CVE-2025-30383 is a high-severity vulnerability identified in Microsoft Office Online Server version 1.0.0, specifically related to Microsoft Office Excel functionality. The vulnerability is classified under CWE-843, which corresponds to 'Access of Resource Using Incompatible Type' or 'Type Confusion.' This type of flaw occurs when a program accesses a resource using a type that is incompatible with the actual type of the resource, potentially leading to unexpected behavior such as memory corruption. In this case, the vulnerability allows an unauthorized attacker to execute code locally on the affected system. The CVSS 3.1 base score of 7.8 reflects a high severity, with the vector indicating local attack vector (AV:L), low attack complexity (AC:L), no privileges required (PR:N), but requiring user interaction (UI:R). The impact on confidentiality, integrity, and availability is rated high, meaning successful exploitation can lead to full system compromise. The vulnerability does not currently have known exploits in the wild, but the potential for local code execution makes it a significant risk. Since the attack vector is local, an attacker must have some form of access to the system or trick a user into interacting with a malicious file or content. The vulnerability likely arises from improper handling of data types within the Excel component of Office Online Server, which can be exploited to execute arbitrary code, potentially allowing an attacker to escalate privileges or move laterally within a network.

For European organizations, the impact of CVE-2025-30383 can be substantial, especially for enterprises and public sector entities relying on Microsoft Office Online Server for collaborative document editing and Excel spreadsheet processing. Successful exploitation could lead to local code execution, enabling attackers to compromise confidentiality by accessing sensitive data, integrity by modifying documents or system configurations, and availability by disrupting services. Given the high integration of Office Online Server in many European corporate and government environments, exploitation could facilitate lateral movement within networks, data exfiltration, or deployment of ransomware. The requirement for local access or user interaction somewhat limits remote exploitation but does not eliminate risk, particularly in environments with weak endpoint security or where users may be tricked into opening malicious Excel files. The absence of known exploits in the wild provides a window for proactive mitigation, but the high severity score necessitates urgent attention to prevent potential targeted attacks, especially in sectors with high-value data such as finance, healthcare, and critical infrastructure.

1. Apply patches or updates from Microsoft as soon as they become available, even though no patch links are currently provided, monitoring official Microsoft security advisories is critical. 2. Implement strict endpoint protection measures, including application whitelisting and behavior-based detection to identify anomalous execution patterns related to Office Online Server or Excel processes. 3. Limit local access to systems running Office Online Server to trusted personnel only and enforce strong authentication and access controls. 4. Educate users on the risks of interacting with untrusted Excel files or links, emphasizing cautious behavior to reduce the risk of user interaction exploitation. 5. Employ network segmentation to isolate Office Online Server environments from less secure network zones, minimizing lateral movement opportunities. 6. Monitor logs and system behavior for signs of exploitation attempts, such as unexpected process launches or memory access violations related to Office Online Server components. 7. Consider disabling or restricting Excel functionality within Office Online Server if not required, reducing the attack surface. 8. Use endpoint detection and response (EDR) tools to detect and respond to suspicious local activity promptly.