CVE-2025-53737 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. The vulnerability arises when the application improperly handles memory allocation on the heap, allowing an attacker to overwrite memory regions beyond the allocated buffer. This can lead to arbitrary code execution with the privileges of the local user. The attack vector requires local access (AV:L), no privileges (PR:N), but user interaction (UI:R), indicating that an attacker must trick a user into triggering the vulnerability, such as opening a malicious Excel file via the Office Online Server interface. The vulnerability affects confidentiality, integrity, and availability (all rated high), as arbitrary code execution could lead to data theft, corruption, or service disruption. The CVSS score of 7.8 reflects a high severity but not critical, due to the local attack vector and required user interaction. No public exploits or patches are currently available, but the vulnerability has been officially published and reserved since July 2025. This vulnerability is significant because Office Online Server is widely used in enterprise environments to provide browser-based Office functionality, and Excel is a commonly used application, making exploitation potentially impactful.

For European organizations, this vulnerability poses a significant risk to data confidentiality, integrity, and service availability, especially in sectors relying heavily on Microsoft Office Online Server for collaborative document editing and sharing. Successful exploitation could allow attackers to execute arbitrary code locally, potentially leading to lateral movement within networks, data exfiltration, or disruption of business processes. Organizations in finance, government, healthcare, and critical infrastructure sectors are particularly vulnerable due to their reliance on Excel for sensitive data processing. The requirement for local access and user interaction somewhat limits remote exploitation but does not eliminate risk, especially in environments where endpoint security is weak or users are susceptible to social engineering. The absence of known exploits currently provides a window for proactive mitigation, but the high severity score necessitates urgent attention to prevent future attacks.

1. Monitor Microsoft security advisories closely and apply official patches immediately once released for Office Online Server version 16.0.0.0. 2. Restrict local access to Office Online Server hosts to trusted personnel only, minimizing the risk of local exploitation. 3. Implement strict user training and awareness programs to reduce the likelihood of users triggering malicious content requiring interaction. 4. Employ endpoint protection solutions capable of detecting anomalous behavior indicative of exploitation attempts, such as unusual memory operations or code injection. 5. Use application whitelisting and privilege restrictions to limit the impact of any successful code execution. 6. Regularly audit and harden Office Online Server configurations, disabling unnecessary features and services to reduce the attack surface. 7. Consider network segmentation to isolate Office Online Server environments from critical infrastructure and sensitive data repositories. 8. Maintain up-to-date backups and incident response plans to quickly recover from potential compromises.