CVE-2025-21387 is a use-after-free vulnerability (CWE-416) identified in Microsoft Office Online Server version 1.0.0, specifically impacting the Excel component. This vulnerability arises when the server improperly manages memory during the processing of Excel files, leading to a use-after-free condition. An attacker can exploit this by crafting a malicious Excel file and convincing a user to open it through the Office Online Server interface. The vulnerability allows remote code execution (RCE) with high impact on confidentiality, integrity, and availability. The CVSS v3.1 score is 7.8, indicating high severity, with an attack vector classified as local (AV:L), low attack complexity (AC:L), no privileges required (PR:N), but requiring user interaction (UI:R). The scope is unchanged (S:U), and the impact on confidentiality, integrity, and availability is high (C:H/I:H/A:H). The exploitability is rated as unproven (E:U), and the remediation level is official (RL:O) with confirmed report confidence (RC:C). No patches are currently available, and no known exploits have been observed in the wild. The vulnerability poses a significant risk for environments where Office Online Server is deployed, especially in enterprise and cloud scenarios where Excel files are frequently processed online. Attackers leveraging this vulnerability could execute arbitrary code on the server, potentially leading to full system compromise or lateral movement within the network.

For European organizations, the impact of CVE-2025-21387 is substantial. Office Online Server is commonly used in enterprise environments to provide browser-based access to Microsoft Office documents, including Excel spreadsheets. A successful exploit could allow attackers to execute arbitrary code remotely, leading to data breaches, disruption of business operations, and potential ransomware deployment. The high impact on confidentiality, integrity, and availability means sensitive corporate data could be exposed or altered, and critical services could be disrupted. Given the reliance on Microsoft cloud and hybrid solutions in Europe, especially in sectors like finance, government, and manufacturing, the vulnerability could be leveraged to target high-value assets. Additionally, the requirement for user interaction means phishing or social engineering campaigns could be used to deliver malicious files, increasing the attack surface. The lack of available patches heightens the urgency for organizations to implement interim mitigations to reduce risk.

1. Implement strict file upload and content filtering policies on Office Online Server to block or quarantine suspicious Excel files before processing. 2. Educate users on the risks of opening unsolicited or unexpected Excel files, especially those accessed via Office Online Server. 3. Restrict access to Office Online Server to trusted networks and users using network segmentation and access control lists (ACLs). 4. Monitor server logs and user activity for unusual behavior indicative of exploitation attempts, such as unexpected file uploads or execution anomalies. 5. Employ endpoint detection and response (EDR) solutions on servers hosting Office Online Server to detect and respond to suspicious processes or memory corruption attempts. 6. Prepare for rapid deployment of official patches from Microsoft once released, including testing in staging environments to minimize downtime. 7. Consider disabling or limiting Excel file processing capabilities on Office Online Server if not essential, to reduce attack surface. 8. Use multi-factor authentication (MFA) and strong identity management to reduce the risk of compromised user credentials facilitating exploitation.