CVE-2025-54898 is a high-severity vulnerability classified as CWE-125 (Out-of-bounds Read) affecting Microsoft Office Online Server, specifically version 16.0.0.0. The vulnerability arises from an out-of-bounds read condition in the Microsoft Office Excel component within the Office Online Server environment. This flaw allows an unauthorized attacker to execute code locally on the affected system. The vulnerability does not require prior authentication (PR:N) but does require user interaction (UI:R), such as opening a maliciously crafted Excel file via the Office Online Server interface. The attack vector is local (AV:L), meaning the attacker must have local access or the ability to induce the user to open the malicious content through the online server. The vulnerability impacts confidentiality, integrity, and availability at a high level (C:H/I:H/A:H), indicating that successful exploitation could lead to full system compromise, data leakage, or denial of service. The CVSS score of 7.8 reflects these factors. Although no known exploits are currently observed in the wild, the vulnerability is publicly disclosed and should be treated with urgency. The lack of available patches at the time of publication increases the risk window. The root cause is an out-of-bounds read, which can lead to memory corruption and subsequent arbitrary code execution. This vulnerability is particularly critical in environments where Office Online Server is used to provide browser-based access to Excel documents, as it expands the attack surface to remote users who can interact with the server-hosted documents.

For European organizations, the impact of CVE-2025-54898 can be significant, especially for enterprises and public sector entities relying on Microsoft Office Online Server to facilitate collaborative document editing and sharing. Exploitation could lead to unauthorized code execution on servers hosting sensitive data, potentially resulting in data breaches, disruption of business operations, and loss of intellectual property. Given the high confidentiality, integrity, and availability impact, attackers could gain persistent access, manipulate or exfiltrate data, or disrupt services critical to business continuity. Organizations in regulated sectors such as finance, healthcare, and government may face compliance violations and reputational damage. The requirement for user interaction means phishing or social engineering attacks could be leveraged to induce users to open malicious Excel files via the online server interface. The absence of known exploits currently provides a window for proactive mitigation, but the public disclosure increases the risk of rapid exploit development. The vulnerability also poses risks to cloud service providers and managed service providers in Europe who offer Office Online Server as part of their service portfolio.

1. Immediate mitigation should include restricting access to Office Online Server to trusted users and networks, employing network segmentation and strict access controls to limit exposure. 2. Implement robust user awareness training to reduce the risk of social engineering attacks that could trigger exploitation via malicious Excel files. 3. Monitor logs and network traffic for unusual activity related to Office Online Server, including unexpected file uploads or execution attempts. 4. Apply the principle of least privilege to service accounts and users interacting with the Office Online Server to minimize potential damage from exploitation. 5. Since no patches are currently available, consider deploying virtual patching or intrusion prevention system (IPS) rules that detect and block attempts to exploit out-of-bounds read patterns in Excel files. 6. Plan for rapid deployment of official patches once released by Microsoft, including testing in staging environments to ensure compatibility and stability. 7. Evaluate alternative document collaboration solutions temporarily if risk exposure is deemed unacceptable until a patch is available. 8. Regularly update and harden the underlying operating system and supporting infrastructure to reduce the overall attack surface.