CVE-2026-74663 in Linux
Summary
by MITRE • 08/22/2026
In the Linux kernel, the following vulnerability has been resolved:
net/sched: reject overly deep qdisc hierarchies
Deep qdisc hierarchies can lead to excessive recursion in qdisc tree walkers and exhaust the kernel stack. The existing loop check does not cover the create-and-graft path, so a hierarchy can still be extended by creating a new child qdisc below an already deep parent.
Store the hierarchy depth in struct Qdisc and update it when qdiscs are grafted. Reject new child qdiscs once the parent is already at the maximum allowed depth.
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Analysis
by VulDB Data Team • 08/22/2026
The Linux kernel networking subsystem contains a critical vulnerability related to queue discipline, or qdisc, management that allows for excessive recursion leading to stack exhaustion and potential system instability. Queue disciplines are fundamental components in the Linux network stack responsible for managing packet queues on network interfaces, handling traffic shaping, prioritization, and congestion control mechanisms such as Token Bucket Filter (TBF), Hierarchical Token Bucket (HTB), or Generic Random Early Detection (GRED). These qdiscs can be organized into hierarchical structures where a parent qdisc manages multiple child qdiscs to implement complex bandwidth allocation policies. The vulnerability arises from the fact that these hierarchies are traversed recursively during packet processing and configuration operations, making them susceptible to stack overflow if the depth of the hierarchy becomes excessive.
The technical flaw lies in an incomplete validation mechanism within the kernel code responsible for creating and grafting new qdiscs onto existing parent nodes. While there was previously a loop detection check intended to prevent circular references or infinite loops during tree traversal, this safeguard did not adequately cover the specific path involved in creating a new child qdisc and attaching it to an already deep parent structure. Consequently, an attacker with local access who possesses sufficient privileges to modify network configuration can repeatedly create nested qdiscs, each one deeper than the last. This action bypasses existing safeguards because the depth check was not applied consistently during the grafting operation where a new child is attached to a parent that may already be several levels deep in the hierarchy.
The operational impact of this vulnerability is severe, primarily manifesting as kernel stack exhaustion due to unbounded recursion. When the qdisc tree walker processes packets or configuration changes across an excessively deep hierarchy, it consumes increasing amounts of kernel stack space with each recursive call. Since Linux kernels have a fixed-size stack for each thread, typically ranging from four to eight kilobytes depending on architecture and configuration, this rapid consumption can quickly lead to a stack overflow condition. Such an event results in a kernel panic or system crash, effectively causing a denial of service against the host machine. In environments where multiple users share access to network configuration tools, such as container orchestration systems or virtualization platforms, this vulnerability could be exploited by unprivileged users who have been granted limited networking capabilities to destabilize the underlying hypervisor or physical server.
This issue is classified under CWE-789: Memory Exhaustion and aligns with ATT&CK technique T1496: Resource Hijacking, specifically through resource exhaustion via denial of service. The vulnerability highlights a common class of issues in systems programming where recursive algorithms lack proper depth limits or boundary checks against available resources like stack space. It underscores the importance of enforcing strict constraints on data structure complexity within kernel-space operations to prevent local privilege escalation scenarios that rely on crashing system services rather than gaining code execution directly.
To mitigate this vulnerability, the Linux kernel developers have implemented a fix that involves storing the hierarchy depth explicitly in the qdisc structure and updating it dynamically whenever new qdiscs are grafted into the tree. The patch introduces a hard limit on the maximum allowed depth for any given parent node. When an attempt is made to create or attach a child qdisc, the system now checks if adding this layer would exceed the predefined threshold. If the parent qdisc has already reached its maximum allowable depth, the operation is rejected with an appropriate error code rather than being processed. This proactive validation ensures that recursion remains bounded and prevents the stack from being exhausted by deeply nested configurations.
Administrators should ensure their systems are updated to include this kernel patch as soon as possible. For distributions providing long-term support kernels, backported fixes may be available in security update channels. In environments where dynamic network configuration is permitted for non-root users, it is advisable to audit existing qdisc hierarchies and restrict the ability to create nested structures through policy enforcement tools like tc (traffic control) wrappers or container runtime configurations that limit networking capabilities. Regular monitoring of system logs for stack overflow warnings can also help detect attempted exploitation before a full crash occurs, although prevention via patching remains the most effective defense strategy against this specific flaw in the network subsystem logic.