CVE-2026-21261 is a medium-severity vulnerability classified as CWE-125 (Out-of-bounds Read) affecting Microsoft Excel in Microsoft 365 Apps for Enterprise version 16.0.1. The vulnerability arises from improper bounds checking when processing Excel files, allowing an attacker to read memory beyond the intended buffer limits. This can lead to disclosure of sensitive information stored in adjacent memory regions. The attack vector is local (AV:L), meaning the attacker must have local access to the system, but no privileges are required (PR:N). However, user interaction (UI:R) is necessary, typically by opening a maliciously crafted Excel file. The vulnerability impacts confidentiality (C:H) but does not affect integrity or availability. The scope is unchanged (S:U), indicating the vulnerability affects only the vulnerable component. The exploitability is limited due to the need for local access and user interaction, and no known exploits have been reported in the wild. No patches have been published yet, but the vulnerability has been officially assigned and published by Microsoft. This vulnerability could be leveraged by attackers to extract sensitive data from memory, potentially exposing confidential information stored in Excel or related processes.

For European organizations, this vulnerability poses a risk of local information disclosure, which could lead to leakage of sensitive corporate or personal data. While the attack requires local access and user interaction, insider threats or social engineering attacks could exploit this flaw. Organizations relying heavily on Microsoft 365 Apps for Enterprise, especially Excel, may face risks of data breaches affecting confidentiality. This could impact sectors handling sensitive data such as finance, healthcare, legal, and government institutions. The medium severity suggests moderate risk, but the potential for information leakage could facilitate further attacks or espionage. The lack of known exploits reduces immediate risk, but the absence of patches means the vulnerability remains exploitable if discovered by attackers. European companies with strict data protection regulations (e.g., GDPR) must consider the compliance implications of potential data exposure.

1. Restrict local access to systems running Microsoft 365 Apps for Enterprise to trusted users only, minimizing the risk of local exploitation. 2. Educate users to avoid opening Excel files from untrusted or unknown sources to prevent triggering the vulnerability. 3. Employ endpoint detection and response (EDR) solutions capable of detecting anomalous behavior related to Excel processes. 4. Implement application whitelisting and macro restrictions to reduce the risk of malicious file execution. 5. Monitor for unusual file access or memory-related anomalies on endpoints using Microsoft 365 Apps. 6. Stay alert for official patches or updates from Microsoft and apply them promptly once available. 7. Consider isolating high-risk users or systems in segmented network zones to limit lateral movement in case of exploitation. 8. Conduct regular security awareness training focused on phishing and social engineering to reduce user interaction risks.