CVE-2025-21402 is a high-severity vulnerability affecting Microsoft Office LTSC for Mac 2021, specifically version 16.0.1. The vulnerability is categorized under CWE-641, which relates to improper restriction of names for files and other resources. This flaw exists in the Microsoft OneNote component of the Office suite, enabling a remote code execution (RCE) attack vector. The vulnerability allows an attacker to craft malicious OneNote files or resources with specially crafted names that bypass the intended restrictions on file naming. When a user opens or interacts with such a malicious file, the vulnerability can be triggered, leading to the execution of arbitrary code with the privileges of the user. The CVSS v3.1 base score is 7.8, indicating a high severity level. The vector string (CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H/E:U/RC:C) shows that the attack requires local access (AV:L), low attack complexity (AC:L), no privileges required (PR:N), but does require 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 are reported in the wild yet, and no patches or mitigation links have been published at the time of this report. The vulnerability was reserved in December 2024 and published in January 2025. This flaw could be exploited by an attacker who can convince a user to open a malicious OneNote file or resource, potentially leading to full system compromise on affected Mac systems running this specific Office version.

For European organizations, this vulnerability poses a significant risk, especially those with Mac environments using Microsoft Office LTSC for Mac 2021. The ability to execute arbitrary code remotely can lead to data breaches, intellectual property theft, ransomware deployment, or lateral movement within corporate networks. Confidentiality, integrity, and availability of sensitive business data could be severely impacted. Organizations in sectors such as finance, healthcare, government, and critical infrastructure, which often handle sensitive personal and operational data, are particularly at risk. The requirement for local access and user interaction somewhat limits the attack surface to targeted phishing or social engineering campaigns, but the high impact on system security means successful exploitation could have severe consequences. Additionally, the lack of available patches increases the window of exposure, necessitating immediate attention to mitigation strategies.

Given the absence of official patches, European organizations should implement specific mitigations beyond generic advice: 1) Restrict the use of Microsoft Office LTSC for Mac 2021 version 16.0.1 on critical systems until a patch is available. 2) Implement strict email filtering and attachment scanning to block or quarantine suspicious OneNote files or files with unusual naming conventions. 3) Educate users about the risks of opening unsolicited or unexpected OneNote files, emphasizing caution with files from unknown or untrusted sources. 4) Employ endpoint detection and response (EDR) solutions capable of monitoring and blocking suspicious process executions triggered by Office applications. 5) Use application whitelisting to prevent unauthorized execution of code spawned by Office processes. 6) Monitor logs and network traffic for anomalous behavior indicative of exploitation attempts. 7) Consider isolating Mac systems running this Office version from sensitive networks or limiting their network privileges to reduce potential lateral movement. 8) Maintain up-to-date backups to enable recovery in case of compromise. 9) Stay alert for official patches or advisories from Microsoft and apply them promptly once available.