CVE-2025-54900 is a high-severity heap-based buffer overflow vulnerability identified in Microsoft Office Online Server, specifically affecting the Excel component. This vulnerability arises due to improper handling of memory buffers on the heap, which can be exploited by an unauthorized attacker to execute arbitrary code locally. The flaw allows an attacker to craft malicious Excel content that, when processed by the vulnerable Office Online Server version 16.0.0.0, triggers the overflow. This can lead to corruption of memory, enabling the attacker to gain control over the execution flow and execute code with the privileges of the Office Online Server process. 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 high, as successful exploitation can lead to full compromise of the affected system. Although no known exploits are currently reported in the wild, the vulnerability is publicly disclosed and patched status is not indicated, suggesting urgency for remediation. Given the nature of Office Online Server as a web-based collaborative platform for document editing, exploitation could affect multiple users and potentially lead to lateral movement within an enterprise network.

For European organizations, this vulnerability poses a significant risk, especially for enterprises and public sector entities relying on Microsoft Office Online Server for collaborative document management and editing. Exploitation could result in unauthorized code execution on servers that often handle sensitive corporate or governmental data, leading to data breaches, disruption of business operations, and potential exposure of confidential information. The local attack vector implies that attackers need some level of access or interaction, such as convincing a user to open a malicious Excel file via the online platform. However, once exploited, the attacker could escalate privileges or move laterally within the network, amplifying the impact. Given the widespread use of Microsoft Office products across Europe, the vulnerability could affect a broad range of sectors including finance, healthcare, government, and critical infrastructure, potentially undermining trust in cloud-based collaboration services and causing regulatory compliance issues under GDPR if personal data is compromised.

European organizations should prioritize the following mitigation steps: 1) Immediate patching: Monitor Microsoft’s official channels for patches or updates addressing CVE-2025-54900 and apply them promptly. 2) Restrict local access: Limit user permissions on servers running Office Online Server to reduce the risk of local exploitation. 3) Harden user interaction: Implement strict content filtering and scanning of Excel files uploaded or accessed via the Office Online Server to detect and block malicious files. 4) Network segmentation: Isolate Office Online Server instances from critical internal networks to contain potential breaches. 5) Monitor logs and behavior: Deploy advanced endpoint detection and response (EDR) tools to identify anomalous activities indicative of exploitation attempts. 6) User awareness: Train users to recognize suspicious files and avoid interacting with untrusted Excel documents. 7) Consider disabling or restricting Office Online Server features that allow uploading or processing of Excel files until a patch is applied, if operationally feasible. These targeted measures go beyond generic advice by focusing on the specific attack vector and environment of this vulnerability.