From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-63993][IMPORTANT] vxlan: do not reuse cached ip_hdr() value after skb_tunnel_check_pmtu() [ Upstream Date: Sun, 19 Jul 2026 21:06:19 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-63993 X-AL-KERNEL-Priority: IMPORTANT X-AL-KERNEL-Severity: IMPORTANT X-AL-KERNEL-Base-Severity: IMPORTANT X-AL-KERNEL-KPANIC: YES X-AL-KERNEL-ActionableScore: 7 X-AL-KERNEL-ActionableScore-Lower: 5 X-AL-KERNEL-Commit: 6b8bfce9d2f774d2c2243e0248e03efb99bba6c0 List-Id: CVE: CVE-2026-63993 Priority: IMPORTANT AL-KERNEL base severity: IMPORTANT KPANIC flag: YES Patch: vxlan: do not reuse cached ip_hdr() value after skb_tunnel_check_pmtu() [ Upstream Commit: 6b8bfce9d2f774d2c2243e0248e03efb99bba6c0 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6b8bfce9d2f774d2c2243e0248e03efb99bba6c0 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63993 Analysis date: Sun, 19 Jul 2026 21:06:19 -0400 ActionableScore: 7 ActionableScore lower bound: 5 Actionable bucket: Strong Important candidate / Actionable Moderate at minimum Manual review required: YES Summary: A UAF can occur in the VXLAN transmit path when a cached IP header pointer is reused after PMTU handling changes `skb->head`, allowing network traffic through a configured VXLAN path to potentially crash the kernel. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-63993 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63993 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: IMPORTANT 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-2026-63993 IMPORTANT CHECK WITH IMPACT FROM ORIG NN IMPORTANT Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H';*CWE-416;CWE-825;*CWE-672;Other CVSS 'AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A use after free can occur in the VXLAN transmit path because vxlan_xmit_one reused a cached ip_hdr pointer after skb_tunnel_check_pmtu. That helper can change skb head, making the saved old_iph pointer stale before it is passed to ip_tunnel_ecn_encap. A remote or adjacent packet source may be able to trigger the path when traffic is forwarded or bridged through a configured VXLAN device and PMTU handling causes skb head changes. For the CVSS the PR:N is used because the attacker does not need a local account on the victim if they can inject traffic into the affected forwarding path. The base score keeps AC:H because the trigger depends on tunnel configuration, packet shape, and PMTU behavior. Impact is at least denial of service via kernel crash. In the paranoid score, limited confidentiality and integrity impact are kept because this is a real UAF read in network code, but no arbitrary write primitive is demonstrated.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7) YES UAF NETWORK SKB LINUS IPV6 KPANIC PACKET NO NO checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=7 ActionableScoreLower=5 ## 1. ActionableScore * Conservative score: 5 * Paranoid score: 7 * Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**:: ## 2. Signal breakdown * Remote reachable / network-triggerable: +2 VXLAN transmit processing can be reached by traffic forwarded or bridged through a configured VXLAN device. This is not a purely local ioctl or admin-only control path. * Generic memory corruption: +2 The commit explicitly states that reusing the cached `old_iph` after `skb_tunnel_check_pmtu()` can cause a UAF. * Real lifetime corruption: +1 `skb_tunnel_check_pmtu()` can change `skb->head`, making the previously cached IP header pointer stale. * Reliable kernel crash / strong DoS: +1 A stale pointer dereference in the network transmit path is a credible kernel crash / DoS condition. * Common packet-processing path: +1 VXLAN is a common overlay networking technology in virtualization, cloud, and container networking environments. * Hard or configuration-dependent trigger: -1 Exploitation depends on VXLAN being configured and on PMTU handling causing `skb->head` changes. * Constrained primitive / no demonstrated write: -1 The visible use is passing a stale IP header pointer into ECN encapsulation logic. The patch does not show arbitrary write, callback control, type confusion, or attacker-controlled object replacement. * Paranoid confidentiality impact plausible: +1 Because this is a real UAF read in packet-processing code, limited stale-memory exposure or influence cannot be fully ruled out without deeper manual review. * Paranoid integrity impact plausible: +1 Integrity impact is weaker than availability, but stale header data could theoretically affect encapsulation metadata. No arbitrary write primitive is demonstrated. ## 3. Reachability analysis An attacker does not need a local account on the host if they can inject packets into a traffic path that is forwarded or bridged through the affected VXLAN device. In CVSS-like terms this supports PR:N for the attacker in exposed overlay-network deployments. The path is not universally default-enabled because VXLAN must be configured by an administrator or orchestration system. However, VXLAN is common in cloud, virtualization, Kubernetes, and data-center overlays, so this should not be treated as a rare subsystem. Namespaces and containers can matter operationally. A tenant workload or container may not configure VXLAN itself, but may still be able to generate traffic that traverses a host-managed VXLAN overlay. The realistic trigger still requires packet shape and PMTU behavior that causes `skb_tunnel_check_pmtu()` to modify `skb->head`, so attack complexity remains high. Call-site confidence: high. The changed call sites are directly in `vxlan_xmit_one()`, and the commit explicitly describes the stale pointer lifetime issue. ## 4. Severity interpretation This behaves above an ordinary Moderate because it is a network-path UAF candidate in a widely deployed tunneling subsystem. The realistic demonstrated impact is primarily DoS through a stale pointer dereference after `skb->head` changes. It should not automatically be treated as full Important with arbitrary code execution or strong LPE because the patch does not show attacker-controlled reclaim, stale object replacement, write-after-free, callback dispatch, or arbitrary write. The paranoid score is justified for manual triage sensitivity because early UAF issues in networking code are historically easy to underestimate, but the conservative score remains Actionable Moderate. ## 5. One-sentence report phrase A UAF can occur in the VXLAN transmit path when a cached IP header pointer is reused after PMTU handling changes `skb->head`, allowing network traffic through a configured VXLAN path to potentially crash the kernel. ## 6. Manual review recommendation MANUAL CHECK REQUIRED This is a network-reachable UAF candidate in VXLAN packet processing. Even though the visible primitive appears constrained and mainly DoS-oriented, the combination of networking reachability, stale pointer lifetime corruption, and common overlay deployments makes it unsafe to auto-close without manual review. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: vxlan: do not reuse cached ip_hdr() value after skb_tunnel_check_pmtu() [ Upstream Commit: 6b8bfce9d2f774d2c2243e0248e03efb99bba6c0 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6b8bfce9d2f774d2c2243e0248e03efb99bba6c0 Changed files: drivers/net/bareudp.c drivers/net/geneve.c drivers/net/vxlan/vxlan_core.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=6b8bfce9d2f774d2c2243e0248e03efb99bba6c0 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-63993 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63993 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:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H The Best / paranoid CVSS score: 7 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: 5 Paranoid ActionableScore: 7 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: IMPORTANT KPANIC detected for this report: YES Published priority for this report (same as in Subject): IMPORTANT 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).