CVE-2025-54900 is a heap-based buffer overflow vulnerability classified under CWE-122, found in Microsoft Office Online Server's Excel component version 16.0.0.0. This vulnerability arises from improper handling of memory buffers, allowing an attacker to overwrite heap memory, potentially leading to arbitrary code execution. The flaw can be triggered by an unauthorized attacker with local access who can induce a user interaction, such as opening a maliciously crafted Excel file through the Office Online Server interface. The CVSS 3.1 base score is 7.8, reflecting high impact on confidentiality, integrity, and availability, with low attack complexity and no privileges required. The vulnerability does not currently have known exploits in the wild, but its presence in a widely deployed enterprise product makes it a critical concern. The lack of an available patch at the time of disclosure necessitates immediate risk mitigation. The vulnerability could allow attackers to execute code with the privileges of the Office Online Server process, potentially leading to system compromise or lateral movement within a network. Given the integration of Office Online Server in many enterprise environments, exploitation could disrupt business operations and expose sensitive data.

European organizations relying on Microsoft Office Online Server for collaborative document editing and sharing face significant risks from this vulnerability. Exploitation could lead to unauthorized code execution on servers, compromising sensitive corporate data and disrupting critical business workflows. The impact extends to confidentiality breaches, data integrity loss, and potential denial of service if the server crashes due to exploitation. Organizations in sectors such as finance, government, healthcare, and critical infrastructure are particularly vulnerable due to their reliance on Office Online Server and the sensitivity of their data. The requirement for local access and user interaction somewhat limits remote exploitation but does not eliminate risk, especially in environments with many users and potential insider threats. The absence of a patch increases exposure time, making proactive mitigation essential. Additionally, the vulnerability could be leveraged as a foothold for further attacks within a network, amplifying its impact on European enterprises.

1. Immediately restrict local access to servers running Microsoft Office Online Server to trusted personnel only, minimizing the attack surface. 2. Implement strict user privilege controls and monitor user activities to detect any suspicious behavior indicative of exploitation attempts. 3. Disable or limit the use of Excel file processing in Office Online Server if feasible until a patch is released. 4. Employ application whitelisting and endpoint protection solutions capable of detecting anomalous code execution patterns related to heap overflows. 5. Monitor system logs and network traffic for unusual activity that may signal exploitation attempts. 6. Prepare for rapid deployment of official patches from Microsoft once available, including testing in controlled environments before production rollout. 7. Educate users about the risks of opening untrusted Excel files, even within internal environments, to reduce the likelihood of triggering the vulnerability. 8. Consider network segmentation to isolate Office Online Server instances from critical infrastructure to limit lateral movement in case of compromise.