CVE-2026-20859 is a use-after-free vulnerability classified under CWE-416 affecting Windows Kernel-Mode Drivers in Microsoft Windows Server 2025, specifically the Server Core installation version 10.0.26100.0. This vulnerability allows an authorized local attacker to elevate privileges by exploiting improper memory management in kernel-mode drivers, leading to use-after-free conditions. The attacker can execute arbitrary code or cause system instability by manipulating freed memory, resulting in full control over the affected system. The CVSS 3.1 base score is 7.8 (high), reflecting the vulnerability's significant impact on confidentiality, integrity, and availability. The attack vector is local (AV:L), requiring low attack complexity (AC:L) and privileges (PR:L), but no user interaction (UI:N). The scope is unchanged (S:U), meaning the impact remains within the vulnerable component. No known exploits have been reported in the wild, and no patches are currently linked, indicating that mitigation relies on access control and monitoring until official updates are released. This vulnerability is particularly critical for environments running Windows Server 2025 Server Core installations, which are often used in enterprise and cloud infrastructure due to their minimal footprint and enhanced security posture. Exploitation could allow attackers to bypass security controls and gain elevated privileges, potentially compromising entire systems or networks.

For European organizations, this vulnerability presents a significant risk, especially for those utilizing Windows Server 2025 Server Core installations in critical infrastructure, data centers, or cloud environments. Successful exploitation could lead to unauthorized privilege escalation, enabling attackers to execute malicious code with kernel-level privileges, thus compromising system confidentiality, integrity, and availability. This could result in data breaches, disruption of services, or lateral movement within networks. Given the local attack vector, insider threats or attackers who have gained initial footholds could leverage this vulnerability to escalate privileges and deepen their access. The lack of current exploits in the wild reduces immediate risk but does not eliminate the threat, as attackers may develop exploits once the vulnerability details are widely known. European organizations with stringent regulatory requirements (e.g., GDPR) could face compliance and reputational damage if affected. The Server Core installation is favored for its reduced attack surface, but this vulnerability undermines that advantage by allowing privilege escalation despite minimal installation footprints.

1. Implement strict access controls to limit local administrative privileges and restrict access to trusted personnel only. 2. Employ robust monitoring and logging of local privilege escalation attempts and unusual kernel-mode activities to detect exploitation attempts early. 3. Use application whitelisting and endpoint protection solutions capable of detecting anomalous behavior indicative of use-after-free exploitation. 4. Isolate critical servers running Windows Server 2025 Server Core installations within segmented network zones to reduce lateral movement opportunities. 5. Prepare for rapid deployment of patches by establishing a vulnerability management process that includes testing and timely application of updates once Microsoft releases a fix. 6. Conduct regular security audits and penetration testing focusing on privilege escalation vectors to identify and remediate potential weaknesses. 7. Educate system administrators and security teams about this specific vulnerability to enhance awareness and incident response readiness. 8. Consider temporary compensating controls such as disabling unnecessary kernel-mode drivers or services if feasible until patches are available.