CVE-2025-29955 is a vulnerability identified in Microsoft Windows Server 2025, specifically affecting the Server Core installation variant. The root cause of this vulnerability is improper input validation within the Windows Hyper-V component. Hyper-V is Microsoft's native hypervisor used for creating and managing virtual machines on Windows servers. Improper input validation means that the system fails to correctly verify or sanitize inputs before processing them, which can lead to unexpected behavior or exploitation. In this case, the vulnerability allows an unauthorized local attacker to cause a denial of service (DoS) condition. The attacker does not require any privileges (PR:N) or user interaction (UI:N) to exploit this flaw, but must have local access to the system (AV:L). The impact is limited to availability (A:H), meaning the attacker can disrupt the service or crash the Hyper-V component, potentially affecting hosted virtual machines or the host server's stability. The CVSS v3.1 base score is 6.2, categorized as medium severity. The vulnerability does not affect confidentiality or integrity, and there are no known exploits in the wild at the time of publication. The affected version is Windows Server 2025 build 10.0.26100.0, and no patches have been linked yet. This vulnerability is classified under CWE-20 (Improper Input Validation), which is a common software weakness leading to various security issues. Given the nature of Hyper-V as a critical virtualization platform, exploitation could disrupt business operations relying on virtualized infrastructure.

For European organizations, the impact of this vulnerability can be significant in environments that utilize Windows Server 2025 with Hyper-V for virtualization, especially in data centers, cloud service providers, and enterprises running critical workloads on virtual machines. A local attacker exploiting this flaw could cause denial of service, leading to downtime of virtual machines or the host server, which may disrupt business continuity, affect service level agreements, and cause operational losses. Although exploitation requires local access, insider threats or compromised accounts could leverage this vulnerability. The lack of impact on confidentiality and integrity reduces the risk of data breaches, but availability disruptions remain a concern. Organizations with automated or remote management tools that rely on Hyper-V might experience cascading effects if the host becomes unstable. Given the medium severity, the threat should be prioritized in patch management cycles, particularly for critical infrastructure and services relying on Windows Server 2025 Server Core installations.

To mitigate this vulnerability, European organizations should take the following specific actions: 1) Monitor for and apply official patches or updates from Microsoft as soon as they become available, as no patch links are currently provided. 2) Restrict local access to Windows Server 2025 hosts running Hyper-V to trusted administrators only, minimizing the risk of local exploitation. 3) Implement strict access controls and auditing on servers to detect unauthorized local access attempts. 4) Use virtualization host hardening best practices, such as disabling unnecessary services and limiting the attack surface of the Server Core installation. 5) Employ host-based intrusion detection systems (HIDS) to monitor for unusual activity or crashes related to Hyper-V. 6) Consider network segmentation to isolate virtualization hosts from less trusted networks or users. 7) Prepare incident response plans for potential denial of service events affecting virtualization infrastructure. 8) Regularly review and update security policies regarding local access and privilege management on critical servers.