CVE-2025-21759 is a high-severity vulnerability in the Linux kernel related to the IPv6 multicast implementation, specifically within the igmp6_send() function. The issue arises because igmp6_send() can be invoked without holding the RTNL (routing netlink) lock or RCU (Read-Copy-Update) protection, which are synchronization mechanisms used to ensure safe concurrent access to kernel data structures. Without these protections, the function may access a network pointer that has been freed, leading to a Use-After-Free (UAF) condition (CWE-416). This vulnerability can cause the kernel to dereference invalid memory, potentially leading to system crashes, privilege escalation, or arbitrary code execution within the kernel context. The fix involves extending RCU protection to safely fetch the network pointer and changing the memory allocation approach from sock_alloc_send_skb() (which uses GFP_KERNEL allocations that can sleep) to alloc_skb() under RCU protection, ensuring safe socket buffer allocation without risking sleeping in atomic contexts. The vulnerability affects specific Linux kernel versions identified by commit hashes and was published on February 27, 2025. The CVSS v3.1 base score is 7.8, indicating high severity, with attack vector local, low attack complexity, requiring low privileges, no user interaction, and impacting confidentiality, integrity, and availability. No known exploits are currently reported in the wild.

For European organizations, this vulnerability poses a significant risk, especially for those running Linux-based servers, network infrastructure devices, or embedded systems that utilize IPv6 multicast networking. Exploitation could allow attackers with local access or limited privileges to escalate their rights to kernel level, compromising system integrity and confidentiality. This could lead to unauthorized data access, disruption of critical services, or persistent backdoors in critical infrastructure. Given the widespread use of Linux in European data centers, telecommunications, and government systems, the impact could be broad, affecting cloud providers, ISPs, and enterprises relying on Linux for networking and server operations. The vulnerability's potential to cause denial of service or kernel panic also threatens availability, which is critical for sectors such as finance, healthcare, and public administration.

European organizations should prioritize patching affected Linux kernel versions as soon as updates become available from trusted sources or distributions. Since the vulnerability requires local access and low privileges, organizations should also enforce strict access controls and limit user privileges on systems running vulnerable kernels. Network segmentation and monitoring for unusual local activity can help detect exploitation attempts. Employing kernel hardening techniques such as Kernel Address Space Layout Randomization (KASLR), and enabling security modules like SELinux or AppArmor can reduce exploitation risk. For environments where immediate patching is not feasible, disabling or restricting IPv6 multicast features temporarily may mitigate exposure. Regularly auditing kernel versions and configurations, combined with incident response readiness, will further reduce risk.