CVE-2025-21402 is a remote code execution vulnerability identified in Microsoft Office LTSC for Mac 2021, version 16.0.1, specifically affecting the OneNote application. The root cause is an improper restriction of names for files and other resources (CWE-641), which means that OneNote does not adequately validate or restrict file names or resource identifiers it processes. This flaw can be exploited by an attacker who convinces a user to open a specially crafted OneNote file or resource, triggering the vulnerability. The CVSS 3.1 base score is 7.8 (high), with an attack vector classified as local (AV:L), low attack complexity (AC:L), no privileges required (PR:N), but requiring user interaction (UI:R). The scope remains unchanged (S:U), and the impact on confidentiality, integrity, and availability is high (C:H/I:H/A:H). This means successful exploitation can lead to complete compromise of the affected system, including unauthorized data access, modification, or system disruption. The vulnerability was published on January 14, 2025, with no known exploits in the wild and no patches currently available. The vulnerability is significant because it affects a widely used productivity suite on Mac platforms, which are increasingly targeted in enterprise environments. The improper validation of file/resource names could allow attackers to bypass security controls and execute arbitrary code remotely, potentially leading to malware installation or lateral movement within networks.

The potential impact of CVE-2025-21402 is severe for organizations using Microsoft Office LTSC for Mac 2021. Successful exploitation can result in remote code execution, allowing attackers to run arbitrary code with the privileges of the user opening the malicious OneNote file. This can lead to full system compromise, data theft, unauthorized data modification, and disruption of business operations. Given the high confidentiality, integrity, and availability impacts, sensitive corporate data and intellectual property could be exposed or altered. The requirement for user interaction means phishing or social engineering campaigns could be used to deliver the exploit. The vulnerability affects Mac users, which are prevalent in many sectors including technology, finance, and creative industries. Enterprises relying on Mac environments for productivity are at risk of targeted attacks, potentially enabling attackers to establish persistence or move laterally within networks. The absence of known exploits currently provides a window for proactive mitigation, but the high severity score indicates that once exploited, damage could be extensive.

To mitigate CVE-2025-21402, organizations should implement the following specific measures: 1) Restrict or disable the opening of OneNote files from untrusted or unknown sources, especially those received via email or external downloads. 2) Educate users about the risks of opening unsolicited or suspicious OneNote files and encourage verification of file origins. 3) Employ application whitelisting and endpoint detection and response (EDR) solutions to monitor and block anomalous OneNote behaviors or unauthorized code execution attempts. 4) Use network segmentation to limit the impact of potential compromises originating from Mac endpoints. 5) Monitor logs and alerts for unusual file access or process execution related to OneNote. 6) Maintain up-to-date backups of critical data to enable recovery in case of compromise. 7) Stay informed about Microsoft’s security advisories and apply patches immediately once they become available. 8) Consider deploying Mac-specific security tools that can detect exploitation attempts targeting Office applications. These steps go beyond generic advice by focusing on controlling OneNote file handling, user awareness, and proactive detection tailored to this vulnerability.