CVE-2025-53737 is a heap-based buffer overflow vulnerability identified in Microsoft Office Online Server, specifically affecting the Excel component. This vulnerability arises from improper handling of memory allocation on the heap, which can lead to overwriting adjacent memory regions. An unauthorized attacker can exploit this flaw by tricking a user into interacting with a maliciously crafted Excel file or content served through Office Online Server. Successful exploitation allows the attacker to execute arbitrary code locally on the affected system with no prior authentication required, although user interaction is necessary. The vulnerability is present in version 1.0.0 of Office Online Server. The CVSS 3.1 base score of 7.8 reflects a high severity level, with attack vector classified as local (AV:L), low attack complexity (AC:L), no privileges required (PR:N), but requiring user interaction (UI:R). The impact on confidentiality, integrity, and availability is high, meaning an attacker can fully compromise the affected system. No known exploits are currently reported in the wild, and no patches have been linked yet, indicating that organizations should prioritize monitoring and mitigation efforts. The vulnerability is categorized under CWE-122, which is a common weakness related to heap-based buffer overflows, a critical class of memory corruption bugs that often lead to remote code execution or privilege escalation. Given the nature of Office Online Server as a web-based collaborative platform for Office documents, exploitation could affect multiple users and systems within an organization if the server is exposed or used internally.

For European organizations, this vulnerability poses a significant risk, especially for enterprises and public sector entities relying on Microsoft Office Online Server for document collaboration and processing. Successful exploitation could lead to local code execution on servers or client machines, potentially allowing attackers to gain control over sensitive data, disrupt business operations, or move laterally within the network. Confidentiality breaches could expose personal data protected under GDPR, leading to regulatory penalties and reputational damage. Integrity and availability impacts could disrupt critical workflows, especially in sectors like finance, healthcare, and government where document handling is essential. Since the attack requires user interaction but no authentication, phishing or social engineering campaigns could be leveraged to trigger the exploit. The lack of a patch at the time of publication increases the urgency for organizations to implement compensating controls. The threat is amplified in environments where Office Online Server is internet-facing or insufficiently segmented from critical infrastructure.

European organizations should implement the following specific mitigations: 1) Immediately audit and inventory all instances of Microsoft Office Online Server version 1.0.0 to identify affected systems. 2) Restrict access to Office Online Server to trusted internal networks and VPNs, minimizing exposure to untrusted users. 3) Employ strict email and web filtering to reduce the risk of malicious Excel files reaching end users. 4) Educate users on the risks of interacting with unsolicited or suspicious Office documents, emphasizing the need for caution with links or embedded content. 5) Monitor logs and network traffic for unusual activity related to Office Online Server, including unexpected process executions or memory anomalies. 6) Implement application whitelisting and endpoint detection and response (EDR) solutions to detect and block exploitation attempts. 7) Prepare for rapid deployment of patches once Microsoft releases an official fix, including testing in controlled environments. 8) Consider isolating Office Online Server in a segmented network zone with limited privileges to contain potential compromise. 9) Regularly back up critical data and verify restoration procedures to mitigate impact of potential attacks.