CVE-2025-21864 is a vulnerability identified in the Linux kernel's TCP/IP networking stack, specifically related to the handling of security paths (secpath) and destination cache (dst) objects within the context of network namespaces (netns) and IPsec transformations (xfrm). The issue arises during the deletion of network namespaces after TCP tests over IPcomp6 (IP payload compression for IPv6) are run. The vulnerability is due to a lingering reference to an xfrm_state object that is not properly deleted when the network namespace is removed. This occurs because the secpath, which holds a reference to the xfrm_state, remains attached to a socket buffer (skb) that is deferred for freeing on a CPU's defer_list. If the defer_list is not flushed before the netns deletion, the reference to the xfrm_state persists unexpectedly. This can lead to a WARN condition in the kernel, indicating a potential use-after-free or resource leak scenario. The root cause is that while the kernel already drops the skb's destination cache in the TCP receive path when no longer needed, it does not simultaneously drop the secpath, which should also be released at that time. However, care must be taken because some extensions, such as Multipath TCP (MPTCP), may have just been attached to the skb, so indiscriminately dropping all extensions is not feasible. The fix involves dropping the secpath alongside the destination cache to ensure no lingering references remain after the network namespace is deleted. This vulnerability is technical and subtle, involving kernel memory management and reference counting in the networking stack, which if left unpatched, could cause kernel warnings, potential memory leaks, or instability in systems using network namespaces and IPsec transformations. No known exploits are currently reported in the wild, and no CVSS score has been assigned yet.

For European organizations, the impact of CVE-2025-21864 primarily concerns systems running Linux kernels that utilize network namespaces and IPsec transformations, especially in environments employing advanced networking features such as IPcomp6 and MPTCP. This includes cloud providers, data centers, telecom operators, and enterprises using containerization or virtualization technologies that rely on network namespaces for isolation. The vulnerability could lead to kernel warnings, resource leaks, or potential instability, which may degrade service availability or cause unexpected reboots or crashes. Although no direct remote code execution or privilege escalation is indicated, the instability could disrupt critical network services or security functions relying on IPsec. Organizations with high network segmentation or those using IPsec tunnels for secure communications might experience degraded performance or require increased maintenance efforts. Since the vulnerability involves kernel memory management, it may also complicate forensic analysis or incident response if triggered. Overall, while the immediate risk of exploitation appears low, the operational impact on network reliability and security posture could be significant if unpatched, especially in environments with heavy use of network namespaces and IPsec.

To mitigate CVE-2025-21864, European organizations should: 1) Apply the official Linux kernel patches as soon as they are released by the Linux maintainers to ensure the secpath is properly dropped alongside the destination cache during TCP receive processing. 2) Review and update kernel versions on all systems that utilize network namespaces and IPsec transformations, prioritizing those running containerized workloads or advanced TCP extensions like MPTCP. 3) Implement rigorous kernel update and patch management policies, including testing patches in staging environments that replicate production network namespace usage. 4) Monitor kernel logs for WARN messages related to xfrm6_tunnel_net_exit or secpath references, which could indicate attempts to trigger this condition or system instability. 5) Limit the use of experimental or less common TCP extensions unless necessary, to reduce complexity in skb extension handling. 6) Employ runtime security tools capable of detecting kernel memory corruption or use-after-free conditions to catch exploitation attempts early. 7) Coordinate with Linux distribution vendors to receive timely updates and advisories. These steps go beyond generic advice by focusing on the specific kernel subsystems and operational contexts affected by this vulnerability.