CVE-2025-54904 is a high-severity use-after-free vulnerability (CWE-416) identified in Microsoft Office Online Server, specifically affecting the Excel component. This vulnerability allows an unauthorized attacker to execute arbitrary code locally on the affected system. The flaw arises from improper handling of memory in Office Excel Online Server version 16.0.0.0, where a previously freed memory region is accessed, leading to potential corruption of memory and execution of attacker-controlled code. Exploitation requires local access and user interaction, such as opening a malicious Excel file via the Office Online Server interface. The CVSS v3.1 base score is 7.8, reflecting high impact on confidentiality, integrity, and availability, with low attack complexity and no privileges required but user interaction necessary. Although no known exploits are currently reported 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 code execution that could lead to full system compromise.

For European organizations, the impact of this vulnerability could be substantial. Office Online Server is widely used in enterprises and public sector organizations across Europe to provide browser-based access to Microsoft Office applications. Successful exploitation could allow attackers to execute arbitrary code on servers hosting Office Online Server, potentially leading to data breaches, disruption of business operations, and lateral movement within corporate networks. Given the high confidentiality, integrity, and availability impact, sensitive information processed through Excel Online could be exposed or manipulated. Additionally, compromised servers could be leveraged to launch further attacks against internal infrastructure. The requirement for local access and user interaction somewhat limits remote exploitation, but phishing or social engineering could facilitate this vector. The absence of a patch at the time of publication increases the urgency for organizations to implement interim mitigations.

European organizations should implement the following specific mitigations: 1) Restrict access to Office Online Server interfaces to trusted internal networks and VPN users only, minimizing exposure to untrusted users. 2) Implement strict user access controls and monitoring to detect anomalous activities related to Excel Online usage. 3) Employ application whitelisting and endpoint protection on servers hosting Office Online Server to detect and block suspicious code execution attempts. 4) Educate users on the risks of opening untrusted Excel files and implement email filtering to reduce phishing attempts delivering malicious documents. 5) Monitor Microsoft security advisories closely for the release of official patches and apply them promptly once available. 6) Consider deploying network segmentation to isolate Office Online Server from critical backend systems to limit lateral movement in case of compromise. 7) Use logging and alerting mechanisms to detect exploitation attempts or unusual memory-related errors in Office Online Server logs.