CVE-2025-54896 is a high-severity use-after-free vulnerability (CWE-416) found in Microsoft Office Online Server, specifically affecting the Excel component. This vulnerability allows an unauthorized attacker to execute code locally on the affected system. The flaw arises when the application improperly manages memory, freeing an object while it is still in use, which can lead to arbitrary code execution. Exploitation requires local access and some user interaction, such as opening a malicious Excel file or interacting with a crafted document served via Office Online Server. The CVSS 3.1 base score of 7.8 reflects the high impact on confidentiality, integrity, and availability, with low attack complexity and no privileges required, but user interaction is necessary. Although no known exploits are currently in the wild, the vulnerability poses a significant risk due to the critical nature of Office Online Server in enterprise environments and the potential for privilege escalation or lateral movement within networks.

For European organizations, this vulnerability could have serious consequences. Office Online Server is widely deployed in enterprises to provide web-based access to Office documents, including Excel spreadsheets. Successful exploitation could lead to unauthorized code execution on servers hosting sensitive documents, potentially compromising confidential business data, intellectual property, and personal data protected under GDPR. The integrity of documents could be altered, and availability of the service disrupted, impacting business continuity. Given the local attack vector, attackers might leverage phishing or social engineering to induce user interaction, increasing the risk of targeted attacks against high-value organizations. The breach of confidentiality and integrity could also lead to regulatory penalties and reputational damage within the European market.

Organizations should prioritize patching as soon as Microsoft releases an official update addressing CVE-2025-54896. In the interim, restrict local access to Office Online Server hosts to trusted personnel only and enforce strict user access controls. Implement application whitelisting and endpoint protection solutions capable of detecting anomalous behavior indicative of exploitation attempts. Disable or limit the ability to open untrusted Excel documents via Office Online Server, and educate users about the risks of interacting with unsolicited or suspicious files. Network segmentation should be employed to isolate Office Online Server from critical infrastructure to contain potential breaches. Regularly audit and monitor logs for unusual activity related to Office Online Server processes. Additionally, consider deploying exploit mitigation technologies such as memory protection features (e.g., Control Flow Guard) to reduce exploitation success.