Microsoft 365 Security Hardening: Complete Guide for Small Businesses

MFA blocks 99.9% of account compromise attacks. Enable Security Defaults in Entra admin center > Identity > Overview > Properties > Manage security defaults. This enforces MFA registration for all users and blocks legacy authentication protocols.

Create 2 break-glass emergency access accounts excluded from Conditional Access. Implement Privileged Identity Management (PIM) for just-in-time admin access if you have Entra ID P2. Minimize permanent Global Admins to 2-4 people. Use least-privilege delegated roles.

Create policies for: Require MFA for All Users, Block Legacy Authentication (this bypasses MFA), Require MFA for Admins with phishing-resistant methods. Apply to all cloud apps except your break-glass accounts.

Configure Anti-Phishing Policies: Enable impersonation protection for executives and finance team. Enable domain protection. Set actions to Quarantine. Configure SPF, DKIM, and DMARC. Create External Email Warning Banner mail flow rule to alert users about outside emails.

In SharePoint admin center > Policies > Sharing, restrict external sharing levels. Disable Anyone links. Set default link type to Specific people. Enable warnings when sharing outside organization. Configure Data Loss Prevention policies to detect sensitive data.

Enable Mobile Device Management requiring password/PIN, device encryption, minimum OS version, and remote wipe capability. Configure App Protection Policies requiring PIN for app access and blocking copy/paste to unmanaged apps.

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