CVE-2025-64666 is a vulnerability identified in Microsoft Exchange Server 2019 Cumulative Update 15 (version 15.02.0.0) involving improper input validation (CWE-20). This flaw allows an attacker who is already authorized with low privileges to exploit the vulnerability over a network to elevate their privileges, potentially gaining higher-level access or administrative control. The vulnerability does not require user interaction but does require the attacker to have some level of authenticated access, which limits exploitation to insiders or compromised accounts. The CVSS 3.1 base score is 7.5, indicating high severity, with attack vector network (AV:N), attack complexity high (AC:H), privileges required low (PR:L), no user interaction (UI:N), and impacts on confidentiality, integrity, and availability all rated high (C:H/I:H/A:H). The attack complexity being high suggests that exploitation is non-trivial, possibly requiring specific conditions or crafted input. No public exploits or patches are currently available, but the vulnerability is published and reserved since November 2025. Given the critical role of Microsoft Exchange Server in enterprise email and collaboration, exploitation could lead to significant data breaches, disruption of communications, and potential lateral movement within networks.

For European organizations, this vulnerability poses a significant risk due to the widespread use of Microsoft Exchange Server 2019 in corporate and governmental environments. Successful exploitation can lead to unauthorized privilege escalation, enabling attackers to access sensitive emails, confidential data, and internal systems. This could result in data breaches, espionage, disruption of business operations, and damage to organizational reputation. Critical sectors such as finance, healthcare, government, and energy, which rely heavily on Exchange Server for communication, are particularly vulnerable. The network-based nature of the exploit means that attackers can potentially leverage compromised low-privilege accounts to escalate privileges without requiring user interaction, increasing the risk of stealthy attacks. The high impact on confidentiality, integrity, and availability could also facilitate ransomware deployment or persistent advanced threats within European networks.

Organizations should immediately inventory their Exchange Server 2019 installations to identify those running Cumulative Update 15 (version 15.02.0.0). Although no patches are currently available, administrators should monitor Microsoft security advisories closely for updates and apply patches promptly once released. In the interim, implement strict network segmentation to limit access to Exchange servers, enforce least privilege principles rigorously, and strengthen authentication mechanisms such as multi-factor authentication (MFA) to reduce the risk of account compromise. Deploy enhanced monitoring and anomaly detection focused on privilege escalation attempts and unusual access patterns on Exchange servers. Regularly review and audit user permissions to ensure no excessive privileges are granted. Consider disabling or restricting legacy protocols and services that could be leveraged to exploit the vulnerability. Additionally, maintain up-to-date backups and incident response plans to mitigate potential impacts of exploitation.