CVE-2025-53739 is a vulnerability classified under CWE-843 (Access of Resource Using Incompatible Type, or type confusion) found in Microsoft Office Online Server, specifically affecting the Excel component. This vulnerability arises when the software incorrectly handles resource types, allowing an attacker to confuse the program into accessing or manipulating data as an incompatible type. This flaw can be exploited by an unauthorized attacker to execute arbitrary 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 v3.1 score is 7.8 (high), with vector AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H, indicating that the attack requires local access and user interaction but no privileges, and can lead to high impact on confidentiality, integrity, and availability. Although no known exploits are currently reported in the wild, the nature of type confusion vulnerabilities often allows attackers to bypass security controls and execute malicious code, potentially leading to full system compromise. The vulnerability is particularly concerning in environments where Office Online Server is used to process Excel files, as maliciously crafted files could trigger the exploit. The lack of available patches at the time of publication increases the urgency for organizations to implement interim mitigations.

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. Successful exploitation could lead to local code execution, allowing attackers to escalate privileges, steal sensitive data, or disrupt services. This impacts confidentiality, integrity, and availability of critical business information and systems. Organizations in sectors such as finance, government, healthcare, and critical infrastructure, which heavily rely on Microsoft Office services, could face operational disruptions and data breaches. The requirement for local access and user interaction somewhat limits remote exploitation but does not eliminate risk, especially in environments where users frequently open Excel files from external sources. The absence of known exploits currently provides a window for proactive defense, but the high severity score indicates that once exploits emerge, the threat could rapidly escalate.

1. Monitor Microsoft’s official channels for patches addressing CVE-2025-53739 and apply them promptly once available. 2. Implement strict access controls to limit local access to systems running Office Online Server, reducing the risk of local exploitation. 3. Educate users on the risks of opening Excel files from untrusted or unknown sources to prevent triggering the vulnerability. 4. Employ application whitelisting and endpoint protection solutions capable of detecting anomalous behavior related to Office applications. 5. Use network segmentation to isolate Office Online Server environments from less trusted networks and users. 6. Regularly audit and monitor logs for unusual activity that could indicate exploitation attempts. 7. Consider disabling or restricting Excel file processing in Office Online Server if feasible until patches are applied. 8. Maintain up-to-date backups to enable recovery in case of compromise.