CVE-2025-59233 is a vulnerability classified under CWE-843 (Access of Resource Using Incompatible Type, also known as type confusion) found in Microsoft Office Online Server, specifically affecting the Excel component in version 16.0.0.0. Type confusion vulnerabilities occur when a program accesses a resource using an incorrect or incompatible type, leading to undefined behavior that attackers can exploit to execute arbitrary code. In this case, an unauthorized attacker can leverage this flaw to execute code locally on the affected system. The CVSS v3.1 score of 7.8 indicates a high-severity issue, with the vector showing local attack vector (AV:L), low attack complexity (AC:L), no privileges required (PR:N), but user interaction is necessary (UI:R). The scope is unchanged (S:U), and the impact on confidentiality, integrity, and availability is high (C:H/I:H/A:H). The vulnerability was reserved in early September 2025 and published in mid-October 2025. No public exploits are known yet, and no patches have been linked at the time of this report. The vulnerability's exploitation requires local access and user interaction, such as opening a malicious Excel file or interacting with a compromised Office Online Server session. Successful exploitation could lead to full compromise of the local system, allowing attackers to execute arbitrary code with the privileges of the user running the Office Online Server service. This could lead to data theft, system manipulation, or disruption of service. The vulnerability is particularly concerning because Office Online Server is often deployed in enterprise environments to provide web-based Office functionality, making it a valuable target for attackers aiming to compromise enterprise infrastructure.

For European organizations, the impact of CVE-2025-59233 is significant due to the widespread use of Microsoft Office Online Server in enterprise and government environments. Exploitation could lead to unauthorized code execution on servers that handle sensitive documents and collaboration workflows, potentially exposing confidential data and disrupting business operations. The high impact on confidentiality, integrity, and availability means attackers could steal or alter sensitive information, deploy ransomware, or cause service outages. Organizations relying on Office Online Server for document collaboration and processing are at risk of operational disruption and reputational damage. The requirement for local access and user interaction somewhat limits remote exploitation but does not eliminate risk, especially in environments where users have access to the server or where phishing or social engineering could induce user interaction. The lack of known exploits currently provides a window for proactive mitigation, but the high severity demands urgent attention. Failure to address this vulnerability could lead to targeted attacks against critical infrastructure, financial institutions, and public sector organizations across Europe.

1. Monitor Microsoft security advisories closely and apply official patches or updates for Office Online Server version 16.0.0.0 as soon as they become available. 2. Restrict local access to Office Online Server hosts to trusted administrators only, minimizing the risk of local exploitation. 3. Implement strict user access controls and least privilege principles for accounts that can interact with Office Online Server. 4. Educate users about the risks of opening untrusted Excel files or interacting with suspicious Office Online Server sessions to reduce the likelihood of user interaction exploitation. 5. Employ application whitelisting and endpoint detection and response (EDR) solutions to detect and block suspicious code execution attempts on servers. 6. Regularly audit and monitor logs for unusual activity related to Office Online Server processes and user sessions. 7. Consider network segmentation to isolate Office Online Server infrastructure from less trusted network zones. 8. Prepare incident response plans specifically addressing potential exploitation of Office Online Server vulnerabilities. 9. Use multi-factor authentication (MFA) for administrative access to reduce the risk of credential compromise leading to local exploitation. 10. If immediate patching is not possible, consider temporarily disabling or limiting Excel functionality in Office Online Server to reduce attack surface.