline manual review recommended / below Important**:: ## 2. Signal breakdown Conservative signals: * Local unprivileged trigger: +1 The problematic path is NF_INET_LOCAL_OUT. A local process may be able to generate IPv6 output that reaches conntrack defragmentation, depending on configuration. * Memory corruption, weak/indirect corruption candidate: +1 The bug corrupts skb metadata state by converting the unset mac_header sentinel into a small apparently valid offset. This is a bogus header pointer / invalid metadata interpretation issue, not a proven UAF or write primitive. * Memory layout invariant restoration: +1 The patch restores consistency between skb header-offset metadata after IPv6 fragment-header removal. * Common networking core path or packet-processing path: +1 This is in netfilter IPv6 conntrack defragmentation, a packet-processing path that deserves manual scrutiny. * Hard or specific triggering conditions: -1 The issue depends on the LOCAL_OUT defrag path, unset MAC header state, IPv6 fragmentation, and later code using skb_mac_header(). Paranoid additional signal: * Availability impact realistic: +1 Later consumers may misinterpret headroom bytes as a MAC header, plausibly causing packet-processing faults, warnings, or crashes in dependent paths, though no concrete crash primitive is shown. Not awarded: * No remote reachable signal. The described vulnerable path is LOCAL_OUT, not inbound remote packet processing. * No strong memory corruption. The patch does not show UAF, double free, OOB write, arbitrary write, type confusion, or refcount takeover. * No LPE signal. There is no demonstrated reclaim, overwrite, callback control, or privilege boundary bypass. * No confidentiality or integrity impact in the conservative score. There is no clear read-back or write primitive. ## 3. Reachability analysis The most realistic trigger is local code that causes IPv6 fragmented output to pass through netfilter conntrack defragmentation on the LOCAL_OUT path. This may be reachable by an unprivileged local user in some configurations, but reliable triggering depends on IPv6, conntrack defrag being active, and packet construction conditions that leave skb->mac_header unset. Namespaces may lower practical privilege requirements if an attacker can generate suitable traffic inside a namespace whose packets still traverse the affected host netfilter path. However, this is not equivalent to a normal remote network attack, because the commit specifically identifies NF_INET_LOCAL_OUT. The path is security-relevant because netfilter and skb metadata are common kernel networking infrastructure, but the affected condition is narrower than a generic IPv6 receive-path parser bug. Call-site confidence: medium. The changed code and immediate skb metadata effects are visible, but downstream consumers of the corrupted mac_header are not shown in the provided patch. ## 4. Severity interpretation This behaves more like a borderline actionable Moderate than an Important-class vulnerability. The theoretical concern is that corrupted skb header metadata can make later code treat headroom bytes as a valid MAC header. The realistic evidence supports metadata corruption and possible DoS, but not a strong memory corruption primitive, privilege escalation, or remote exploitation. The paranoid score is kept at 4 because this is a netfilter skb metadata bug in a packet path and should not be auto-closed without review. It should not be raised to Important based only on the current patch because no UAF, OOB write, arbitrary write, or ownership-boundary bypass is demonstrated. ## 5. One-sentence report phrase An IPv6 netfilter defragmentation bug can incorrectly convert an unset skb MAC header sentinel into a valid-looking offset on the LOCAL_OUT path, causing later packet-processing code to interpret headroom as a MAC header and potentially leading to local DoS. ## 6. Manual review recommendation MANUAL CHECK RECOMMENDED This should receive manual review because it is a networking skb metadata corruption issue in netfilter, but the available evidence supports only weak/indirect corruption and possible DoS rather than a proven LPE or remote primitive. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: netfilter: nf_conntrack_reasm: guard mac_header adjustment after IPv6 defrag Commit: 6e8cd710ca35c576f5f2e5a396047c9ac61f75e5 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6e8cd710ca35c576f5f2e5a396047c9ac61f75e5 Commit description: nf_ct_frag6_reasm() slides the packet head forward to drop the IPv6 fragment header and then unconditionally advances skb->mac_header: skb->mac_header += sizeof(struct frag_hdr); On the NF_INET_LOCAL_OUT defrag path the skb has no link-layer header yet, so skb->mac_header is still the "not set" sentinel (u16)~0U. Adding sizeof(struct frag_hdr) wraps it to a small value (0xffff + 8 == 7), after which skb_mac_header_was_set() wrongly reports a MAC header is present and skb_mac_header() points into the headroom. The reassembler has done this unconditional add since it was introduced; it was harmless while mac_header was a bare pointer, but wrong once mac_header became a u16 offset whose unset state is the ~0U sentinel tested by skb_mac_header_was_set(). The sibling net/ipv6/reassembly.c does the same relocation and does guard the adjustment; mirror the guard here. Fixes: 9fb9cbb ("[NETFILTER]: Add nf_conntrack subsystem.") Cc: stable@vger.kernel.org Reported-by: Weiming Shi Assisted-by: Claude:claude-opus-4-8 Signed-off-by: Xiang Mei Signed-off-by: Florian Westphal Signed-off-by: Greg Kroah-Hartman Changed files: net/ipv6/reassembly.c net/ipv6/netfilter/nf_conntrack_reasm.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=6e8cd710ca35c576f5f2e5a396047c9ac61f75e5 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-72250 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72250 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:L/UI:N/S:U/C:L/I:N/A:L 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: 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).