CVE-2026-21261 is a medium-severity vulnerability classified under CWE-125 (Out-of-bounds Read) affecting Microsoft Office 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 out-of-bounds read can lead to disclosure of sensitive information stored in adjacent memory areas. The flaw can be exploited locally by an unauthorized attacker without requiring privileges or authentication, but it does require user interaction, such as opening a specially crafted Excel file. The CVSS 3.1 vector (AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:N) reflects that the attack vector is local, with low attack complexity, no privileges required, but user interaction is necessary. The scope is unchanged, and the confidentiality impact is high, while integrity and availability are unaffected. No known exploits have been reported in the wild, and no 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 for Enterprise in corporate environments, this vulnerability poses a risk of sensitive data leakage if exploited. The lack of remote exploitability limits the attack surface, but insider threats or compromised local users could leverage this flaw. The absence of patches necessitates vigilance and interim mitigations until updates are available.

The primary impact of CVE-2026-21261 is unauthorized disclosure of sensitive information from memory due to an out-of-bounds read in Microsoft Excel. This can lead to leakage of confidential data, potentially including credentials, personal information, or proprietary business data. Since the vulnerability requires local access and user interaction, the risk is mainly from malicious insiders, compromised endpoints, or social engineering attacks that trick users into opening crafted Excel files. The confidentiality breach could facilitate further attacks such as privilege escalation or lateral movement within an organization. However, the vulnerability does not affect data integrity or system availability, limiting its destructive potential. Organizations with extensive use of Microsoft 365 Apps for Enterprise, especially in sectors handling sensitive or regulated data, face increased risk. The lack of known exploits reduces immediate threat but also means attackers may develop exploits in the future. Overall, the vulnerability could undermine trust in endpoint security and data confidentiality if not addressed promptly.

1. Monitor Microsoft security advisories closely and apply official patches immediately once released to remediate the vulnerability. 2. Restrict local access to systems running the affected Microsoft 365 Apps for Enterprise version to trusted users only, minimizing exposure to unauthorized local attackers. 3. Implement strict endpoint security controls, including application whitelisting and behavioral monitoring, to detect and prevent execution of malicious or suspicious Excel files. 4. Educate users about the risks of opening unsolicited or unexpected Excel documents, especially from untrusted sources, to reduce the likelihood of user interaction exploitation. 5. Employ data loss prevention (DLP) solutions to monitor and block unauthorized data exfiltration attempts that could result from memory disclosure. 6. Use endpoint detection and response (EDR) tools to identify anomalous local activity indicative of exploitation attempts. 7. Consider isolating high-risk users or sensitive environments to limit the impact of potential local exploits. 8. Regularly audit and update software inventories to ensure vulnerable versions are identified and remediated promptly.