CVE-2026-26108 is a heap-based buffer overflow vulnerability classified under CWE-122, affecting Microsoft Office Excel in Microsoft 365 Apps for Enterprise version 16.0.1. The vulnerability arises from improper handling of memory buffers when processing Excel files, allowing an attacker to overflow a heap buffer and overwrite critical memory structures. This memory corruption can lead to arbitrary code execution with the privileges of the user opening the malicious file. Exploitation requires user interaction, specifically opening a crafted Excel document, but does not require any prior authentication or elevated privileges. The vulnerability has a CVSS v3.1 base score of 7.8, indicating high severity, with impacts on confidentiality, integrity, and availability. The attack vector is local (AV:L), with low attack complexity (AC:L), no privileges required (PR:N), but user interaction is necessary (UI:R). The scope is unchanged (S:U), meaning the exploit affects only the vulnerable component. Currently, there are no known exploits in the wild, and no patches have been published, leaving organizations exposed. The vulnerability was reserved in February 2026 and published in March 2026. Given the critical role of Microsoft Excel in enterprise environments and the widespread deployment of Microsoft 365 Apps, this vulnerability represents a significant security risk.

If exploited, this vulnerability allows attackers to execute arbitrary code on affected systems with the privileges of the logged-in user, potentially leading to full system compromise. This can result in unauthorized access to sensitive data, disruption of business operations, and installation of persistent malware or ransomware. Since Microsoft Excel is widely used in enterprises globally, successful exploitation could facilitate lateral movement within networks, data exfiltration, and further attacks on critical infrastructure. The requirement for user interaction limits remote exploitation but does not eliminate risk, as phishing campaigns or malicious document distribution remain effective attack vectors. The absence of patches increases the window of exposure, heightening the urgency for mitigation. Organizations relying heavily on Microsoft 365 Apps for Enterprise, especially those in sectors with high data sensitivity such as finance, healthcare, and government, face substantial risk from this vulnerability.

Until an official patch is released, organizations should implement several targeted mitigations: 1) Enforce strict email filtering and attachment scanning to block or quarantine suspicious Excel files, reducing the chance of malicious documents reaching users. 2) Educate users to recognize and avoid opening unexpected or suspicious Excel attachments, emphasizing the risk of enabling macros or content in untrusted files. 3) Utilize Microsoft Defender Exploit Guard or similar endpoint protection features to enable exploit mitigation techniques such as heap protection and control flow guard. 4) Restrict execution privileges of Microsoft 365 Apps processes using application control policies like AppLocker or Windows Defender Application Control to limit damage from potential exploits. 5) Monitor endpoint and network logs for unusual behavior indicative of exploitation attempts, including anomalous process creation or memory corruption events. 6) Maintain regular backups and test recovery procedures to mitigate impact from potential ransomware or destructive payloads delivered via this vulnerability. 7) Plan for rapid deployment of the official patch once available, prioritizing high-risk systems and users with elevated privileges.