CVE-2025-53735 is a use-after-free vulnerability classified under CWE-416 found in Microsoft Office Online Server, specifically impacting the Excel component. Use-after-free vulnerabilities occur when a program continues to use memory after it has been freed, leading to undefined behavior including potential arbitrary code execution. In this case, an unauthorized attacker can exploit this flaw to execute code locally on the affected system. The vulnerability affects version 16.0.0.0 of Office Online Server and was published on August 12, 2025. The CVSS 3.1 base score is 7.8, indicating high severity, with vector AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H, meaning the attack requires local access, low attack complexity, no privileges, but some user interaction, and impacts confidentiality, integrity, and availability substantially. No public exploit code or active exploitation has been reported yet. The vulnerability likely arises from improper memory management in Excel Online's handling of certain operations, allowing attackers to manipulate freed memory to execute arbitrary code. This can lead to full compromise of the affected server, data leakage, or service disruption. Since Office Online Server is often deployed in enterprise environments to provide browser-based Office functionality, exploitation could impact multiple users and connected services.

For European organizations, the impact of CVE-2025-53735 can be significant. Office Online Server is commonly used in enterprises to provide collaborative document editing and viewing capabilities. Successful exploitation could allow attackers to execute arbitrary code on the server hosting Office Online Server, potentially leading to full system compromise. This jeopardizes the confidentiality of sensitive documents, the integrity of business data, and the availability of critical collaboration services. Organizations handling regulated data under GDPR could face compliance risks and reputational damage if breaches occur. The requirement for local access limits remote exploitation but insider threats or attackers who gain initial footholds could leverage this vulnerability to escalate privileges and move laterally. Disruption of Office Online Server services could impair productivity across departments relying on Excel Online, especially in finance, accounting, and data analysis teams. The absence of known exploits in the wild 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 or updates for Office Online Server version 16.0.0.0 as soon as they are released. 2. Restrict local access to servers running Office Online Server by enforcing strict access control policies, including network segmentation and multi-factor authentication for administrative access. 3. Implement application whitelisting and endpoint protection solutions to detect and block suspicious behavior indicative of exploitation attempts. 4. Educate users about the risks of interacting with untrusted content or links that could trigger the vulnerability, as user interaction is required. 5. Regularly audit and harden the configuration of Office Online Server deployments, disabling unnecessary features and services to reduce attack surface. 6. Employ network monitoring and intrusion detection systems to identify anomalous activities around Office Online Server hosts. 7. Prepare incident response plans that include steps for containment and recovery in case of exploitation. 8. Consider isolating Office Online Server environments from critical infrastructure to limit potential impact.