CVE-2025-37863 is a vulnerability identified in the Linux kernel's overlay filesystem (overlayfs) implementation. Overlayfs allows multiple filesystem layers to be combined, typically used in container environments and other scenarios requiring union mounts. The vulnerability arises from improper handling of 'datadir-only' configurations, where the upper layer of the overlayfs could theoretically refer directly to a data layer without a corresponding lower directory (lowerdir). Initially, the Linux kernel did not allow data-only layers without a lowerdir, but the introduction of the 'datadir+' feature attempted to support this use case without fully implementing the necessary handling logic. This incomplete implementation led to a kernel Oops (a kernel crash or panic) when such configurations were used. The fix involved disallowing the use of datadir without a lowerdir, thereby preventing the kernel from entering an unstable state due to this misconfiguration. While no known exploits are currently reported in the wild, the vulnerability could be triggered by malicious or misconfigured overlayfs mounts, potentially leading to denial of service via kernel crashes. The affected versions are specific Linux kernel commits prior to the fix, and the vulnerability was publicly disclosed in May 2025. No CVSS score has been assigned yet, and the vulnerability does not appear to allow privilege escalation or code execution directly but impacts system stability and availability.

For European organizations, this vulnerability primarily poses a risk to system availability and stability, especially for those heavily utilizing Linux-based containerization technologies, such as Docker or Kubernetes, which rely on overlayfs for efficient image layering. A successful exploitation could result in kernel crashes, leading to denial of service conditions on critical infrastructure, cloud services, or enterprise servers. This could disrupt business operations, cause downtime, and potentially impact service level agreements (SLAs). Although it does not directly compromise confidentiality or integrity, the resulting instability could be leveraged by attackers as part of a broader attack chain, for example, to cause disruptions during an incident response or to mask other malicious activities. Organizations running custom or older Linux kernel versions without the patch are at higher risk. Given the widespread use of Linux in European data centers, cloud providers, and enterprise environments, the impact could be significant if not addressed promptly.

European organizations should immediately verify their Linux kernel versions and apply the latest patches that disallow datadir-only overlayfs mounts without a lowerdir. Specifically, system administrators should: 1) Audit container and overlayfs configurations to ensure no unsupported datadir-only setups exist. 2) Update Linux kernels to versions released after the fix for CVE-2025-37863, ensuring that the overlayfs implementation correctly enforces the restriction. 3) Implement monitoring for kernel Oops or crashes related to overlayfs usage to detect potential exploitation attempts or misconfigurations. 4) For container orchestration platforms, validate that container images and runtime configurations do not rely on unsupported overlayfs features. 5) Engage with Linux distribution vendors and cloud providers to confirm that patches are included in their kernel updates and coordinate timely deployment. 6) Consider implementing kernel crash dump analysis to facilitate rapid diagnosis if crashes occur. These steps go beyond generic patching by emphasizing configuration audits, monitoring, and integration with container management practices.