CVE-2026-20844 is a use-after-free vulnerability identified in the Windows Clipboard Server component of Microsoft Windows 10 Version 1809 (build 10.0.17763.0). Use-after-free (CWE-416) vulnerabilities occur when a program continues to use memory after it has been freed, leading to undefined behavior including potential code execution. In this case, the Clipboard Server improperly manages memory, allowing an unauthorized local attacker to exploit this flaw to elevate their privileges on the system. The vulnerability does not require prior authentication or user interaction, but the attacker must have local access to the machine. Exploitation could lead to full system compromise by executing arbitrary code with SYSTEM-level privileges, impacting confidentiality, integrity, and availability of the affected system. The CVSS v3.1 base score is 7.4, reflecting high severity with attack vector local (AV:L), attack complexity high (AC:H), no privileges required (PR:N), and no user interaction (UI:N). Although no public exploits are known at this time, the vulnerability is critical for environments still running this older Windows 10 version, which is no longer mainstream supported. The lack of available patches at the time of publication increases the urgency for mitigation. The vulnerability is categorized under CWE-416 (Use After Free) and also tagged with CWE-362 (Race Condition), indicating potential timing issues contributing to the flaw. This vulnerability highlights risks associated with legacy Windows versions and the importance of timely patching and system upgrades.

For European organizations, the impact of CVE-2026-20844 can be significant, especially in sectors relying on legacy Windows 10 Version 1809 systems such as manufacturing, healthcare, and government agencies. Successful exploitation allows local attackers to escalate privileges to SYSTEM level, potentially leading to full system compromise, unauthorized data access, and disruption of critical services. This could result in data breaches, operational downtime, and regulatory non-compliance under GDPR. The vulnerability's local attack vector limits remote exploitation but raises concerns in environments with shared workstations, remote desktop access, or insider threats. Organizations with insufficient endpoint protection or weak local access controls are particularly vulnerable. The absence of known exploits reduces immediate risk but does not eliminate the threat, as weaponization could occur in the future. Legacy systems that cannot be upgraded promptly remain at high risk, making this vulnerability a critical consideration for European entities maintaining older Windows infrastructures.

1. Apply official Microsoft patches as soon as they become available for Windows 10 Version 1809 to remediate the use-after-free vulnerability. 2. If patching is not immediately possible, restrict local access to affected systems by enforcing strict physical and logical access controls, including limiting user accounts with local login privileges. 3. Employ application whitelisting and endpoint detection and response (EDR) solutions capable of detecting anomalous behavior related to privilege escalation attempts. 4. Disable or restrict clipboard sharing features in environments where it is not essential, reducing the attack surface of the Clipboard Server component. 5. Conduct regular system audits to identify and upgrade legacy Windows 10 systems to supported versions with ongoing security updates. 6. Implement network segmentation to isolate critical systems and limit lateral movement in case of compromise. 7. Educate users and administrators about the risks of local privilege escalation vulnerabilities and the importance of maintaining updated systems. 8. Monitor security advisories from Microsoft and threat intelligence feeds for any emerging exploit activity related to this CVE.