CVE-2025-53759 is a vulnerability classified under CWE-908 (Use of Uninitialized Resource) affecting Microsoft Office Online Server version 16.0.0.0, specifically within the Microsoft Office Excel component. The flaw stems from the software using resources that have not been properly initialized before use, which can lead to unpredictable behavior. An attacker can exploit this by crafting a malicious Excel file that, when opened by a user on the Office Online Server, triggers local code execution without requiring prior authentication. The CVSS v3.1 score of 7.8 reflects a high severity, with an attack vector of local (AV:L), low attack complexity (AC:L), no privileges required (PR:N), but requiring 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). This means an attacker can fully compromise the affected system, potentially leading to data theft, data manipulation, or service disruption. The vulnerability was reserved in early July 2025 and published in August 2025, with no known exploits in the wild yet. The absence of patches at the time of reporting indicates that organizations must be vigilant and prepare to deploy updates promptly once available. The vulnerability is particularly concerning for environments where Office Online Server is used to provide web-based document editing capabilities, as it could allow attackers to bypass typical network protections by leveraging user interaction. The lack of required privileges means any user with access to the server or the ability to induce a user to open a malicious file could trigger the exploit.

For European organizations, the impact of CVE-2025-53759 could be significant, especially for enterprises and public sector entities relying heavily on Microsoft Office Online Server for collaborative document editing and workflow. Successful exploitation could lead to unauthorized code execution on critical servers, resulting in data breaches, loss of data integrity, and potential service outages. This could disrupt business operations, lead to regulatory non-compliance (e.g., GDPR violations due to data exposure), and damage organizational reputation. The high impact on confidentiality, integrity, and availability means sensitive corporate or governmental data could be exposed or altered. Additionally, local code execution could serve as a foothold for lateral movement within networks, amplifying the threat. Given the increasing reliance on cloud and hybrid environments in Europe, the vulnerability could also affect hosted Office Online Server deployments, increasing the attack surface. Organizations in sectors such as finance, healthcare, government, and critical infrastructure are particularly at risk due to the sensitivity of their data and the potential consequences of disruption.

1. Monitor Microsoft security advisories closely and apply patches or updates for Office Online Server 16.0.0.0 immediately upon release to remediate the vulnerability. 2. Restrict access to Office Online Server instances to trusted users and networks, employing network segmentation and firewall rules to limit exposure. 3. Implement strict file upload and content scanning policies to detect and block malicious Excel files before they reach the server. 4. Employ application whitelisting and endpoint protection on servers hosting Office Online Server to detect and prevent unauthorized code execution. 5. Educate users about the risks of opening untrusted Excel files, especially in environments where Office Online Server is accessible. 6. Enable and review detailed logging and monitoring on Office Online Server to detect anomalous activities indicative of exploitation attempts. 7. Consider deploying additional security controls such as sandboxing or containerization for Office Online Server to limit the impact of potential exploits. 8. Conduct regular vulnerability assessments and penetration testing focused on Office Online Server deployments to identify and address security gaps proactively.