CVE-2025-21971 is a vulnerability identified in the Linux kernel's network scheduler (net_sched) subsystem, specifically related to the handling of traffic control (tc) classes. The vulnerability arises from the function qdisc_tree_reduce_backlog(), which traverses the queueing discipline (qdisc) tree to update parent backlog counters. This function uses the special class identifier TC_H_ROOT (0xFFFFFFFF) as a termination condition to identify the root of the qdisc tree. However, the vulnerability occurs because it is possible to create a qdisc class with the classid set to TC_H_ROOT. When this happens, the traversal prematurely terminates at this class rather than reaching the actual root qdisc. This leads to incorrect maintenance of parent backlog statistics. In particular, for the Deficit Round Robin (DRR) qdisc, this miscalculation can cause a kernel crash, as reported by the researcher Mingi Cho. The root cause is that the system does not prevent the creation of classes with the reserved classid TC_H_ROOT, which should be unique and reserved for the root qdisc. The fix implemented prevents the creation of any qdisc class with the classid TC_H_ROOT across all qdisc types, thereby ensuring the traversal logic functions correctly and preventing potential crashes. This vulnerability affects multiple versions of the Linux kernel identified by the commit hash 066a3b5b2346febf9a655b444567b7138e3bb939. No known exploits are currently reported in the wild, and no CVSS score has been assigned yet.

For European organizations, this vulnerability poses a risk primarily to systems running vulnerable Linux kernel versions that utilize the network scheduler subsystem, especially those employing the Deficit Round Robin (DRR) qdisc for traffic shaping and quality of service. A successful exploitation could lead to kernel crashes, resulting in denial of service (DoS) conditions. This could disrupt critical network services, degrade performance, or cause outages in environments relying on Linux-based routers, firewalls, or servers handling network traffic management. In sectors such as telecommunications, finance, healthcare, and critical infrastructure—where Linux is widely deployed—such disruptions could have significant operational and financial consequences. Additionally, while no known exploits exist currently, the vulnerability's nature could allow a local attacker or a malicious process with sufficient privileges to trigger the crash, potentially as part of a broader attack to degrade system availability or cause instability. The incorrect backlog statistics could also impact network traffic management accuracy, potentially leading to suboptimal network performance or security monitoring blind spots.

European organizations should prioritize updating their Linux kernel to versions that include the patch preventing the creation of classes with the TC_H_ROOT classid. Since the vulnerability is rooted in the kernel's network scheduler, kernel updates from trusted Linux distributions should be applied promptly. Network administrators should audit their systems to identify any custom or third-party qdisc configurations that might attempt to create classes with reserved classids and remove or correct them. Additionally, organizations should implement strict access controls to limit who can modify qdisc configurations, as exploitation requires the ability to create or modify traffic control classes. Monitoring kernel logs for unusual qdisc-related errors or crashes can provide early detection of attempted exploitation. For critical systems, consider deploying kernel live patching solutions to minimize downtime during patch application. Finally, maintain a robust incident response plan to quickly address any service disruptions potentially caused by this vulnerability.