CVE-2025-29977 is a high-severity use-after-free vulnerability (CWE-416) identified in Microsoft Office Online Server, specifically affecting version 1.0.0. This vulnerability arises from improper handling of memory in Microsoft Office Excel components 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 leading to arbitrary code execution, memory corruption, or system crashes. In this case, an unauthorized attacker can exploit this vulnerability to execute code locally on the affected system. The CVSS v3.1 base score is 7.8, indicating a high impact with the 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 does require user interaction. The vulnerability impacts confidentiality, integrity, and availability with high severity. Although no known exploits are currently reported in the wild, the vulnerability's nature and impact make it a significant risk. The lack of available patches at the time of publication increases the urgency for mitigation. The vulnerability was reserved in March 2025 and published in May 2025, indicating recent discovery and disclosure. The flaw specifically affects Office Online Server 1.0.0, a product used to provide browser-based access to Microsoft Office applications, including Excel, which is widely used in enterprise environments for document collaboration and editing.

For European organizations, this vulnerability poses a considerable risk, especially for enterprises relying on Microsoft Office Online Server for collaborative document editing and sharing. Successful exploitation could lead to local code execution, allowing attackers to escalate privileges, execute arbitrary commands, or deploy malware within the corporate network. This could result in data breaches, intellectual property theft, disruption of business operations, and potential lateral movement within the network. Given the high confidentiality, integrity, and availability impacts, organizations may face regulatory consequences under GDPR if sensitive personal data is compromised. The requirement for local access and user interaction somewhat limits remote exploitation but does not eliminate risk, as phishing or social engineering could be used to trick users into triggering the vulnerability. The absence of known exploits currently provides a window for proactive defense, but the high severity score necessitates immediate attention to prevent future exploitation.

European organizations should implement a multi-layered mitigation strategy beyond generic patching advice. First, restrict local access to systems running Office Online Server through strict network segmentation and access controls, limiting user permissions to the minimum necessary. Employ application whitelisting and endpoint protection solutions capable of detecting anomalous behavior indicative of exploitation attempts. Educate users about the risks of interacting with untrusted content or links that could trigger the vulnerability, emphasizing phishing awareness. Monitor logs and system behavior for unusual activity related to Office Online Server processes. Since no patches are currently available, consider deploying temporary workarounds such as disabling or restricting Excel functionality within Office Online Server if feasible. Maintain up-to-date backups and incident response plans tailored to potential exploitation scenarios. Finally, stay informed on vendor updates and apply official patches immediately upon release to remediate the vulnerability definitively.