CVE-2025-29977 is a high-severity use-after-free vulnerability (CWE-416) found in Microsoft Office Online Server, specifically affecting version 1.0.0. This vulnerability arises from improper memory management in the handling of Microsoft Office Excel files within the Office Online Server environment. A use-after-free flaw occurs when a program continues to use a pointer after the memory it points to has been freed, potentially allowing attackers to execute arbitrary code. In this case, an unauthorized attacker can exploit this vulnerability to execute code locally on the server hosting Office Online Server by crafting malicious Excel files that trigger the use-after-free condition. The CVSS 3.1 base score is 7.8, indicating a high severity level, with the vector AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C. This means the attack requires local access (AV:L), low attack complexity (AC:L), no privileges (PR:N), and user interaction (UI:R). The impact on confidentiality, integrity, and availability is high, and the scope remains unchanged. The vulnerability was published on May 13, 2025, and no known exploits in the wild have been reported yet. No patches or mitigations have been linked at the time of publication, so organizations must monitor for updates from Microsoft. The vulnerability is critical because it allows code execution, potentially leading to full system compromise of the Office Online Server, which is often used in enterprise environments to provide web-based Office document editing and collaboration.

For European organizations, the impact of CVE-2025-29977 could be significant, especially for enterprises and public sector entities that rely on Microsoft Office Online Server to provide web-based document editing and collaboration services. Successful exploitation could lead to unauthorized code execution on the server, resulting in data breaches, service disruption, or lateral movement within the network. Confidential information processed or stored via Office Online Server could be exposed or altered, impacting data privacy compliance obligations such as GDPR. The availability of the service could also be compromised, affecting business continuity. Since the attack requires local access and user interaction, the threat vector may involve phishing or social engineering to trick users into opening malicious Excel files. However, once exploited, the attacker could gain elevated control over the server environment, posing a severe risk to organizational security posture.

European organizations should implement the following specific mitigation measures: 1) Restrict local access to Office Online Server environments to trusted administrators only, minimizing the attack surface. 2) Enforce strict user training and awareness programs to reduce the risk of users opening malicious Excel files, emphasizing caution with files from untrusted sources. 3) Employ application whitelisting and endpoint protection solutions on servers hosting Office Online Server to detect and block suspicious activities related to memory corruption exploits. 4) Monitor logs and network traffic for unusual behavior indicative of exploitation attempts, such as unexpected process launches or memory access violations. 5) Segregate Office Online Server infrastructure from critical network segments to limit lateral movement in case of compromise. 6) Regularly check for and apply official security patches or updates from Microsoft as soon as they become available. 7) Consider deploying additional runtime memory protection technologies (e.g., Control Flow Guard, Data Execution Prevention) on servers to mitigate exploitation of use-after-free vulnerabilities.