In this video, we will explain how to effectively block legacy authentication in a Microsoft 365 environment to enhance security and reduce the risk of credential-based attacks. We’ll walk through how to check sign-in logs to identify if legacy authentication is still in use within your organisation. Then, we’ll demonstrate how to block legacy...
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Introduction of How To BLOCK Legacy Authentication Using Conditional Access

In this video, we will explain how to effectively block legacy authentication in a Microsoft 365 environment to enhance security and reduce the risk of credential-based attacks. We’ll walk through how to check sign-in logs to identify if legacy authentication is still in use within your organisation. Then, we’ll demonstrate how to block legacy authentication using Conditional Access policies while also covering scenarios where exceptions may be necessary, such as for service accounts. Additionally, we’ll share best practices for securing these exceptions to minimise the attack surface. To further strengthen security, we’ll explain how to disable SMTP authentication and completely turn off other legacy authentication protocols in your environment. By following these steps, you can ensure your Microsoft 365 environment is more secure and resilient against modern threats.
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Last Updated: May 22, 2026
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