CVE-2026-21509 is a vulnerability identified in Microsoft Office 2019 version 19.0.0, categorized under CWE-807, which involves reliance on untrusted inputs in a security decision. This flaw allows an unauthorized local attacker to bypass security features by manipulating inputs that the software trusts improperly. The vulnerability does not require elevated privileges but does require local access and user interaction, such as opening a maliciously crafted document. The CVSS v3.1 score of 7.8 reflects high severity, with impacts on confidentiality, integrity, and availability (all rated high). The exploitability is facilitated by low attack complexity and no privileges required, but user interaction is necessary. No public exploits or patches are currently available, indicating a window of exposure. The vulnerability could enable attackers to execute unauthorized actions or access sensitive information within Office documents, potentially leading to data leakage or disruption of document workflows. The issue stems from improper validation or sanitization of inputs that influence security decisions within Office, a critical productivity suite widely used in enterprises. The flaw's local nature limits remote exploitation but still poses a significant risk in environments where local access is possible, such as shared workstations or compromised endpoints.

For European organizations, this vulnerability threatens the confidentiality, integrity, and availability of data processed through Microsoft Office 2019. Given Office's ubiquitous use in business, government, and education sectors, exploitation could lead to unauthorized data access, modification, or denial of service in document handling. This is particularly critical for organizations handling sensitive personal data under GDPR, intellectual property, or classified information. The local attack vector means insider threats or attackers with physical or remote desktop access could leverage this vulnerability. Disruption of document workflows could impact operational continuity. The absence of patches increases exposure duration, necessitating interim controls. The impact is amplified in sectors with high reliance on Office documents for critical operations, such as finance, legal, and public administration. Additionally, the requirement for user interaction means phishing or social engineering could facilitate exploitation, increasing risk in environments with less security awareness.

Until an official patch is released, European organizations should implement strict access controls to limit local access to systems running Microsoft Office 2019. Employ application whitelisting and endpoint protection to detect and block execution of suspicious documents or macros. Enhance user training to recognize and avoid opening untrusted or unexpected Office files, reducing the risk of user interaction exploitation. Deploy network segmentation to isolate critical systems and restrict lateral movement. Use monitoring and logging to detect anomalous Office document activity indicative of exploitation attempts. Consider disabling or restricting features that process untrusted inputs within Office, such as macros or embedded content, where feasible. Maintain up-to-date backups of critical documents to mitigate availability impacts. Prepare for rapid deployment of patches once Microsoft releases them, including testing in controlled environments. Engage with Microsoft security advisories for updates and guidance. Finally, conduct regular vulnerability assessments and penetration testing focused on local privilege escalation and input validation weaknesses.