CVE-2025-59257 is a vulnerability classified under CWE-1287, indicating improper validation of the specified type of input within the Windows Local Session Manager (LSM) component of Microsoft Windows Server 2025, specifically the Server Core installation variant. The flaw arises because LSM fails to correctly validate certain input types received over the network, which can be manipulated by an authorized attacker with low privileges to trigger a denial of service condition. The vulnerability affects Windows Server 2025 version 10.0.26100.0. Exploitation requires network access and privileges to interact with LSM but does not require user interaction. The impact is limited to availability, as the attacker can cause the LSM service or the server to become unresponsive or crash, denying legitimate users access to session management services. The CVSS v3.1 base score is 6.5 (medium), reflecting network attack vector, low attack complexity, required privileges, no user interaction, and impact limited to availability. No known exploits have been reported in the wild, and no official patches have been released as of the publication date. The vulnerability is significant for environments using Server Core installations, which are commonly deployed in enterprise and cloud data centers for their reduced footprint and attack surface. Improper input validation vulnerabilities in session management components can lead to service disruptions, affecting dependent applications and services.

For European organizations, the primary impact is denial of service on Windows Server 2025 Server Core installations, potentially disrupting critical services relying on session management. This can affect data centers, cloud service providers, and enterprises using Server Core for hosting applications, domain controllers, or infrastructure services. Availability loss can lead to operational downtime, impacting business continuity and service level agreements. Although confidentiality and integrity are not directly affected, the disruption of availability can indirectly affect compliance with regulations such as GDPR if services handling personal data become inaccessible. Organizations with exposed network access to LSM services are at higher risk. The lack of known exploits reduces immediate risk, but the medium severity score and the presence of required privileges mean insider threats or compromised accounts could leverage this vulnerability. The Server Core installation’s popularity in European enterprise environments means the scope of affected systems could be significant, especially in sectors like finance, healthcare, and government.

1. Restrict network access to the Windows Local Session Manager service by implementing strict firewall rules and network segmentation, limiting exposure to trusted hosts only. 2. Monitor network traffic and system logs for unusual or repeated LSM session requests that could indicate exploitation attempts. 3. Enforce the principle of least privilege to minimize the number of users or services with the required privileges to interact with LSM. 4. Deploy intrusion detection/prevention systems (IDS/IPS) tuned to detect anomalous LSM traffic patterns. 5. Prepare for patch deployment by inventorying all Windows Server 2025 Server Core installations and establishing rapid update procedures once Microsoft releases a fix. 6. Consider temporary compensating controls such as disabling non-essential network services on Server Core installations if feasible. 7. Educate system administrators about this vulnerability and the importance of monitoring and restricting access to session management components.