CVE-2025-21858 is a high-severity use-after-free vulnerability in the Linux kernel's geneve network tunneling driver. The issue arises in the geneve_find_dev() function due to improper handling of linked lists that track geneve devices across network namespaces. Specifically, when a geneve device is destroyed, its linked list pointer (geneve_dev.next) remains linked to the backend UDP socket's network namespace list. If the network namespace associated with the device is dismantled and the device freed, subsequent creation of a new geneve device in the same namespace can trigger a use-after-free condition when geneve_find_dev() accesses the stale pointer. This vulnerability was reported by the syzkaller fuzzing tool and confirmed by kernel address sanitizer (KASAN) reports indicating slab-use-after-free errors. The root cause is that geneve_exit_batch_rtnl() calls unregister_netdevice_queue() to free devices but does not unlink geneve_dev.next from the geneve_list, leading to dangling pointers. The fix involves calling geneve_dellink() in geneve_destroy_tunnels() to properly unlink devices before freeing them. Exploitation requires local privileges with the ability to create or destroy geneve devices, which typically requires at least limited kernel privileges. The vulnerability impacts confidentiality, integrity, and availability as it can lead to kernel memory corruption, potentially enabling privilege escalation or denial of service. The CVSS 3.1 score is 7.8 (high), reflecting local attack vector, low complexity, required privileges, no user interaction, and high impact on confidentiality, integrity, and availability. This vulnerability affects Linux kernel versions containing the vulnerable geneve driver code prior to the patch. No known exploits are reported in the wild yet.

For European organizations, this vulnerability poses a significant risk particularly to environments running Linux servers or network infrastructure that utilize geneve tunnels for network virtualization or overlay networks. Geneve is commonly used in cloud, data center, and container orchestration platforms to encapsulate Layer 2 traffic over Layer 3 networks. Exploitation could allow attackers with limited local privileges to corrupt kernel memory, potentially leading to privilege escalation, unauthorized data access, or system crashes. This could disrupt critical services, compromise sensitive data, and impact business continuity. Organizations relying on Linux-based virtualized infrastructure, NFV (Network Function Virtualization), or SDN (Software Defined Networking) solutions that employ geneve tunnels are especially at risk. The vulnerability's exploitation could also affect multi-tenant cloud environments, increasing the risk of cross-tenant attacks. Given the high impact on confidentiality, integrity, and availability, European enterprises must prioritize patching to maintain compliance with data protection regulations such as GDPR and to safeguard critical infrastructure.

1. Apply the official Linux kernel patches that fix the geneve use-after-free vulnerability as soon as they become available. Monitor kernel updates from trusted sources and test patches in staging environments before production deployment. 2. Restrict access to network namespace and geneve device creation to trusted administrators only, minimizing the risk of local exploitation. 3. Implement kernel hardening features such as Kernel Address Sanitizer (KASAN) and Kernel Page Table Isolation (KPTI) where possible to detect and mitigate memory corruption issues. 4. Employ strict container and virtualization security policies to limit container escape or privilege escalation attempts via geneve devices. 5. Monitor system logs and kernel messages for unusual activity related to geneve devices or network namespace operations. 6. Use security tools that can detect anomalous kernel behavior or memory corruption attempts. 7. In environments where immediate patching is not possible, consider disabling geneve tunneling if it is not essential or replacing it with alternative tunneling protocols until patched.