CVE-2026-20859 is a use-after-free vulnerability classified under CWE-416 found in kernel-mode drivers of Microsoft Windows Server 2025, specifically the Server Core installation version 10.0.26100.0. Use-after-free vulnerabilities occur when a program continues to use memory after it has been freed, which can lead to memory corruption, crashes, or arbitrary code execution. In this case, the vulnerability allows an authorized attacker with low privileges on the local system to exploit the flaw to elevate their privileges to SYSTEM level. The vulnerability does not require user interaction and has a low attack complexity, but does require local access and some privileges. The CVSS v3.1 base score is 7.8, indicating a high severity due to the potential for full confidentiality, integrity, and availability compromise. The flaw resides in kernel-mode drivers, which operate at a high privilege level, making exploitation particularly dangerous. No public exploits or active exploitation have been reported yet, but the vulnerability is publicly disclosed and patched status is pending. The Server Core installation is a minimal installation option for Windows Server, often used in enterprise environments for improved security and reduced attack surface, but this vulnerability undermines that security by allowing privilege escalation. The vulnerability was reserved in December 2025 and published in January 2026, indicating recent discovery and disclosure.

For European organizations, this vulnerability poses a significant risk especially to enterprises and critical infrastructure relying on Windows Server 2025 Server Core installations. Successful exploitation allows attackers with limited local access to gain full system privileges, potentially leading to unauthorized access to sensitive data, disruption of services, and deployment of further malicious payloads such as ransomware or persistent backdoors. The impact on confidentiality, integrity, and availability is high, as attackers can manipulate system processes, access protected resources, and cause system instability or denial of service. Organizations with remote or shared access environments, such as managed service providers or cloud-hosted Windows Server instances, are particularly vulnerable. The lack of user interaction requirement increases the risk of automated or stealthy exploitation once local access is obtained. Although no known exploits are currently active, the vulnerability’s disclosure may prompt attackers to develop exploits, increasing the urgency for mitigation. The Server Core installation is often used in security-sensitive environments, so this vulnerability undermines the security posture of such deployments.

1. Apply official security patches from Microsoft immediately once they become available for Windows Server 2025 Server Core installations. 2. Restrict local access to Windows Server systems to trusted administrators only, minimizing the risk of local privilege escalation. 3. Implement strict access controls and monitoring on administrative accounts and local user sessions to detect suspicious privilege escalation attempts. 4. Use endpoint detection and response (EDR) solutions capable of detecting anomalous kernel-mode activity or memory corruption indicative of exploitation attempts. 5. Employ application whitelisting and kernel-mode driver signing enforcement to reduce the risk of malicious code execution. 6. Regularly audit and harden server configurations, disabling unnecessary services and features to reduce attack surface. 7. Consider network segmentation to limit lateral movement if a server is compromised. 8. Educate administrators about the risks of local privilege escalation vulnerabilities and the importance of timely patching. 9. Maintain up-to-date backups and incident response plans to mitigate potential damage from exploitation.