CVE-2025-54900 is a high-severity heap-based buffer overflow vulnerability identified in Microsoft Office Online Server, specifically affecting the Excel component. The 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 on the affected system. The flaw is classified under CWE-122, indicating a heap-based buffer overflow, which typically allows attackers to overwrite memory regions, potentially leading to code execution, data corruption, or system crashes. The vulnerability affects version 16.0.0.0 of Office Online Server. According to the CVSS 3.1 vector (7.8), the attack requires local access (AV:L), no privileges (PR:N), and user interaction (UI:R), with low attack complexity (AC:L). The impact on confidentiality, integrity, and availability is high (C:H/I:H/A:H), meaning successful exploitation could lead to full compromise of the affected system. Although no known exploits are currently in the wild, the vulnerability is publicly disclosed and classified as critical enough to warrant immediate attention. The lack of available patches at the time of disclosure increases the urgency for mitigation. The vulnerability allows execution of arbitrary code locally, which could be leveraged by attackers who have some form of access to the system, such as through phishing or malicious document delivery, to escalate privileges or move laterally within a network.

For European organizations, this vulnerability poses a significant risk, especially for enterprises and public sector entities that rely on Microsoft Office Online Server for collaborative document editing and sharing. Exploitation could lead to unauthorized code execution on servers hosting Office Online Server, potentially compromising sensitive business data, intellectual property, and personal data protected under GDPR. The high impact on confidentiality, integrity, and availability means attackers could disrupt business operations, steal or alter data, and establish persistent footholds within networks. Given the local attack vector and requirement for user interaction, phishing campaigns or insider threats could be used to trigger exploitation. Organizations in sectors such as finance, healthcare, government, and critical infrastructure are particularly at risk due to the sensitive nature of their data and regulatory requirements. Additionally, the vulnerability could be leveraged to bypass existing security controls if attackers gain initial access, increasing the risk of broader network compromise.

European organizations should implement the following specific mitigations: 1) Immediately audit and monitor all systems running Microsoft Office Online Server version 16.0.0.0 to identify affected instances. 2) Restrict local access to Office Online Server hosts to trusted personnel only, minimizing the risk of unauthorized local exploitation. 3) Implement strict user access controls and multi-factor authentication to reduce the likelihood of unauthorized user interaction that could trigger the vulnerability. 4) Employ application whitelisting and endpoint detection and response (EDR) solutions to detect and block suspicious activities related to code execution attempts. 5) Educate users on the risks of interacting with untrusted documents or links that could lead to local exploitation. 6) Monitor for unusual process behavior or memory anomalies indicative of heap overflow exploitation attempts. 7) Since no patches are currently available, consider isolating Office Online Server environments or deploying them in hardened virtualized environments to limit potential damage. 8) Stay updated with Microsoft advisories and apply patches immediately once released. 9) Conduct penetration testing and vulnerability scanning focused on Office Online Server to proactively identify exploitation attempts.