CVE-2025-59235 is an out-of-bounds read vulnerability (CWE-125) identified in Microsoft Office Online Server, specifically within the Excel processing component. This vulnerability allows an attacker with local access to read memory beyond the intended buffer boundaries, potentially disclosing sensitive information stored in adjacent memory areas. The flaw arises from improper bounds checking during Excel file processing, which can be triggered by a specially crafted Excel document. Exploitation requires local access and user interaction, but no privileges or authentication are necessary, making it accessible to low-privileged users. The vulnerability impacts confidentiality by exposing potentially sensitive data and availability by causing application instability or crashes. The CVSS 3.1 score of 7.1 (AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:H) indicates a high severity due to the high confidentiality impact and availability impact, combined with low attack complexity. No public exploit code or active exploitation has been reported yet. The affected version is Microsoft Office Online Server 16.0.0.0. Since Office Online Server is widely used in enterprise environments for collaborative document editing, this vulnerability poses a significant risk if left unpatched.

For European organizations, this vulnerability could lead to unauthorized disclosure of sensitive information processed within Excel files on Office Online Server. Given the local access requirement, insider threats or compromised endpoints could exploit this flaw to extract confidential data, impacting data privacy and compliance with regulations such as GDPR. The availability impact could disrupt business operations relying on Office Online Server for document collaboration. Organizations in sectors handling sensitive financial, governmental, or personal data are particularly at risk. The lack of known exploits currently reduces immediate risk, but the presence of a high-severity vulnerability in a widely deployed Microsoft product necessitates prompt attention. Failure to address this vulnerability could lead to data breaches, regulatory penalties, and reputational damage.

1. Monitor Microsoft security advisories closely and apply official patches or updates for Office Online Server 16.0.0.0 as soon as they become available. 2. Restrict local access to servers running Office Online Server to trusted personnel only, employing strict access controls and endpoint security measures. 3. Implement application whitelisting and endpoint detection to identify and block attempts to exploit this vulnerability. 4. Conduct regular audits of user activity on Office Online Server to detect unusual behavior indicative of exploitation attempts. 5. Isolate Office Online Server environments from less trusted networks and users to minimize exposure. 6. Educate users about the risks of opening untrusted Excel files and enforce policies to limit the use of potentially malicious documents. 7. Consider deploying additional memory protection mechanisms or runtime application self-protection (RASP) tools to mitigate out-of-bounds read risks.