CVE-2025-59233 is a vulnerability categorized under CWE-843 (Access of Resource Using Incompatible Type, or 'Type Confusion') found in Microsoft Office Online Server, specifically impacting the Excel component. This vulnerability arises when the software incorrectly handles resource types, allowing an attacker to access or manipulate resources using incompatible types. The flaw enables an unauthorized attacker to execute arbitrary code locally on the affected system. The vulnerability affects version 16.0.0.0 of Office Online Server. The CVSS 3.1 base score is 7.8, indicating high severity, with the 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 it impacts confidentiality, integrity, and availability to a high degree. The vulnerability was reserved in September 2025 and published in October 2025. No public exploits are known at this time, but the potential for local code execution makes it a critical concern for environments where Office Online Server is deployed, especially in multi-user or shared environments. The lack of an available patch at the time of publication necessitates immediate attention to access controls and monitoring to mitigate risk.

The vulnerability allows an attacker with local access and the ability to induce user interaction to execute arbitrary code on the affected system. This can lead to full compromise of the Office Online Server instance, potentially exposing sensitive documents and data processed through Excel Online. For European organizations, this could mean unauthorized data disclosure, data manipulation, or disruption of critical document collaboration services. Given the high integration of Microsoft Office products in European enterprises and governments, exploitation could impact confidentiality, integrity, and availability of business-critical information. The local access requirement limits remote exploitation but does not eliminate risk in environments where multiple users share access or where attackers can gain foothold through other means. The absence of known exploits currently reduces immediate risk but does not preclude future weaponization. The vulnerability could also be leveraged as a stepping stone for lateral movement within networks, increasing overall organizational risk.

1. Monitor Microsoft security advisories closely and apply official patches immediately once released for Office Online Server version 16.0.0.0. 2. Restrict local access to Office Online Server hosts to trusted administrators only, minimizing the risk of unauthorized local code execution. 3. Implement strict user interaction policies and educate users about the risks of interacting with untrusted content on Office Online Server. 4. Employ application whitelisting and endpoint protection solutions to detect and prevent unauthorized code execution on servers. 5. Use network segmentation to isolate Office Online Server infrastructure from less trusted network zones. 6. Enable detailed logging and monitoring on Office Online Server to detect anomalous behavior indicative of exploitation attempts. 7. Conduct regular security assessments and penetration testing focusing on local privilege escalation and code execution vectors. 8. Consider temporary mitigation by disabling or limiting Excel Online functionality if patching is delayed and risk is high.