Kill M365 Password Spraying: Why You Must Disable Exchange SMTP Basic Auth
🛡️ Summary Password spraying attacks thrive on legacy protocols like Basic Authentication. While Microsoft disabled Basic Auth for most Exchange Online endpoints in 2022, SMTP AUTH remains a critical exception—and a prime target. This post explains why, and how to audit, restrict, and shut it down. 🔍 What Are Exchange Authentication Policies? Exchange authentication policies are configuration objects in Exchange Online that allow administrators to disable Basic Authentication per protocol for specific users or groups. These policies operate at the protocol layer , preventing Basic Auth requests from ever reaching Microsoft Entra ID. This is different from Conditional Access, which evaluates sign-in attempts at the identity layer . Authentication policies are more effective for legacy protocol hardening because they cut off Basic Auth at the source . 🔓 Why Basic Authentication Is Dangerous Sends credentials in plaintext (Base64), easily intercepted. Doesn’t support M...