CVE-2025-21390 is a heap-based buffer overflow vulnerability (CWE-122) identified in Microsoft Office Online Server version 1.0.0, specifically impacting the Excel component. The vulnerability arises from improper handling of memory buffers when processing Excel files, which can be exploited by an attacker to execute arbitrary code remotely. The attack vector requires the victim to interact with a maliciously crafted Excel document served through the Office Online Server, but does not require any prior authentication, increasing the attack surface. The CVSS v3.1 base score is 7.8, reflecting high severity due to the potential for complete compromise of the affected system’s confidentiality, integrity, and availability. The vulnerability is exploitable with low complexity and no privileges required, but user interaction is necessary. No public exploits or patches are currently available, indicating the need for vigilance and proactive mitigation. The flaw could allow attackers to execute code under the context of the Office Online Server service account, potentially leading to lateral movement within networks or data exfiltration. Given the widespread deployment of Microsoft Office Online Server in enterprise environments, this vulnerability represents a significant risk, especially in sectors relying on cloud-based document collaboration and processing.

For European organizations, the impact of CVE-2025-21390 is substantial. Exploitation could lead to remote code execution on servers that handle sensitive document processing, potentially exposing confidential business data and intellectual property. The compromise of Office Online Server could serve as a foothold for attackers to move laterally within corporate networks, escalate privileges, and disrupt business operations. Critical sectors such as finance, government, healthcare, and manufacturing that rely on Microsoft Office Online Server for document collaboration and workflow automation are particularly vulnerable. The disruption or data breach resulting from this vulnerability could lead to regulatory penalties under GDPR and damage organizational reputation. Additionally, the requirement for user interaction means phishing or social engineering campaigns could be leveraged to trigger exploitation, increasing the risk profile. The lack of available patches at this time necessitates immediate risk management and mitigation efforts to prevent potential attacks.

1. Monitor Microsoft security advisories closely and apply official patches or updates for Office Online Server as soon as they are released. 2. Restrict access to Office Online Server to trusted networks and users via network segmentation and firewall rules to reduce exposure. 3. Implement strict email filtering and user awareness training to reduce the likelihood of users interacting with malicious Excel files. 4. Employ application whitelisting and endpoint detection and response (EDR) solutions to detect anomalous behavior indicative of exploitation attempts. 5. Regularly audit and harden Office Online Server configurations, disabling unnecessary features and services to minimize attack surface. 6. Use network intrusion detection systems (NIDS) to monitor for suspicious traffic patterns related to Office Online Server. 7. Consider deploying web application firewalls (WAF) with custom rules to detect and block exploit attempts targeting this vulnerability. 8. Maintain up-to-date backups and incident response plans to enable rapid recovery in case of compromise.