CVE-2025-21761 is a high-severity vulnerability in the Linux kernel specifically affecting the Open vSwitch (OVS) component. The vulnerability arises from improper use of Read-Copy-Update (RCU) synchronization in the function ovs_vport_cmd_fill_info(). This function can be invoked without holding the necessary RTNL (rtnetlink) lock or RCU protection, leading to a potential Use-After-Free (UAF) condition. UAF vulnerabilities occur when a program continues to use a pointer after the memory it points to has been freed, which can lead to memory corruption, crashes, or arbitrary code execution. In this case, the lack of RCU protection and dev_net_rcu() usage means that concurrent access to network device structures can cause unsafe memory access. The vulnerability affects multiple versions of the Linux kernel identified by the commit hash 9354d452034273a50a4fd703bea31e5d6b1fc20b. The CVSS v3.1 score is 7.8 (high), reflecting the vulnerability's potential to impact confidentiality, integrity, and availability with low attack complexity and requiring low privileges but no user interaction. Although no known exploits are currently reported in the wild, the vulnerability's nature and impact make it a critical concern for systems running Linux kernels with Open vSwitch. The fix involves ensuring proper RCU protection and usage of dev_net_rcu() to prevent the UAF condition.

For European organizations, the impact of CVE-2025-21761 can be significant, especially for those relying on Linux-based infrastructure and Open vSwitch for network virtualization, software-defined networking (SDN), or cloud environments. Exploitation could allow attackers with low privileges to execute arbitrary code in kernel context, leading to full system compromise, data breaches, or denial of service. This can disrupt critical services, including telecommunications, financial services, government operations, and industrial control systems. Given the widespread use of Linux in European data centers, cloud providers, and enterprise environments, the vulnerability poses a risk to confidentiality (data exposure), integrity (unauthorized modification), and availability (service disruption). The absence of required user interaction and the relatively low complexity of exploitation increase the likelihood of targeted attacks or automated exploitation once proof-of-concept code becomes available. Organizations with virtualized network environments or those using Open vSwitch extensively are particularly at risk.

European organizations should prioritize patching Linux kernels to versions that include the fix for CVE-2025-21761. Since the vulnerability is in the Open vSwitch kernel module, updating both the kernel and Open vSwitch packages is critical. Network administrators should audit their environments to identify systems running vulnerable kernel versions and Open vSwitch instances. Employ kernel live patching solutions where possible to minimize downtime. Additionally, implement strict access controls to limit low-privilege user access to systems running Open vSwitch, as the vulnerability requires low privileges but not root. Monitoring kernel logs and network device events for anomalies can help detect exploitation attempts. For environments where immediate patching is not feasible, consider isolating vulnerable systems or disabling Open vSwitch features that invoke ovs_vport_cmd_fill_info(), if operationally possible. Finally, maintain up-to-date intrusion detection and prevention systems tuned to detect exploitation patterns related to UAF vulnerabilities in the Linux kernel.