CVE-2025-59235 is an out-of-bounds read vulnerability categorized under CWE-125 affecting Microsoft Office Online Server, specifically its Excel component. This vulnerability arises from improper bounds checking during memory reads, allowing an attacker to read memory locations outside the intended buffer. The flaw enables unauthorized local attackers to disclose sensitive information residing in memory, potentially exposing confidential data. The vulnerability requires local access to the system and some user interaction but does not require privileges or elevated permissions, making it accessible to low-privileged users on the affected system. The CVSS 3.1 base score is 7.1, indicating high severity, with vector metrics AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:H, meaning local attack vector, low attack complexity, no privileges required, user interaction required, unchanged scope, high confidentiality impact, no integrity impact, and high availability impact. The vulnerability affects Microsoft Office Online Server version 16.0.0.0, a platform used to provide browser-based Office applications including Excel Online. Exploitation could lead to information disclosure and potential denial of service due to memory corruption effects. No public exploits or patches are currently available, but the vulnerability is published and should be addressed promptly once mitigations are released. The flaw is significant because Office Online Server is widely deployed in enterprise environments for collaborative document editing and processing, making the confidentiality of data critical. Attackers exploiting this vulnerability could gain access to sensitive information stored in memory buffers during Excel document processing, potentially leading to data leaks or further attacks.

For European organizations, the impact of CVE-2025-59235 is primarily the risk of unauthorized disclosure of sensitive information processed through Microsoft Office Online Server, particularly Excel documents. This could affect confidentiality of corporate data, intellectual property, and personal information, leading to compliance violations under GDPR and other data protection regulations. The availability impact could disrupt business operations relying on Office Online Server for document collaboration and processing, causing productivity losses. Since the attack requires local access and user interaction, insider threats or compromised endpoints pose a significant risk vector. Organizations in sectors such as finance, healthcare, government, and critical infrastructure that rely heavily on Microsoft Office Online Server are particularly vulnerable. The lack of current public exploits reduces immediate risk but also means organizations must proactively patch and monitor to prevent future exploitation. The vulnerability could also be leveraged as part of a multi-stage attack chain to escalate access or move laterally within networks, increasing overall risk exposure.

1. Monitor Microsoft security advisories closely and apply official patches or updates for Office Online Server version 16.0.0.0 as soon as they become available. 2. Restrict local access to Office Online Server hosts to trusted administrators and users only, minimizing the attack surface. 3. Implement strict endpoint security controls to prevent unauthorized local access and reduce the risk of malicious user interaction. 4. Employ application whitelisting and behavior monitoring on servers hosting Office Online Server to detect anomalous activities indicative of exploitation attempts. 5. Conduct regular security audits and vulnerability assessments focusing on Office Online Server deployments. 6. Educate users about the risks of interacting with untrusted content or executing unknown actions within Office Online Server environments. 7. Utilize network segmentation to isolate Office Online Server infrastructure from less secure network zones. 8. Maintain comprehensive logging and alerting to identify suspicious access patterns or potential exploitation attempts. 9. Consider deploying additional memory protection mechanisms or runtime application self-protection (RASP) if supported by the environment. 10. Prepare incident response plans specifically addressing potential exploitation of memory corruption vulnerabilities in Office Online Server.