CVE-2025-26642 is a high-severity vulnerability identified in Microsoft Office Online Server version 1.0.0. The vulnerability is classified as an out-of-bounds read (CWE-125), which occurs when the software reads data outside the boundaries of allocated memory. This flaw can be exploited by an unauthorized attacker to execute code locally on the affected system. The vulnerability requires local access (Attack Vector: Local) but does not require privileges (Privileges Required: None), though it does require user interaction (User Interaction: Required). The vulnerability impacts confidentiality, integrity, and availability, all rated as high, indicating that successful exploitation could lead to full compromise of the affected system. The CVSS 3.1 base score is 7.8, reflecting a high severity level. The out-of-bounds read likely allows an attacker to manipulate memory in a way that leads to arbitrary code execution, potentially enabling them to run malicious code with the same privileges as the Office Online Server process. Since the vulnerability is in Office Online Server, which is used to provide browser-based access to Office documents, exploitation could allow attackers to compromise document processing workflows or gain footholds within enterprise environments. No known exploits are currently reported in the wild, and no patches have been linked yet, indicating that organizations should prioritize monitoring and prepare for imminent patch deployment. The vulnerability was reserved in February 2025 and published in April 2025, showing recent discovery and disclosure.

For European organizations, the impact of CVE-2025-26642 could be significant, especially for enterprises and public sector entities relying on Microsoft Office Online Server to facilitate collaborative document editing and sharing. Successful exploitation could lead to unauthorized code execution on servers handling sensitive documents, risking data breaches, intellectual property theft, and disruption of business operations. The compromise of Office Online Server could also serve as a pivot point for attackers to move laterally within networks, escalating privileges and accessing other critical systems. Given the high confidentiality, integrity, and availability impact, organizations could face regulatory repercussions under GDPR if personal data is exposed. Additionally, disruption of document services could affect productivity and trust in digital collaboration tools. The requirement for local access and user interaction somewhat limits remote exploitation, but insider threats or compromised user accounts could still trigger attacks. The lack of current exploits provides a window for proactive defense, but the absence of patches necessitates immediate risk mitigation.

European organizations should implement several targeted mitigation strategies: 1) Restrict local access to Office Online Server hosts strictly to trusted administrators and service accounts to reduce the risk of local exploitation. 2) Employ application whitelisting and endpoint protection solutions on servers running Office Online Server to detect and block suspicious code execution attempts. 3) Monitor user interactions and audit logs related to Office Online Server usage to identify anomalous behavior that could indicate exploitation attempts. 4) Isolate Office Online Server instances within segmented network zones with strict access controls to limit lateral movement if compromised. 5) Prepare for rapid deployment of official patches from Microsoft once available by maintaining an up-to-date asset inventory and patch management process. 6) Educate users and administrators about the risks of interacting with untrusted content or executing unknown files within Office Online Server environments. 7) Consider deploying additional runtime protections such as memory protection technologies (e.g., DEP, ASLR) if not already enabled to mitigate exploitation of memory corruption vulnerabilities. These measures go beyond generic advice by focusing on access control, monitoring, and preparation for patching in the specific context of Office Online Server.