From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-63913][MODERATE 7.0] netfilter: conntrack: tcp: do not force CLOSE on invalid-seq RST without direction check Date: Sun, 19 Jul 2026 19:49:28 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-63913 X-AL-KERNEL-Priority: MODERATE 7.0 X-AL-KERNEL-Severity: MODERATE 7.0 X-AL-KERNEL-Base-Severity: MODERATE X-AL-KERNEL-KPANIC: YES X-AL-KERNEL-ActionableScore: 5 X-AL-KERNEL-ActionableScore-Lower: 4 X-AL-KERNEL-Commit: 2006979a15af5404bf932a325357683c0bac1656 List-Id: CVE: CVE-2026-63913 Priority: MODERATE 7.0 AL-KERNEL base severity: MODERATE KPANIC flag: YES Patch: netfilter: conntrack: tcp: do not force CLOSE on invalid-seq RST without direction check Commit: 2006979a15af5404bf932a325357683c0bac1656 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2006979a15af5404bf932a325357683c0bac1656 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63913 Analysis date: Sun, 19 Jul 2026 19:49:28 -0400 ActionableScore: 5 ActionableScore lower bound: 4 Actionable bucket: Actionable Moderate at minimum Manual review required: NO Summary: A logic flaw in netfilter TCP conntrack allows crafted invalid-sequence RST traffic to force a tracked connection into TCP_CONNTRACK_CLOSE without validating packet direction, potentially causing premature NAT or conntrack entry teardown and network DoS. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-63913 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63913 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: NO A detailed explanation of the methodology and priority rules is included at the end of this message. ====================================================================== AL-KERNEL CLASSIFICATION RESULT ====================================================================== CVE-2026-63913 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:L/A:H';*CWE-20;CWE-345;CWE-754;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:L';BEST CVSS score: '6.5';DESCR 'A logic flaw in the netfilter TCP conntrack state machine can allow an invalid sequence RST packet to force a tracked connection into TCP_CONNTRACK_CLOSE without confirming that the packet is in the expected reply direction. This can prematurely terminate a conntrack or NAT entry after a crafted SYN and invalid RST sequence, making connection teardown easier than intended. For the CVSS the PR:N is used because the attacker does not need local privileges when crafted TCP packets can reach the conntrack or NAT path. AC:H is used because reliable impact against a victim flow usually depends on matching the relevant flow state and packet context. The issue is network reachable on systems using TCP conntrack or NAT, but it is not a memory corruption bug. Impact is denial of service and limited integrity impact against connection tracking state rather than privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5) YES REMOTE DANGER SIMPLEFIX NETFILTER NETWORK KPANIC PACKET NO NO checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=5 ActionableScoreLower=4 ## 1. ActionableScore * Conservative score: 4 * Paranoid score: 5 * Final recommended bucket: **Actionable Moderate at minimum**:: ## 2. Signal breakdown Triggered signals: * Remote reachable / network-triggerable: +2 Crafted TCP packets can reach the conntrack/NAT packet path when the system uses TCP conntrack or NAT. * Common networking core path or packet-processing path: +1 The bug is in netfilter conntrack TCP state handling, which is a central packet-processing path on firewall/NAT systems. * Integrity impact plausible: +1 The attacker can alter conntrack state by forcing TCP_CONNTRACK_CLOSE earlier than intended, affecting NAT/session state correctness. * Availability impact realistic: +1 in paranoid score only Premature NAT entry teardown can disrupt active or establishing connections. This is not a kernel crash, so I count it only for the paranoid/actionable score. Subtracted signals: * Hard or unreliable sequencing / context requirement: -1 Reliable impact depends on packet ordering and matching a relevant conntrack state after SYN observation, so this is not a trivial one-packet universal DoS. Not triggered: * No generic memory corruption. * No UAF, OOB write, double free, refcount takeover, or page-cache corruption. * No privilege escalation primitive. * No kernel crash or host-wide panic shown by the patch. ## 3. Reachability analysis The issue can be triggered by a remote or adjacent attacker able to send crafted TCP packets through a firewall/NAT path using nf_conntrack. No local account is required in the network-facing interpretation, so PR:N is reasonable for CVSS-style reasoning. Namespaces and containers do not matter for the basic remote packet path, but local containerized workloads may also trigger it if their traffic traverses host conntrack/NAT. The affected functionality is common on Linux firewalls, routers, container hosts, and NAT gateways, though impact depends on whether the attacker can influence packet flow state sufficiently. ## 4. Severity interpretation This behaves like an actionable Moderate networking logic flaw rather than an Important memory-safety vulnerability. The practical impact is premature conntrack/NAT teardown, causing connection disruption and limited state-integrity impact. There is no evidence of memory corruption, kernel code execution, information disclosure, or local privilege escalation. The paranoid score reaches 5 because the bug is network reachable in a common packet-processing subsystem and can affect NAT/firewall state. It should not be auto-closed as a trivial Moderate, but it also does not justify Important-class handling without evidence of broader connection hijacking, persistent firewall bypass, or memory corruption. ## 5. One-sentence report phrase A logic flaw in netfilter TCP conntrack allows crafted invalid-sequence RST traffic to force a tracked connection into TCP_CONNTRACK_CLOSE without validating packet direction, potentially causing premature NAT or conntrack entry teardown and network DoS. ## 6. Manual review recommendation MANUAL CHECK RECOMMENDED Reason: network-reachable netfilter/NAT behavior can have deployment-specific impact, especially on firewalls, NAT gateways, and container hosts. However, the patch supports a logic/state DoS only, not memory corruption or privilege escalation. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: netfilter: conntrack: tcp: do not force CLOSE on invalid-seq RST without direction check Commit: 2006979a15af5404bf932a325357683c0bac1656 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2006979a15af5404bf932a325357683c0bac1656 Commit description: An unintended behavior in the TCP conntrack state machine allows a connection to be forced into the CLOSE state using an RST packet with an invalid sequence number. Specifically, after a SYN packet is observed, an RST with an invalid SEQ can transition the conntrack entry to TCP_CONNTRACK_CLOSE, regardless of whether the RST corresponds to the expected reply direction. The relevant code path assumes the RST is a response to an outgoing SYN, but does not validate packet direction or ensure that a matching SYN was actually sent in the opposite direction. As a result, a crafted packet sequence consisting of a SYN followed by an invalid-sequence RST can prematurely terminate an active NAT entry. This makes connection teardown easier than intended. So, tighten the state transition logic to ensure that RST-triggered CLOSE transitions only occur when the RST is a valid response to a previously observed SYN in the correct direction. Cc: stable@vger.kernel.org Fixes: 9fb9cbb ("[NETFILTER]: Add nf_conntrack subsystem.") Signed-off-by: Hamza Mahfooz Signed-off-by: Florian Westphal Signed-off-by: Greg Kroah-Hartman Changed files: net/netfilter/nf_conntrack_proto_tcp.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=2006979a15af5404bf932a325357683c0bac1656 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-63913 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63913 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:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:L The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:H The Best / paranoid CVSS score: 6.5 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: 4 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: YES 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).