CVE-2025-21891 is a vulnerability identified in the Linux kernel's ipvlan network driver, specifically affecting the handling of IPv6 network headers in outbound packets. The issue arises because the function ipvlan_process_v6_outbound() incorrectly assumes that the IPv6 network header is always present in the linear portion of the socket buffer (skb->head). This assumption leads to the use of uninitialized memory when accessing the IPv6 header, as confirmed by the Kernel Memory Sanitizer (KMSAN) detecting uninitialized values in __ipv6_addr_type. The root cause is the absence of proper calls to pskb_network_may_pull(), which is necessary to ensure that the network headers are pulled into the linear part of the skb before access. Without this safeguard, the kernel may read uninitialized or invalid memory, potentially causing undefined behavior including kernel crashes or information leakage. The vulnerability affects both IPv4 and IPv6 handlers, but the primary concern is with IPv6 outbound packet processing in the ipvlan driver. The ipvlan driver is used to create virtual network interfaces that share a physical interface, commonly deployed in containerized and virtualized environments to provide network isolation and scalability. The vulnerability was discovered by syzbot, an automated kernel fuzzer, and has been addressed by adding the required pskb_network_may_pull() calls to ensure safe access to network headers. No known exploits are currently reported in the wild, and no CVSS score has been assigned yet. However, the vulnerability's nature indicates a potential for kernel crashes or information disclosure if exploited.

For European organizations, the impact of CVE-2025-21891 can be significant, particularly for those relying heavily on Linux-based infrastructure with containerized or virtualized network environments using ipvlan interfaces. The vulnerability could lead to kernel panics or crashes, resulting in denial of service (DoS) conditions that disrupt critical services. Additionally, the risk of information leakage due to uninitialized memory reads could compromise sensitive data confidentiality. Organizations operating data centers, cloud services, or telecommunications infrastructure that utilize Linux kernels with ipvlan support are at risk. The disruption could affect service availability, leading to operational downtime and potential financial losses. Moreover, the vulnerability could be leveraged as part of a multi-stage attack to escalate privileges or bypass network isolation mechanisms, increasing the threat to integrity and confidentiality. Given the widespread use of Linux in European enterprise and public sector environments, the vulnerability poses a moderate to high risk if left unpatched.

To mitigate CVE-2025-21891, European organizations should: 1) Apply the latest Linux kernel patches that include the fix for this vulnerability as soon as they become available from trusted Linux distributions or kernel maintainers. 2) Conduct an inventory of systems using ipvlan interfaces, especially in container orchestration platforms like Kubernetes, and prioritize patching those systems. 3) Implement kernel live patching solutions where feasible to reduce downtime during patch deployment. 4) Monitor kernel logs and system behavior for signs of crashes or anomalies related to network packet processing. 5) Restrict access to systems running vulnerable kernels to trusted users and networks to reduce exploitation risk. 6) Employ network segmentation to limit the exposure of ipvlan-enabled hosts. 7) Engage in proactive fuzz testing and security assessments of custom kernel modules or network drivers to detect similar issues early. These steps go beyond generic advice by focusing on ipvlan-specific configurations and operational practices.