CVE-2025-37921 is a vulnerability identified in the Linux kernel's VXLAN (Virtual Extensible LAN) implementation, specifically within the 'vnifilter' mode of VXLAN devices. VXLAN is a network virtualization technology widely used to encapsulate Layer 2 Ethernet frames within Layer 3 UDP packets, enabling scalable network segmentation in cloud and data center environments. The vulnerability arises when a Virtual Network Identifier (VNI) is deleted from a VXLAN device operating in vnifilter mode. During this deletion process, the Forwarding Database (FDB) entry associated with the default remote endpoint is removed without properly acquiring the necessary hash lock. This improper locking leads to a race condition and triggers warnings from the kernel's lock dependency checker (lockdep), indicating potential concurrency issues. The root cause is the unlocked deletion of the default FDB entry, which violates synchronization requirements in the kernel's networking code. The issue was introduced by a prior commit that created wrappers for FDB lookup but failed to ensure locking during deletion. The vulnerability is reproducible using specific ip and bridge commands that add and then delete a VNI on a VXLAN device configured with vnifilter mode. Although the vulnerability does not appear to cause immediate crashes or memory corruption, the lack of proper locking can lead to inconsistent FDB state, kernel warnings, and potentially undefined behavior under concurrent operations. The fix involves acquiring the appropriate hash lock before deleting the FDB entry and releasing it afterward, ensuring thread-safe manipulation of the FDB. This vulnerability affects Linux kernel versions containing the specified commit hashes prior to the fix and is relevant to environments utilizing VXLAN with vnifilter mode, which is common in advanced network virtualization setups.

For European organizations, the impact of CVE-2025-37921 depends largely on their use of Linux-based infrastructure employing VXLAN with vnifilter mode. Many enterprises and cloud providers in Europe rely on Linux servers for network virtualization and container orchestration platforms such as Kubernetes, which may leverage VXLAN for overlay networking. Improper locking in the FDB deletion process could lead to kernel warnings and potential instability in network virtualization components, possibly causing intermittent network disruptions or degraded performance. While no direct exploit or remote code execution is indicated, the vulnerability could be leveraged as part of a broader attack chain to destabilize network services or cause denial of service in critical infrastructure. This is particularly concerning for sectors with high reliance on virtualized networks, such as telecommunications, finance, and cloud service providers. Additionally, the presence of kernel warnings and race conditions complicates troubleshooting and may increase operational risk. Given the foundational role of Linux in European IT infrastructure, unpatched systems could face reliability issues impacting business continuity and service availability.

To mitigate CVE-2025-37921, European organizations should prioritize updating their Linux kernels to versions where this vulnerability has been patched. Since the issue stems from improper locking in the VXLAN vnifilter code, applying the official kernel patches that acquire the hash lock before FDB deletion is essential. Organizations should: 1) Identify all systems running Linux kernels with VXLAN vnifilter enabled, especially those using the affected commit versions. 2) Test and deploy updated kernel versions from trusted Linux distributions that include the fix. 3) If immediate patching is not feasible, consider disabling vnifilter mode on VXLAN devices as a temporary workaround to avoid triggering the vulnerability. 4) Monitor kernel logs for lockdep warnings related to vxlan_find_mac and FDB deletions to detect potential exploitation or instability. 5) Implement robust change management and network configuration auditing to ensure VXLAN configurations follow best practices and minimize exposure. 6) Engage with Linux distribution vendors and security mailing lists to stay informed about updates and any emerging exploit techniques related to this vulnerability.