CVE-2025-53739 is a vulnerability classified under CWE-843 (Access of Resource Using Incompatible Type, or 'type confusion') found in Microsoft Office Online Server, specifically impacting the Excel component. The vulnerability arises when the software improperly handles data types, allowing an attacker to access resources using incompatible types. This flaw can be exploited to execute arbitrary code locally on the affected system without requiring prior authentication privileges, although user interaction is necessary to trigger the exploit. The vulnerability affects version 16.0.0.0 of Office Online Server and was published on August 12, 2025. The CVSS v3.1 base score is 7.8, indicating high severity, with vector AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H, meaning the attack requires local access and user interaction but no privileges, and can compromise confidentiality, integrity, and availability fully. No patches were listed at the time of publication, and no known exploits are reported in the wild. The vulnerability's exploitation could allow attackers to run arbitrary code, potentially leading to system compromise, data theft, or disruption of services. Given the widespread use of Microsoft Office Online Server in enterprise environments, this vulnerability represents a significant risk, especially in environments where local user access is common or where users might be tricked into interacting with malicious content.

For European organizations, the impact of CVE-2025-53739 could be substantial. Many enterprises, government agencies, and critical infrastructure operators in Europe rely on Microsoft Office Online Server for document collaboration and productivity. Successful exploitation could lead to unauthorized code execution on servers or user machines, resulting in data breaches, loss of data integrity, and potential service outages. Confidential information processed or stored via Office Online Server could be exposed or altered. The requirement for local access and user interaction somewhat limits remote exploitation but does not eliminate risk, especially in environments with shared workstations or where social engineering could induce user interaction. The high impact on confidentiality, integrity, and availability could disrupt business operations and erode trust. Additionally, regulatory frameworks such as GDPR impose strict data protection requirements, and exploitation leading to data breaches could result in significant legal and financial penalties for affected organizations.

1. Monitor Microsoft security advisories closely and apply official patches or updates immediately once released for Office Online Server version 16.0.0.0. 2. Restrict local access to systems running Office Online Server to trusted personnel only, minimizing the attack surface. 3. Implement strict user privilege management to prevent unauthorized users from executing code or accessing sensitive resources locally. 4. Educate users about the risks of interacting with untrusted content or links that could trigger the vulnerability. 5. Employ application whitelisting and endpoint protection solutions that can detect and block suspicious code execution attempts. 6. Regularly audit and monitor logs for unusual activity indicative of exploitation attempts. 7. Consider network segmentation to isolate Office Online Server infrastructure from less secure network zones. 8. Use multi-factor authentication and strong access controls to reduce the risk of unauthorized local access. 9. Prepare incident response plans specifically addressing potential exploitation scenarios of this vulnerability.