CVE-2025-59233 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. Type confusion vulnerabilities occur when a program accesses a resource using an incorrect or incompatible data type, leading to undefined behavior that can be exploited to execute arbitrary code. In this case, the flaw allows an unauthorized attacker to execute code locally on the affected system. The vulnerability affects version 16.0.0.0 of Office Online Server. The CVSS v3.1 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. This means the attack requires local access (AV:L), low attack complexity (AC:L), no privileges (PR:N), but user interaction (UI:R) is needed. 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 September 2025 and published in October 2025. No patches or known exploits are currently available. The vulnerability could allow attackers who gain local access and trick users into interaction to execute arbitrary code, potentially leading to full system compromise. This is particularly concerning for environments where Office Online Server is deployed to provide Excel services, as it could be a vector for lateral movement or privilege escalation.

For European organizations, the impact of CVE-2025-59233 could be significant, especially in sectors relying heavily on Microsoft Office Online Server for document collaboration and processing, such as finance, government, healthcare, and critical infrastructure. Successful exploitation could lead to local code execution, allowing attackers to compromise confidentiality by accessing sensitive data, integrity by modifying documents or system configurations, and availability by disrupting services. Given that Office Online Server is often deployed in enterprise environments, exploitation could facilitate lateral movement within networks, potentially leading to broader compromise. The requirement for local access and user interaction somewhat limits remote exploitation but does not eliminate risk, particularly in environments with shared workstations or where attackers have gained initial footholds. The absence of known exploits currently provides a window for mitigation, but the high impact score underscores the urgency of addressing this vulnerability to prevent future attacks.

1. Monitor Microsoft’s official channels for patches addressing CVE-2025-59233 and apply them immediately upon release. 2. Restrict local access to systems running Office Online Server to trusted personnel only, employing strict access controls and network segmentation to limit exposure. 3. Implement application whitelisting and endpoint protection solutions capable of detecting anomalous behavior indicative of exploitation attempts. 4. Educate users about the risks of interacting with untrusted content or prompts within Office Online Server environments to reduce the likelihood of successful user interaction exploitation. 5. Regularly audit and monitor logs for unusual activity related to Office Online Server processes, especially those involving Excel services. 6. Consider deploying additional security controls such as sandboxing or containerization for Office Online Server instances to contain potential exploitation. 7. Review and harden configurations of Office Online Server to minimize attack surface, disabling unnecessary features or components where possible.