CVE-2025-37953 is a vulnerability identified in the Linux kernel's Hierarchical Token Bucket (HTB) queuing discipline implementation, specifically within the functions htb_deactivate() and htb_next_rb_node(). The issue arises due to a NULL pointer dereference in htb_next_rb_node() triggered after modifications made to make htb_qlen_notify() idempotent. The vulnerability is rooted in the sequence of function calls during packet dequeue operations: htb_dequeue_tree() calls fq_codel_dequeue(), which calls qdisc_tree_reduce_backlog(), then htb_qlen_notify(), and subsequently htb_deactivate(). The problem occurs because htb_deactivate() can be called multiple times on the same data structure without proper checks, leading to the clprio[prio]->ptr pointer being set to NULL prematurely. When htb_next_rb_node() subsequently accesses this pointer without verifying its validity, a NULL pointer dereference occurs, potentially causing a kernel crash or denial of service. The fix involves making htb_deactivate() idempotent—ensuring it returns immediately if already called—and adding safety checks in htb_next_rb_node() to handle NULL pointers gracefully. This vulnerability affects multiple Linux kernel versions identified by specific commit hashes and was publicly disclosed on May 20, 2025. No known exploits are currently reported in the wild, and no CVSS score has been assigned yet.

For European organizations, the impact of CVE-2025-37953 could be significant, especially for those relying heavily on Linux-based infrastructure for networking, cloud services, and critical applications. The vulnerability can lead to kernel crashes resulting in denial of service (DoS), which may disrupt network traffic management and degrade service availability. This is particularly critical for ISPs, data centers, and enterprises running Linux servers that use HTB for traffic shaping and quality of service (QoS). While this vulnerability does not directly lead to privilege escalation or remote code execution, the resulting instability could be exploited as part of a broader attack chain or cause operational disruptions. Organizations with high availability requirements or those operating critical infrastructure may experience service outages, impacting business continuity and potentially violating regulatory requirements such as the EU NIS Directive. Given the Linux kernel's widespread use in European IT environments, the scope of affected systems is broad, increasing the potential impact.

To mitigate CVE-2025-37953, European organizations should: 1) Apply the official Linux kernel patches that make htb_deactivate() idempotent and add NULL pointer checks in htb_next_rb_node() as soon as they are available from trusted Linux distribution vendors or the Linux kernel mainline. 2) Prioritize patching on systems that handle significant network traffic shaping or QoS functions, especially routers, firewalls, and network appliances running Linux. 3) Implement robust kernel crash monitoring and alerting to detect and respond quickly to any DoS events potentially related to this vulnerability. 4) Conduct thorough testing in staging environments to ensure that kernel updates do not disrupt existing network traffic management policies. 5) Consider temporary network traffic management adjustments to reduce reliance on HTB queuing disciplines if patching cannot be immediately applied. 6) Maintain up-to-date backups and disaster recovery plans to minimize downtime in case of exploitation. 7) Engage with Linux distribution security advisories and subscribe to vulnerability feeds to stay informed about patch releases and exploit developments.