From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-63983][MODERATE REGULAR] net/sched: fix packet loop on netem when duplicate is on [ Upstream Date: Sun, 19 Jul 2026 17:09:14 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-63983 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: 1 X-AL-KERNEL-Commit: 1a298a514ce766c6d0c232991a390fec67af81ad List-Id: CVE: CVE-2026-63983 Priority: MODERATE REGULAR AL-KERNEL base severity: MODERATE KPANIC flag: NO Patch: net/sched: fix packet loop on netem when duplicate is on [ Upstream Commit: 1a298a514ce766c6d0c232991a390fec67af81ad Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1a298a514ce766c6d0c232991a390fec67af81ad Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63983 Analysis date: Sun, 19 Jul 2026 17:09:14 -0400 ActionableScore: 4 ActionableScore lower bound: 1 Actionable bucket: Borderline manual review recommended / Actionable Moderate if exposed Manual review required: YES Summary: A packet loop in netem duplication can recursively re-enqueue duplicated skbs through another netem qdisc until stack or memory exhaustion occurs, causing a conditional local or network-triggered denial of service when a vulnerable qdisc hierarchy is configured. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-63983 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63983 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-2026-63983 MODERATE CHECK WITH IMPACT FROM ORIG NN IMPORTANT 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:N/A:H';CWE-674;**CWE-400;CWE-362;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'netem packet duplication can create a recursive enqueue loop when a duplicated skb is re inserted at the root qdisc and then traverses another netem qdisc that duplicates it again. This can continue until kernel stack or memory is exhausted, causing denial of service. For the CVSS the PR:L paranoid interpretation is used because an untrusted local user or delegated service may be able to generate traffic through an already configured vulnerable netem hierarchy, while creating that hierarchy normally requires CAP_NET_ADMIN. The issue can be indirectly triggered by network traffic only in deployments where attacker controlled packets traverse the affected qdisc tree. It is not a generic remote attack against default kernels. Impact is denial of service only, with no clear evidence of information disclosure or privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review recommended / Actionable Moderate if exposed (with actual score 3) YES REMOTE LOCK SIMPLEFIX NETWORK SKB QDISC PACKET LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5 NO NO checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=4 ActionableScoreLower=1 ## 1. ActionableScore * Conservative score: 1 * Paranoid score: 4 * Final recommended bucket: **Borderline manual review recommended / Actionable Moderate if exposed** ## 2. Signal breakdown Paranoid scoring: * Remote reachable / network-triggerable: +2 Conditional only. If attacker-controlled packets traverse an already configured vulnerable netem qdisc hierarchy, packet input can trigger the loop. * Reliable kernel crash / strong DoS: +1 Commit explicitly says recursion can continue until stack or memory is exhausted. * Availability impact realistic: +1 Stack or memory exhaustion is host-wide availability impact, not just a warning. * Common packet-processing path: +1 The bug is in qdisc enqueue handling for packets traversing traffic control. * Rare / non-default configuration: -1 Requires netem duplicate and another netem in the qdisc tree. This is not a default network path. Conservative scoring: * Reliable kernel crash / strong DoS: +1 * Availability impact realistic: +1 * Common packet-processing path: +1 * Requires CAP_NET_ADMIN to configure vulnerable topology: -2 * Rare / non-default configuration: -1 No memory corruption, UAF, arbitrary write, info leak, or privilege escalation primitive is shown by the patch. ## 3. Reachability analysis A fully conservative trigger requires a local administrator or CAP_NET_ADMIN capable actor to configure a netem hierarchy with duplication enabled. After such a hierarchy exists, the actual trigger may be just packet traversal, so a local unprivileged process or a remote peer may be able to trigger the DoS if its traffic crosses the affected qdisc tree. Namespace delegation matters. A container or service with CAP_NET_ADMIN in a network namespace may be able to configure qdisc state, and the resulting kernel crash would still affect the host kernel. In typical hardened deployments, untrusted containers are not given CAP_NET_ADMIN, so exposure is configuration dependent. The path is not default-enabled. netem is commonly available but normally used for testing, traffic shaping, or lab environments rather than default production forwarding. ## 4. Severity interpretation This behaves like a resource-exhaustion DoS rather than a memory corruption vulnerability. The realistic impact is kernel stack or memory exhaustion leading to system crash or severe availability loss. It is not an Important-class candidate based on the patch alone because there is no evidence of UAF, object reuse, controlled overwrite, information disclosure, or privilege escalation. It is still worth manual review when netem is exposed through delegated CAP_NET_ADMIN or when attacker-controlled network traffic can traverse a preconfigured affected qdisc tree. ## 5. One-sentence report phrase A packet loop in netem duplication can recursively re-enqueue duplicated skbs through another netem qdisc until stack or memory exhaustion occurs, causing a conditional local or network-triggered denial of service when a vulnerable qdisc hierarchy is configured. ## 6. Manual review recommendation MANUAL CHECK RECOMMENDED Manual review is recommended because this is a packet-triggered kernel DoS in a networking path, but the issue is configuration dependent and does not show a memory corruption or privilege escalation primitive. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: net/sched: fix packet loop on netem when duplicate is on [ Upstream Commit: 1a298a514ce766c6d0c232991a390fec67af81ad Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1a298a514ce766c6d0c232991a390fec67af81ad Changed files: net/sched/sch_netem.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=1a298a514ce766c6d0c232991a390fec67af81ad ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-63983 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63983 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:N/A:H The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H The Best / paranoid CVSS score: 5.5 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: 1 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).