CVE-2025-21356 is a high-severity vulnerability identified in Microsoft Office 2019, specifically affecting the Visio component. The vulnerability is classified as CWE-843, which corresponds to 'Access of Resource Using Incompatible Type,' commonly known as a type confusion vulnerability. This type of flaw occurs when a program accesses a resource using a type that is incompatible with the actual type of the resource, potentially leading to unpredictable behavior such as memory corruption. In this case, the vulnerability allows an attacker to execute arbitrary code remotely by exploiting the way Microsoft Office Visio processes certain crafted files. The CVSS 3.1 base score is 7.8, indicating a high level of severity. The vector string (CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C) reveals 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). The exploitability is currently unknown (E:U), and the vulnerability is officially recognized and documented (RL:O, RC:C). No known exploits are reported in the wild yet, and no patch links are provided at this time. The vulnerability was reserved in December 2024 and published in January 2025. This vulnerability could be triggered by convincing a user to open a malicious Visio file, leading to remote code execution with the privileges of the user, potentially allowing full system compromise.

For European organizations, this vulnerability poses a significant risk, especially for those heavily reliant on Microsoft Office 2019 and Visio for business operations, including sectors such as engineering, architecture, and project management where Visio is commonly used. Successful exploitation could lead to unauthorized disclosure of sensitive information, data manipulation, and disruption of business continuity through system compromise. Given the high impact on confidentiality, integrity, and availability, attackers could deploy malware, ransomware, or conduct espionage activities. The requirement for local access and user interaction somewhat limits the attack vector to scenarios where an attacker can deliver a malicious file to the victim, such as phishing campaigns or insider threats. However, once exploited, the attacker gains significant control, which can affect critical infrastructure, intellectual property, and personal data protected under GDPR. The absence of known exploits in the wild currently reduces immediate risk but does not eliminate the threat, as weaponization could occur rapidly after public disclosure.

European organizations should implement targeted mitigations beyond generic advice. First, restrict the use of Microsoft Visio 2019 to trusted users and environments, and consider disabling Visio file preview features in email clients to reduce the risk of accidental execution. Employ strict email filtering and attachment sandboxing to detect and block malicious Visio files. Educate users about the risks of opening unsolicited or unexpected Visio documents, emphasizing the need for caution with files from unknown or untrusted sources. Implement application whitelisting to prevent unauthorized execution of Visio or related processes. Monitor endpoint behavior for anomalies indicative of exploitation attempts, such as unusual process spawning or memory access patterns. Since no patch is currently available, consider deploying virtual desktop infrastructure (VDI) or isolated environments for users who must handle Visio files, limiting potential damage. Maintain up-to-date backups and incident response plans tailored to rapid containment of such exploits. Finally, stay alert for official patches or updates from Microsoft and apply them promptly once released.