CVE-2025-59224 is a use-after-free vulnerability classified under CWE-416, discovered in Microsoft Office Online Server version 16.0.0.0. The vulnerability resides in the Microsoft Office Excel component of the server, where improper memory management leads to a use-after-free condition. This flaw can be triggered when a user opens or interacts with a specially crafted Excel file hosted or processed by the Office Online Server. The use-after-free condition allows an attacker to execute arbitrary code locally on the server environment, potentially leading to full system compromise. The CVSS v3.1 base score is 7.8, indicating high severity, with attack vector being local (AV:L), low attack complexity (AC:L), no privileges required (PR:N), but requiring user interaction (UI:R). The scope is unchanged (S:U), and the impact on confidentiality, integrity, and availability is high (C:H/I:H/A:H). No known exploits have been reported in the wild, and no official patches have been released as of the publication date. The vulnerability was reserved on 2025-09-11 and published on 2025-10-14. The absence of patches means organizations must rely on mitigations and monitoring to reduce risk. The vulnerability is particularly concerning for environments where Office Online Server is exposed to users who can upload or open Excel files, as this increases the attack surface for exploitation.

For European organizations, the impact of CVE-2025-59224 can be significant, especially for enterprises and public sector entities relying heavily on Microsoft Office Online Server for collaborative document editing and sharing. Successful exploitation could lead to local code execution on the server, allowing attackers to escalate privileges, access sensitive data, disrupt services, or pivot within the network. This could compromise confidentiality of sensitive documents, integrity of data processed by the server, and availability of critical collaboration services. Sectors such as finance, government, healthcare, and critical infrastructure that use Office Online Server are at heightened risk. The local attack vector and requirement for user interaction limit remote exploitation but do not eliminate risk, as insider threats or social engineering could facilitate attack delivery. The lack of patches increases exposure time, and the high impact on all security triad components makes this vulnerability a critical concern for European organizations aiming to maintain regulatory compliance and operational continuity.

Until an official patch is released, European organizations should implement several specific mitigations: 1) Restrict local access to Office Online Server hosts to trusted administrators only, minimizing the risk of local exploitation. 2) Enforce strict file upload and validation policies to prevent malicious Excel files from being processed by the server. 3) Implement application whitelisting and endpoint protection on servers hosting Office Online Server to detect and block suspicious code execution attempts. 4) Monitor server logs and network traffic for unusual activity indicative of exploitation attempts, such as unexpected process launches or memory corruption indicators. 5) Educate users about the risks of interacting with untrusted Excel files, especially in collaborative environments. 6) Isolate Office Online Server environments from critical network segments to limit lateral movement in case of compromise. 7) Prepare incident response plans specific to Office Online Server compromise scenarios. 8) Stay alert for Microsoft’s official patch release and apply it promptly. These targeted actions go beyond generic advice by focusing on controlling local access, file handling, and monitoring specific to this vulnerability’s exploitation vector.