CVE-2025-21865 is a vulnerability in the Linux kernel's GPRS Tunneling Protocol (GTP) implementation, specifically within the gtp_net_exit_batch_rtnl() function. The issue arises from improper handling of device destruction across multiple network namespaces (netns). The vulnerability is triggered when the function attempts to destroy GTP devices associated with UDP sockets that span different network namespaces. The flawed logic causes the device's deletion routine (gtp_dellink()) to be called multiple times on the same device during the network namespace cleanup process. This leads to list corruption in the kernel's linked list management, particularly when CONFIG_DEBUG_LIST is enabled, causing kernel BUGs and system crashes. The root cause is that the for_each_netdev() loop in gtp_net_exit_batch_rtnl() redundantly calls the device deletion callback for devices that are linked to multiple netns, resulting in double unlinking and corruption of kernel data structures. The fix involves removing the redundant for_each_netdev() loop and delegating device destruction to the default_device_exit_batch() function, aligning with the approach used in other tunneling protocols like bareudp. This vulnerability can cause system instability, kernel panics, and denial of service due to corrupted kernel lists and invalid memory operations during network namespace teardown. It affects Linux kernel versions prior to the patch commit eb28fd76c0a0 and is particularly relevant for systems using GTP tunnels, commonly found in mobile network infrastructure and telecommunications equipment. No known exploits are reported in the wild as of the publication date.

For European organizations, the impact of CVE-2025-21865 can be significant, especially for those operating telecommunications infrastructure, mobile network equipment, or cloud environments that utilize Linux-based systems with GTP tunneling. The vulnerability can lead to kernel crashes and denial of service, potentially disrupting critical network services and communications. This is particularly relevant for telecom operators, internet service providers, and enterprises relying on virtualized network functions or containers that use network namespaces extensively. The instability caused by kernel list corruption could also lead to broader system outages, affecting service availability and operational continuity. Given the widespread use of Linux in European data centers and telecom infrastructure, unpatched systems could face increased risk of service degradation or downtime. While no direct exploitation is known, the complexity and severity of kernel panics make this a high-impact vulnerability for environments requiring high availability and reliability. Additionally, the vulnerability could be leveraged as part of a multi-stage attack to escalate privileges or disrupt network functions if combined with other vulnerabilities.

To mitigate CVE-2025-21865, European organizations should: 1) Immediately apply the official Linux kernel patches that address the vulnerability, specifically the commit eb28fd76c0a0 or later stable kernel releases containing the fix. 2) Conduct an inventory of systems running affected Linux kernel versions, focusing on those utilizing GTP tunnels or network namespaces, such as telecom infrastructure, mobile core network elements, and containerized environments. 3) Enable kernel debugging and monitoring tools to detect abnormal kernel list operations or unexpected device deletions during network namespace cleanup. 4) Implement strict change management and testing procedures for kernel updates in production environments to minimize downtime. 5) Consider isolating or segmenting network namespaces and GTP tunnel usage to limit the blast radius of potential kernel crashes. 6) Engage with Linux distribution vendors and telecom equipment providers to ensure timely updates and coordinated vulnerability response. 7) Monitor security advisories for any emerging exploits or related vulnerabilities in the GTP kernel module. These steps go beyond generic advice by focusing on the specific affected components, operational contexts, and proactive detection strategies.