CVE-2025-30376 is a high-severity heap-based buffer overflow vulnerability identified in Microsoft Office Online Server, specifically affecting version 1.0.0. The vulnerability arises from improper handling of memory buffers within the Microsoft Office Excel component of the Office Online Server. An attacker can exploit this flaw by crafting malicious Excel content that, when processed by the vulnerable server, triggers a heap overflow condition. This overflow can corrupt adjacent memory, potentially allowing an unauthorized attacker to execute arbitrary code locally on the server hosting Office Online Server. The vulnerability does not require privileges (PR:N) but does require user interaction (UI:R), meaning the attacker must convince a user to open or process a malicious Excel file via the online server. The attack vector is local (AV:L), indicating exploitation requires local access or interaction with the server environment. The vulnerability impacts confidentiality, integrity, and availability, as successful exploitation can lead to full code execution, data compromise, or service disruption. The CVSS 3.1 base score is 7.8, reflecting high severity. No known exploits are currently reported in the wild, and no patches have been linked yet, indicating that mitigation may rely on workarounds or monitoring until official updates are released. The vulnerability is classified under CWE-122 (Heap-based Buffer Overflow), a common and dangerous memory corruption issue that can lead to arbitrary code execution.

For European organizations, this vulnerability poses a significant risk, especially for enterprises and public sector entities relying on Microsoft Office Online Server to provide web-based Office document editing and collaboration. Exploitation could allow attackers to gain code execution on critical servers, leading to data breaches, unauthorized access to sensitive documents, and potential lateral movement within networks. Given the widespread use of Microsoft Office products across Europe, organizations using Office Online Server as part of their collaboration infrastructure are at risk of service disruption and data compromise. The requirement for user interaction means phishing or social engineering campaigns could be leveraged to trigger the exploit. The impact is heightened in sectors with strict data protection regulations such as GDPR, where data breaches can result in severe legal and financial penalties. Additionally, the vulnerability could be leveraged by advanced persistent threat (APT) groups targeting European governmental or financial institutions, potentially affecting national security or economic stability.

Until an official patch is released, European organizations should implement specific mitigations: 1) Restrict access to Office Online Server to trusted internal networks and users only, minimizing exposure to untrusted external actors. 2) Employ strict input validation and scanning of Excel files uploaded or processed by the server to detect and block potentially malicious content. 3) Increase monitoring and logging on Office Online Server for unusual file processing activities or crashes indicative of exploitation attempts. 4) Educate users about the risks of opening untrusted Excel files via the online server and implement policies to limit file sharing from unknown sources. 5) Use application whitelisting and endpoint protection solutions on servers hosting Office Online Server to detect and prevent unauthorized code execution. 6) Prepare for rapid deployment of patches once Microsoft releases an update by maintaining an up-to-date inventory of affected systems and testing patch deployment procedures. 7) Consider network segmentation to isolate Office Online Server from critical infrastructure to limit potential lateral movement.