CVE-2026-21259 is a heap-based buffer overflow vulnerability classified under CWE-122, found in Microsoft Office Excel within Microsoft 365 Apps for Enterprise version 16.0.1. The vulnerability arises from improper handling of heap memory, allowing an attacker to overwrite memory buffers beyond their allocated size. This flaw can be triggered by a local attacker who convinces a user to open a specially crafted Excel file, leading to memory corruption. The corrupted memory state can be leveraged to escalate privileges locally, granting the attacker higher system rights than initially permitted. The CVSS 3.1 base score of 7.8 reflects a high severity due to the potential for complete compromise of confidentiality, integrity, and availability on affected systems. The attack vector is local (AV:L), requiring low attack complexity (AC:L), no privileges (PR:N), but 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). No known exploits have been reported in the wild, and no official patches have been released at the time of publication. The vulnerability was reserved in December 2025 and published in February 2026. Given the widespread use of Microsoft 365 Apps in enterprise environments, this vulnerability poses a significant risk if exploited, especially in environments where users have local access but limited privileges. The absence of patches necessitates immediate mitigation through access controls and monitoring.

For European organizations, the impact of CVE-2026-21259 is substantial due to the extensive adoption of Microsoft 365 Apps across both private and public sectors. Successful exploitation can lead to local privilege escalation, enabling attackers to gain administrative control over affected systems. This can result in unauthorized access to sensitive data, disruption of business operations, and potential lateral movement within networks. Critical sectors such as finance, healthcare, government, and infrastructure are particularly vulnerable, as compromise could lead to data breaches, service outages, and regulatory non-compliance under GDPR. The requirement for local access and user interaction somewhat limits remote exploitation but does not eliminate risk, especially in environments with shared or poorly secured workstations. The lack of available patches increases the window of exposure, necessitating proactive defensive measures. Additionally, the high impact on confidentiality, integrity, and availability underscores the potential for severe operational and reputational damage.

1. Restrict local access to systems running Microsoft 365 Apps, especially Excel, by enforcing strict user account controls and limiting administrative privileges. 2. Implement application whitelisting and endpoint protection solutions capable of detecting anomalous behavior related to memory corruption exploits. 3. Educate users to avoid opening Excel files from untrusted or unknown sources, emphasizing the risk of privilege escalation attacks. 4. Employ network segmentation to isolate critical systems and reduce the risk of lateral movement following local compromise. 5. Monitor system logs and endpoint telemetry for signs of suspicious activity indicative of exploitation attempts, such as unusual process behavior or privilege escalations. 6. Prepare for rapid deployment of official patches once released by Microsoft by maintaining an up-to-date asset inventory and patch management process. 7. Use virtualization or sandboxing technologies to open untrusted Excel files in isolated environments to prevent system-wide impact. 8. Review and tighten group policies related to file handling and macro execution within Microsoft 365 Apps to minimize attack surface.