CVE-2025-37797 is a Use-After-Free (UAF) vulnerability discovered in the Linux kernel's network scheduler component, specifically within the Hierarchical Fair Service Curve (HFSC) queuing discipline (qdisc) class handling. The vulnerability arises from a time-of-check/time-of-use (TOCTOU) race condition in the function hfsc_change_class(). This function initially checks whether a class queue has packets by evaluating the queue length (q.qlen != 0). It then calls qdisc_peek_len(), which for certain child qdiscs such as netem or codel, may drop packets and empty the queue. However, the code erroneously continues under the assumption that the queue remains non-empty and proceeds to add the class to the virtual time tree (vttree). This violates the HFSC scheduler's assumption that only non-empty classes are present in the vttree. Consequently, when the class is later destroyed, this leads to a Use-After-Free condition, where the system attempts to access memory that has already been freed. The vulnerability could potentially be exploited by an attacker with the ability to manipulate network traffic shaping parameters or inject traffic into affected qdiscs, causing kernel memory corruption. The patch for this vulnerability introduces a second queue length check after qdisc_peek_len() to ensure the queue was not emptied before proceeding, thereby preventing the UAF condition. This vulnerability affects Linux kernel versions prior to the patch and is relevant to systems using HFSC with child qdiscs like netem or codel for network traffic control and shaping.

For European organizations, the impact of CVE-2025-37797 could be significant, especially for those relying on Linux-based infrastructure for critical network functions such as traffic shaping, quality of service (QoS) enforcement, or network virtualization. Exploitation of this vulnerability could lead to kernel memory corruption, potentially causing system crashes (denial of service), privilege escalation, or arbitrary code execution within the kernel context. This could compromise the confidentiality, integrity, and availability of affected systems. Organizations operating data centers, telecommunications infrastructure, cloud services, or industrial control systems that utilize Linux with HFSC and child qdiscs like netem or codel are particularly at risk. The vulnerability could be leveraged by attackers to disrupt network performance or gain unauthorized control over systems, impacting business continuity and sensitive data protection. Given the widespread use of Linux in European enterprises and public sector entities, failure to patch this vulnerability promptly could expose them to targeted attacks or opportunistic exploitation, especially in environments where network traffic control is critical.

To mitigate CVE-2025-37797, European organizations should: 1) Immediately apply the official Linux kernel patches that address this vulnerability once available, ensuring all affected systems are updated to a secure kernel version. 2) Audit and inventory all systems using HFSC qdisc with child qdiscs such as netem or codel to identify exposure. 3) Restrict administrative access to network configuration interfaces to trusted personnel only, minimizing the risk of malicious manipulation of qdisc parameters. 4) Implement kernel live patching solutions where feasible to reduce downtime during patch deployment. 5) Monitor kernel logs and network subsystem behavior for anomalies indicative of exploitation attempts, such as unexpected crashes or unusual packet drops. 6) Employ network segmentation and strict firewall rules to limit exposure of vulnerable systems to untrusted networks. 7) Conduct penetration testing and vulnerability scanning focused on network scheduler components to verify patch effectiveness and detect residual risks. These steps go beyond generic advice by focusing on the specific network scheduler context and operational practices relevant to this vulnerability.