CVE-2025-55340 is a vulnerability classified under CWE-287 (Improper Authentication) affecting Microsoft Windows 11 Version 25H2, specifically version 10.0.26200.0. The flaw resides in the Windows Remote Desktop Protocol (RDP) authentication mechanism, allowing an attacker with authorized local access and low privileges to bypass certain security features. This bypass does not require user interaction but has a high attack complexity, meaning exploitation demands specific conditions or expertise. The vulnerability impacts confidentiality, integrity, and availability, potentially enabling unauthorized access to sensitive data, manipulation of system settings, or denial of service. Although no public exploits are known yet, the vulnerability is published and rated with a CVSS v3.1 score of 7.0 (high severity), indicating a serious threat. The lack of available patches at the time of publication necessitates immediate attention to mitigating controls. The vulnerability’s local attack vector and requirement for low privileges suggest that insider threats or compromised local accounts could leverage this flaw to escalate privileges or bypass security controls within RDP sessions. Given the widespread use of Windows 11 in enterprise environments and the critical role of RDP for remote administration, this vulnerability poses a significant risk to organizational security.

For European organizations, the impact of CVE-2025-55340 can be substantial. Many enterprises and public sector entities rely on Windows 11 and RDP for remote management and telework, especially post-pandemic. Exploitation could lead to unauthorized access to sensitive systems, data breaches, and disruption of critical services. Confidentiality is at risk as attackers might access sensitive information through bypassed authentication. Integrity could be compromised if attackers modify system configurations or deploy malware. Availability may also be affected if attackers disrupt RDP services or escalate privileges to disable security controls. Sectors such as finance, healthcare, government, and critical infrastructure are particularly vulnerable due to their reliance on secure remote access. The local access requirement limits remote exploitation but does not eliminate risk, as insider threats or compromised endpoints could serve as attack vectors. The high attack complexity reduces the likelihood of widespread exploitation but does not negate the need for urgent mitigation.

1. Apply official patches from Microsoft immediately once released for Windows 11 Version 25H2 (build 10.0.26200.0). 2. Until patches are available, restrict local access to systems running the affected Windows version, especially those with RDP enabled. 3. Implement strict access controls and monitoring for local user accounts, minimizing the number of users with local access privileges. 4. Use network-level authentication (NLA) and multi-factor authentication (MFA) for RDP sessions to add layers of defense. 5. Monitor event logs and RDP session activity for unusual behavior indicative of authentication bypass attempts. 6. Employ endpoint detection and response (EDR) solutions to detect and respond to suspicious local privilege escalations. 7. Consider disabling RDP on systems where it is not essential or restrict RDP access via firewall rules and network segmentation. 8. Educate users and administrators about the risks of local privilege misuse and enforce strong password policies to reduce insider threat risks.