CVE-2025-21364 is a high-severity vulnerability classified under CWE-502, which pertains to the deserialization of untrusted data. This vulnerability affects Microsoft 365 Apps for Enterprise, specifically version 16.0.1, and involves a security feature bypass in Microsoft Excel. Deserialization vulnerabilities occur when an application processes untrusted serialized data without sufficient validation, potentially allowing an attacker to manipulate the deserialization process to execute arbitrary code, escalate privileges, or cause denial of service. In this case, the vulnerability allows an attacker to bypass Excel's security features, which could lead to the execution of malicious payloads embedded within specially crafted Excel files. The CVSS 3.1 base score of 7.8 indicates a high level of severity, with 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 specifying 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 with no known exploits in the wild, and no patches have been linked yet. This vulnerability could be exploited by convincing a user to open a malicious Excel file, leading to full compromise of the affected system through arbitrary code execution and potentially allowing attackers to bypass existing security controls within Excel.

For European organizations, this vulnerability poses a significant risk due to the widespread use of Microsoft 365 Apps for Enterprise across various sectors including government, finance, healthcare, and critical infrastructure. Successful exploitation could lead to unauthorized access to sensitive data, disruption of business operations, and potential lateral movement within corporate networks. The high impact on confidentiality, integrity, and availability means that attackers could steal confidential information, alter or destroy data, and disrupt services. Given the requirement for local access and user interaction, phishing campaigns or social engineering attacks could be effective vectors, which are common tactics in Europe. The lack of a patch at the time of disclosure increases the window of exposure, and organizations with less mature security awareness or endpoint protection may be particularly vulnerable. Additionally, compliance with GDPR and other data protection regulations means that exploitation could result in legal and financial consequences for affected entities.

Beyond standard advice such as applying patches when available and maintaining updated antivirus solutions, European organizations should implement several specific measures. First, enforce strict email filtering and attachment scanning to detect and block malicious Excel files before reaching end users. Deploy endpoint detection and response (EDR) tools capable of identifying suspicious behaviors related to deserialization attacks and Excel process anomalies. Conduct targeted user awareness training focusing on the risks of opening unsolicited or unexpected Excel attachments, emphasizing the need for caution even with files from known contacts. Utilize application whitelisting and sandboxing techniques to restrict the execution of untrusted macros or embedded objects within Excel files. Employ network segmentation to limit the potential lateral movement if a system is compromised. Monitor logs for unusual Excel activity and implement strict privilege management to minimize the impact of any successful exploit. Finally, prepare incident response plans specifically addressing deserialization and Office document-based attacks to ensure rapid containment and remediation.