CVE-2025-54900 is a heap-based buffer overflow vulnerability classified under CWE-122, affecting Microsoft Office Online Server version 16.0.0.0, specifically within the Excel component. This vulnerability arises from improper handling of memory buffers when processing Excel files, allowing an attacker to overwrite heap memory. Exploitation requires the victim to interact with a specially crafted Excel document, which triggers the overflow and enables execution of arbitrary code with the privileges of the user running the Office Online Server. The CVSS 3.1 score of 7.8 reflects a high severity due to the potential for complete compromise of the affected system's confidentiality, integrity, and availability. The attack vector is local (AV:L), meaning the attacker must have local access or trick a user into opening a malicious file, but no privileges are required beforehand (PR:N). User interaction is necessary (UI:R), and the scope remains unchanged (S:U). The vulnerability is currently published but lacks known exploits in the wild, indicating that while weaponization is possible, it has not yet been observed in active attacks. The absence of patches at the time of reporting highlights the urgency for organizations to prepare mitigations and monitor for updates from Microsoft. This vulnerability is particularly concerning for environments relying on Office Online Server for collaborative document editing and sharing, as it could lead to unauthorized code execution and potential lateral movement within networks.

For European organizations, the impact of CVE-2025-54900 could be substantial. Office Online Server is widely used in enterprise environments for document collaboration, and a successful exploit could lead to unauthorized code execution on servers handling sensitive documents. This threatens the confidentiality of intellectual property and personal data, the integrity of documents and workflows, and the availability of critical collaboration services. Attackers could leverage this vulnerability to deploy malware, establish persistence, or move laterally within corporate networks. Given the high reliance on Microsoft products across Europe, especially in sectors like finance, government, and healthcare, the disruption or compromise of Office Online Server could have cascading effects on business operations and regulatory compliance, including GDPR. The requirement for user interaction somewhat limits exploitation but does not eliminate risk, as phishing or social engineering could be used to deliver malicious Excel files. The lack of known exploits currently provides a window for proactive defense, but the high severity score underscores the need for immediate attention.

1. Monitor Microsoft security advisories closely and apply official patches or updates as soon as they become available for Office Online Server version 16.0.0.0. 2. Implement strict file upload and content filtering policies on Office Online Server to block or quarantine suspicious Excel files, especially those from untrusted sources. 3. Educate users and administrators about the risks of opening unsolicited or unexpected Excel documents, emphasizing caution with files received via email or external sources. 4. Employ application whitelisting and endpoint protection solutions that can detect and block exploitation attempts targeting heap-based buffer overflows. 5. Restrict permissions and run Office Online Server services with the least privilege necessary to limit the impact of potential code execution. 6. Enable detailed logging and monitoring on Office Online Server to detect anomalous behavior indicative of exploitation attempts. 7. Consider network segmentation to isolate Office Online Server from critical infrastructure to reduce lateral movement opportunities. 8. Use sandboxing or virtualized environments to open and analyze suspicious Excel files safely before allowing them into production environments.