CVE-2025-21791 is a high-severity vulnerability in the Linux kernel related to the handling of routing virtual function (vrf) devices within the l3mdev_l3_out() function. The vulnerability arises because l3mdev_l3_out() can be invoked without holding the Read-Copy-Update (RCU) lock, which is necessary to safely access certain kernel data structures concurrently. Specifically, the call chain raw_sendmsg() -> ip_push_pending_frames() -> ip_send_skb() -> ip_local_out() -> __ip_local_out() -> l3mdev_ip_out() leads to l3mdev_l3_out() being called without RCU protection. This lack of synchronization can result in a Use-After-Free (UAF) condition (CWE-416), where the kernel accesses memory that has already been freed, potentially leading to memory corruption. The fix involves adding rcu_read_lock() and rcu_read_unlock() pairs around the vulnerable code to ensure proper synchronization and prevent the UAF. The vulnerability has a CVSS 3.1 score of 7.8, indicating high severity, with an attack vector of local access (AV:L), low attack complexity (AC:L), requiring low privileges (PR:L), no user interaction (UI:N), unchanged scope (S:U), and high impact on confidentiality, integrity, and availability (C:H/I:H/A:H). No known exploits are currently reported in the wild. This vulnerability affects multiple Linux kernel versions identified by specific commit hashes, indicating it is present in recent kernel builds before the patch. Given the Linux kernel's widespread use in servers, desktops, embedded systems, and cloud infrastructure, this vulnerability poses a significant risk if exploited locally by an attacker with limited privileges to escalate rights or cause denial of service via kernel memory corruption.

For European organizations, the impact of CVE-2025-21791 can be substantial due to the pervasive use of Linux in enterprise servers, cloud environments, telecommunications infrastructure, and critical industrial systems. Exploitation could allow a local attacker to execute arbitrary code in kernel space, leading to privilege escalation, data breaches, or system crashes. This could disrupt business operations, compromise sensitive data, and affect service availability. Organizations relying on Linux-based network devices or virtual routing functions are particularly at risk. The high impact on confidentiality, integrity, and availability means that critical sectors such as finance, healthcare, government, and energy could face severe operational and reputational damage. Additionally, the lack of required user interaction and low privilege requirements lower the barrier for attackers, increasing the threat likelihood. Although no exploits are known in the wild yet, the vulnerability's nature and severity warrant urgent attention to prevent potential targeted attacks or exploitation by insider threats.

European organizations should prioritize patching affected Linux kernel versions as soon as vendor updates become available. Until patches are applied, implement strict access controls to limit local user privileges and restrict access to systems running vulnerable kernel versions. Employ kernel hardening techniques such as SELinux or AppArmor to contain potential exploitation. Regularly audit and monitor system logs for unusual kernel activity or crashes indicative of exploitation attempts. Use intrusion detection systems capable of detecting kernel-level anomalies. For environments where patching is delayed, consider isolating vulnerable systems from critical networks and sensitive data. Engage with Linux distribution vendors to obtain timely security updates and verify that kernel versions in use are not affected. Additionally, review and update incident response plans to include scenarios involving kernel-level vulnerabilities. Avoid running untrusted code or allowing unprivileged users to execute code on critical Linux hosts.