CVE-2025-21867 is a high-severity use-after-free vulnerability in the Linux kernel's BPF (Berkeley Packet Filter) subsystem, specifically within the eth_skb_pkt_type() function. This function is responsible for determining the packet type of Ethernet frames. The vulnerability arises because eth_skb_pkt_type() accesses the socket buffer's (skb) data assuming it contains a valid Ethernet header, but under certain conditions, it does not. This occurs when the bpf_prog_test_run_xdp() function passes an invalid user_data argument to bpf_test_init(), which leads to eth_skb_pkt_type() operating on freed memory. The root cause is that bpf_test_init() did not properly validate that user_data was at least the size of an Ethernet header (ETH_HLEN), allowing invalid values to trigger the use-after-free. The kernel memory sanitizer (KMSAN) detected this flaw, which can cause memory corruption, crashes, or potentially arbitrary code execution in kernel space. The fix involves returning an error if user_data is less than ETH_HLEN and removing an unnecessary size check. The vulnerability affects Linux kernel versions including commit be3d72a2896cb24090f268dce4aa8a304d40bc23 and likely others in the 6.x series. The CVSS 3.1 score is 7.8 (high), reflecting local attack vector with low complexity, requiring low privileges but no user interaction, and impacting confidentiality, integrity, and availability. No known exploits are reported in the wild yet. This vulnerability is categorized under CWE-416 (Use After Free).

For European organizations, this vulnerability poses a significant risk, especially for those relying heavily on Linux-based infrastructure such as servers, network appliances, cloud environments, and embedded systems. Exploitation could allow an attacker with local access and low privileges to escalate privileges, execute arbitrary code in kernel context, or cause denial of service through kernel crashes. This can lead to data breaches, service outages, and compromise of critical systems. Given the widespread use of Linux in European data centers, telecommunications, financial institutions, and government agencies, the impact could be severe. Additionally, environments using eBPF for network monitoring, security, or performance tuning are particularly exposed. The lack of required user interaction and the low complexity of exploitation increase the risk of rapid exploitation once a public exploit becomes available. The vulnerability could also affect virtualized environments and containers running Linux kernels, which are common in European cloud deployments.

European organizations should prioritize patching affected Linux kernels to versions where this vulnerability is fixed. Specifically, update to kernel versions that include the fix for CVE-2025-21867 or apply vendor-provided patches promptly. In environments where immediate patching is not feasible, consider restricting access to systems running vulnerable kernels to trusted users only, as exploitation requires local privileges. Disable or limit the use of BPF test interfaces such as bpf_prog_test_run_xdp() if not needed, to reduce attack surface. Employ kernel hardening techniques like SELinux or AppArmor to restrict kernel module loading and BPF program execution. Monitor system logs and kernel messages for unusual activity related to BPF or skb processing. For cloud and container environments, ensure host kernels are patched and consider runtime security tools that detect anomalous kernel behavior. Finally, maintain an inventory of Linux kernel versions in use across the organization to identify vulnerable systems quickly.