CVE-2025-54903 is a use-after-free vulnerability classified under CWE-416, found in Microsoft Office Online Server, specifically within the Microsoft Office Excel component. This vulnerability arises when the software improperly manages memory, freeing an object while it is still in use, which can lead to arbitrary code execution. An attacker can exploit this flaw by tricking a user into interacting with a maliciously crafted Excel document hosted or processed by the Office Online Server. The vulnerability requires user interaction but does not require the attacker to have any prior privileges on the system. Exploitation results in local code execution, potentially allowing the attacker to execute arbitrary code with the privileges of the user running the Office Online Server process. The CVSS v3.1 score of 7.8 reflects high impact on confidentiality, integrity, and availability, with low attack complexity and no privileges required, but user interaction is necessary. The affected version is 16.0.0.0, and as of the publication date, no patches or known exploits have been reported. This vulnerability poses a significant risk to environments where Office Online Server is deployed, especially in enterprise settings where Excel documents are frequently processed or shared online.

For European organizations, this vulnerability presents a serious risk as it can lead to unauthorized code execution on servers handling Office Online Server workloads. This could result in data breaches, unauthorized access to sensitive information, disruption of business operations, and potential lateral movement within corporate networks. Given the widespread use of Microsoft Office products in Europe, especially in government, finance, and large enterprises, exploitation could have severe consequences including loss of confidentiality of personal and corporate data, integrity violations of critical documents, and availability impacts due to potential service disruptions. The requirement for user interaction means phishing or social engineering could be vectors for exploitation, increasing the risk in environments with less stringent user awareness or email filtering. The lack of known exploits currently provides a window for proactive mitigation, but the high severity score demands urgent attention.

Until an official patch is released, European organizations should implement several specific mitigations: 1) Restrict access to Office Online Server to trusted users and networks only, employing network segmentation and firewall rules to limit exposure. 2) Disable or restrict the ability to open or process Excel files from untrusted sources within Office Online Server environments. 3) Enhance user awareness training focused on recognizing phishing attempts and suspicious documents to reduce the risk of user interaction with malicious content. 4) Monitor logs and network traffic for unusual activity related to Office Online Server, including unexpected process launches or memory anomalies. 5) Employ application whitelisting and endpoint detection and response (EDR) tools to detect and block exploitation attempts. 6) Prepare for rapid deployment of patches once Microsoft releases updates addressing this vulnerability. 7) Consider deploying additional layers of security such as sandboxing or content disarm and reconstruction (CDR) for documents processed by Office Online Server.