Your Microsoft 365 tenant is one of your organization’s most critical assets — and most tenants go live with default settings that leave serious security gaps. The good news: most high-impact hardening actions are free, available in your existing license, and can be implemented without disrupting users.

Here are the 10 steps our Microsoft-certified specialists recommend for every M365 tenant, ranked by impact.

Step 1: Enforce Multi-Factor Authentication (MFA) for All Users

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Enable MFA via Security Defaults or Conditional Access

MFA blocks over 99.9% of account compromise attacks. Navigate to Entra ID → Properties → Manage Security Defaults. For more control, use Conditional Access policies to require MFA based on location, device compliance, and risk level.

Key fact: Microsoft reports that MFA prevents 99.9% of automated cyberattacks on accounts. If you implement nothing else on this list, implement MFA.

Step 2: Configure Conditional Access Policies

Conditional Access is the Zero Trust enforcement engine in Microsoft 365. It evaluates every sign-in attempt against conditions you define — requiring MFA, blocking legacy authentication, or restricting access from unmanaged devices.

  • Block legacy authentication protocols (SMTP Auth, POP3, IMAP) — these bypass MFA entirely
  • Require compliant devices for access to sensitive applications
  • Block sign-ins from high-risk locations using Named Locations
  • Implement sign-in risk and user risk policies via Entra ID Protection

Step 3: Enable Microsoft Defender for Office 365

Defender for Office 365 (MDO) protects your email, Teams, SharePoint, and OneDrive against phishing, malware, BEC, and zero-day exploits. Configure Safe Links and Safe Attachments policies as a priority — these scan every link and attachment in real time before delivery.

Step 4: Review and Harden Your Microsoft Secure Score

Microsoft Secure Score is your security posture dashboard. Go to security.microsoft.com → Secure Score and review your improvement actions. Focus on the high-impact, low-effort items first — most organizations can improve their score by 20–30 points in a single day.

Step 5: Enable Privileged Identity Management (PIM)

PIM converts standing admin access into just-in-time access. Instead of accounts permanently holding Global Admin or Security Admin roles, PIM requires users to activate roles on-demand, with approval workflows and time limits. This dramatically reduces the blast radius of a compromised admin account.

Step 6: Configure Data Loss Prevention (DLP) Policies

DLP policies prevent sensitive data — credit card numbers, passport numbers, health records — from being shared outside your organization. Start with the pre-built templates for your industry and regulatory requirements (GDPR, PCI-DSS, HIPAA), then customize based on your specific data types.

Step 7: Enable Sensitivity Labels (MIP)

Microsoft Information Protection (MIP) sensitivity labels allow you to classify and protect documents and emails based on their content. Once applied, labels travel with the document — enforcing encryption, watermarks, and access restrictions regardless of where the file goes.

Step 8: Audit and Remove Unused Guest Accounts

Guest accounts in your Entra ID tenant are a common overlooked attack surface. Run an access review in Entra ID to identify stale guest accounts and remove them. Implement a regular review cadence — quarterly at minimum.

Step 9: Enable Unified Audit Logging

Unified audit logs capture user and admin activity across M365 services — Exchange, SharePoint, Teams, Entra ID, and more. Ensure audit logging is enabled (it’s on by default for most tenants) and set log retention to at least 90 days. This is your forensic trail when an incident occurs.

Step 10: Configure Alert Policies for Critical Events

Set up alert policies in the Microsoft 365 Compliance Center for critical events: new inbox rules forwarding email externally, impossible travel sign-ins, mass file deletion, and admin role changes. These alerts should notify your IT team immediately — minutes matter in a breach scenario.

PDI Tip: Start with Steps 1, 2, and 4 — they provide the highest security ROI and can be completed in a single working day without user disruption.

Next Steps

Implementing these 10 steps will significantly improve your Microsoft 365 security posture. If you’re unsure where to start or want a professional assessment of your current tenant configuration, PDI’s M365 Security Assessment gives you a prioritized roadmap based on your specific environment — benchmarked against Microsoft Secure Score and Zero Trust principles.