CVE-2026-21258 is a vulnerability identified in Microsoft 365 Apps for Enterprise, specifically affecting Microsoft Excel version 16.0.1. The root cause is improper input validation (CWE-20), which allows an attacker to craft malicious Excel files that, when opened by a user, can lead to unauthorized local disclosure of sensitive information. The vulnerability does not require any privileges (PR:N) but does require user interaction (UI:R), meaning the victim must open or interact with a malicious Excel document. The attack vector is local (AV:L), indicating that the attacker must have local access or deliver the malicious file to the victim. The vulnerability impacts confidentiality (C:H) but does not affect integrity or availability. The CVSS v3.1 base score is 5.5, categorizing it as medium severity. No known exploits have been reported in the wild, and no official patches have been linked yet, though Microsoft has acknowledged the issue. The vulnerability could be leveraged to disclose sensitive information stored or processed within Excel, potentially exposing business-critical or personal data. This flaw is particularly concerning for environments where Excel files are widely shared or where sensitive data is handled within spreadsheets. The improper input validation suggests that Excel fails to properly sanitize or verify certain inputs, leading to information leakage. The vulnerability is classified under CWE-20, which relates to improper input validation, and also tagged with CWE-125, indicating potential out-of-bounds read issues. Given the local attack vector and requirement for user interaction, exploitation is limited to scenarios where an attacker can convince a user to open a malicious file or has local access to the system.

For European organizations, the primary impact of CVE-2026-21258 is the potential unauthorized disclosure of sensitive information stored or processed in Microsoft Excel spreadsheets. This could lead to leakage of confidential business data, personal information, or intellectual property, which may result in regulatory non-compliance (e.g., GDPR violations), reputational damage, and financial loss. Since the vulnerability requires local access or delivery of a malicious file and user interaction, the risk is higher in environments with lax endpoint security, insufficient user training, or where Excel files are frequently exchanged without validation. Sectors such as finance, healthcare, government, and critical infrastructure in Europe, which heavily rely on Microsoft 365 productivity tools, could be particularly vulnerable. The confidentiality breach could facilitate further attacks, such as social engineering or targeted phishing, by exposing internal data. However, the lack of impact on integrity and availability limits the scope of damage to information disclosure only. The absence of known exploits in the wild currently reduces immediate risk but does not eliminate the threat, especially as attackers may develop exploits once the vulnerability becomes widely known.

1. Implement strict controls on the handling of Excel files, especially those received from untrusted or external sources. 2. Educate users to avoid opening unsolicited or suspicious Excel documents and to verify the source before enabling content or macros. 3. Employ endpoint security solutions that can detect and block malicious Office documents or unusual file behaviors. 4. Restrict local access to systems running Microsoft 365 Apps to trusted personnel only, minimizing the risk of local exploitation. 5. Monitor for official patches or security updates from Microsoft and apply them promptly once available. 6. Use application whitelisting or sandboxing techniques to isolate Excel processes and limit the impact of potential exploits. 7. Regularly audit and review Excel file usage and sharing policies within the organization to reduce exposure. 8. Consider deploying Data Loss Prevention (DLP) tools to detect and prevent unauthorized disclosure of sensitive information through Office documents. 9. Maintain up-to-date backups of critical data to mitigate potential indirect impacts of information disclosure. 10. Collaborate with IT security teams to integrate this vulnerability into risk assessments and incident response plans.