CVE-2025-21648 is a vulnerability identified in the Linux kernel's netfilter subsystem, specifically within the connection tracking (conntrack) component. The issue arises from improper handling of the maximum size of the conntrack hashtable. The vulnerability is due to the absence of a clamp on the maximum hashtable size to INT_MAX (the maximum value for a signed 32-bit integer). Without this clamp, resizing the hashtable can trigger a WARN_ON_ONCE warning in the kernel function __kvmalloc_node_noprof(), because the __GFP_NOWARN flag is not set during the memory allocation attempt. This can lead to kernel warnings or potentially unstable behavior when the hashtable is resized, which is only possible from the initial network namespace (init_netns). The root cause is related to memory allocation flags and the handling of oversized allocations in the kernel's memory management subsystem. The fix involves enforcing INT_MAX as the maximum size for the conntrack hashtable, preventing the kernel from attempting to allocate an excessively large hashtable that could cause warnings or instability. This vulnerability does not have any known exploits in the wild at the time of publication and affects certain Linux kernel versions identified by specific commit hashes. The vulnerability is technical and low-level, affecting kernel memory management and network connection tracking, which is critical for firewall and NAT operations in Linux-based systems.

For European organizations, this vulnerability could impact any systems running affected Linux kernel versions that utilize netfilter's conntrack functionality, which is common in servers, network appliances, and cloud infrastructure. If exploited or triggered, the vulnerability could cause kernel warnings or instability, potentially leading to denial of service conditions or degraded network performance. This could disrupt critical services, especially in environments relying heavily on Linux-based firewalls, routers, or containerized workloads using network namespaces. While no active exploits are known, the risk lies in potential system crashes or degraded reliability, which could affect availability and operational continuity. Confidentiality and integrity impacts are minimal since the vulnerability does not directly enable privilege escalation or data leakage. However, availability impacts could be significant in high-demand or critical infrastructure environments. European organizations with large-scale Linux deployments, including cloud providers, telecommunications, and enterprises using Linux-based network security solutions, should be particularly attentive to this vulnerability.

Organizations should promptly apply the official Linux kernel patches that clamp the conntrack hashtable size to INT_MAX. Since the vulnerability is related to kernel memory allocation during hashtable resizing, updating to a patched kernel version is the most effective mitigation. Additionally, organizations should audit their Linux systems to identify affected kernel versions and prioritize patching those in critical network roles. Monitoring kernel logs for WARN_ON_ONCE messages related to __kvmalloc_node_noprof() can help detect attempts to trigger the issue. Network administrators should also review conntrack table sizes and configurations to avoid unnecessarily large hashtables that could trigger resizing. In containerized or virtualized environments using network namespaces, special attention should be given to init_netns configurations. Finally, implementing robust system and network monitoring can help detect anomalies that may indicate exploitation attempts or instability caused by this vulnerability.