CVE-2025-53735 is a use-after-free vulnerability classified under CWE-416 found in Microsoft Office Online Server's Excel component, specifically version 16.0.0.0. This vulnerability arises from improper handling of memory, where a program continues to use memory after it has been freed, leading to undefined behavior. An attacker can exploit this flaw to execute arbitrary code locally on the affected system without requiring any privileges but does require user interaction, such as opening a malicious Excel file or triggering a specific action within the Office Online Server environment. The vulnerability impacts confidentiality, integrity, and availability by allowing code execution that could lead to data theft, system manipulation, or denial of service. The CVSS v3.1 base score is 7.8, indicating high severity, with attack vector local (AV:L), low attack complexity (AC:L), no privileges required (PR:N), and user interaction required (UI:R). The scope is unchanged (S:U), and the impact on confidentiality, integrity, and availability is high (C:H/I:H/A:H). Although no public exploits are known yet, the vulnerability's characteristics suggest it could be weaponized in targeted attacks. The lack of available patches at the time of publication necessitates immediate risk mitigation through alternative controls. This vulnerability is particularly relevant for organizations relying on Office Online Server for Excel document processing and collaboration, as it could be leveraged to compromise internal systems.

For European organizations, this vulnerability poses a significant risk due to the widespread use of Microsoft Office Online Server in enterprise environments for document collaboration and processing. Exploitation could lead to unauthorized code execution on servers handling sensitive Excel documents, potentially exposing confidential business data, intellectual property, and personal information protected under GDPR. The ability to execute code locally without privileges but requiring user interaction means that social engineering or malicious document delivery could be effective attack vectors. Disruption of Office Online Server services could impact business continuity and productivity. Organizations in sectors such as finance, government, healthcare, and critical infrastructure, which heavily rely on Microsoft Office ecosystems, face heightened risks. The vulnerability could also facilitate lateral movement within networks if exploited, increasing the scope of compromise. The absence of known exploits currently provides a window for proactive defense, but the high severity score underscores the urgency of mitigation.

1. Monitor Microsoft security advisories closely and apply official patches immediately once released for Office Online Server version 16.0.0.0. 2. Until patches are available, restrict local access to Office Online Server hosts to trusted administrators only, minimizing the attack surface. 3. Implement strict file upload and document validation controls to prevent malicious Excel files from being processed by the server. 4. Educate users and administrators about the risks of opening untrusted Excel documents and the importance of verifying document sources. 5. Employ endpoint detection and response (EDR) solutions to monitor for suspicious behaviors indicative of use-after-free exploitation attempts. 6. Use application whitelisting and privilege restrictions on servers hosting Office Online Server to limit the impact of potential code execution. 7. Regularly audit and review logs for anomalous activities related to Excel document processing. 8. Consider network segmentation to isolate Office Online Server infrastructure from critical internal systems to contain potential breaches.