CVE-2026-56191 is a critical vulnerability affecting Microsoft Exchange Online. Improper authentication mechanisms allow unauthenticated attackers to perform unauthorized tampering operations over a network. This flaw carries a CVSS score of 10.0, indicating the highest severity level. All users of Microsoft Exchange Online are affected, as the vulnerability does not require user interaction or privileged access to exploit. Successful exploitation could result in data modification, service disruption, or lateral movement within compromised environments. Organizations are strongly advised to apply the latest security updates provided by Microsoft to eliminate exposure. Until patched, restrict network access to Exchange services using strict firewall rules and monitor for suspicious activity, including unexpected API calls or unauthorized configuration changes. This vulnerability underscores the importance of timely patch management for cloud-based platforms. No evidence of active exploitation has been reported at the time of publication.