CVE-2025-54898 is a high-severity vulnerability identified in Microsoft Office Online Server, specifically affecting the Excel component. The vulnerability is classified as CWE-125, which corresponds to an out-of-bounds read error. This type of flaw occurs when a program reads data past the boundary of allocated memory, potentially leading to the exposure of sensitive information or enabling further exploitation. In this case, the vulnerability allows an unauthorized attacker to execute code locally on the affected system. The CVSS v3.1 base score is 7.8, indicating a high level of severity. The vector string (CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H) reveals that the attack requires local access (AV:L), low attack complexity (AC:L), no privileges (PR:N), and user interaction (UI:R). The scope is unchanged (S:U), and the impact on confidentiality, integrity, and availability is high (C:H/I:H/A:H). The vulnerability was reserved on July 31, 2025, and published on September 9, 2025. No known exploits are currently in the wild, and no patches have been linked yet. The affected version is 16.0.0.0 of Microsoft Office Online Server. The vulnerability could be exploited by tricking a user into opening a malicious Excel file or interacting with a compromised Office Online Server instance, leading to local code execution. This could allow attackers to escalate privileges, steal sensitive data, or disrupt services.

For European organizations, this vulnerability poses significant risks, especially those heavily reliant on Microsoft Office Online Server for collaborative document editing and sharing. Successful exploitation could lead to unauthorized code execution on servers or user machines, potentially compromising confidential business data, intellectual property, and personal information protected under GDPR. The high impact on confidentiality, integrity, and availability means attackers could manipulate or destroy critical documents, disrupt business operations, or use compromised systems as footholds for further network intrusion. Sectors such as finance, healthcare, government, and critical infrastructure, which often use Microsoft Office Online Server for document workflows, are particularly at risk. Additionally, the requirement for local access and user interaction suggests that phishing or social engineering campaigns could be used to deliver malicious files or links, increasing the threat surface. The absence of known exploits in the wild currently provides a window for proactive mitigation before widespread attacks occur.

European organizations should prioritize the following specific mitigation steps: 1) Immediately inventory and identify all instances of Microsoft Office Online Server version 16.0.0.0 in their environment. 2) Monitor official Microsoft channels closely for the release of security patches addressing CVE-2025-54898 and apply them promptly once available. 3) Implement strict access controls to limit local access to Office Online Server hosts, including the use of network segmentation and least privilege principles. 4) Educate users about the risks of opening unsolicited or suspicious Excel files, emphasizing the need for caution with links or attachments received via email or collaboration platforms. 5) Deploy endpoint detection and response (EDR) solutions capable of detecting anomalous local code execution behaviors related to Office applications. 6) Utilize application whitelisting to prevent unauthorized code execution on critical systems. 7) Regularly audit and harden Office Online Server configurations to minimize attack vectors. 8) Employ advanced email filtering and anti-phishing technologies to reduce the likelihood of malicious file delivery. These targeted actions go beyond generic advice by focusing on the specific attack vector and environment of this vulnerability.