10 Microsoft 365 Security Best Practices

Microsoft 365 is critical for productivity, but its centrality makes it a prime target for cyberattacks. Robust security is non-negotiable. In this blog, I will outline my top 10 Microsoft 365 security practices, combining Microsoft’s best practices with expert insights.

Let’s jump in.

1. Implement Multi-Factor Authentication (MFA)

MFA is your first line of defense. It adds extra verification beyond passwords, making it significantly harder for attackers to compromise accounts, even if they know the password. This simple step can block up to 99.9% of account compromise attacks, dramatically reducing your organization’s risk.

Implementation:

Conditional Access policies: Enforce MFA based on user roles, locations, and devices.
Security Defaults: Microsoft’s built-in policies with MFA.
Per-user MFA: Individual MFA configuration.

Best Practice:

Prioritize passwordless authentication (e.g., Microsoft Authenticator with number matching) for enhanced security and user experience.
Apply MFA to all users, especially admins.

2. Secure Administrator Accounts

Admin accounts are highly privileged and require stringent controls. These accounts hold the “keys to the kingdom,” and if compromised, an attacker could gain full control of your Microsoft 365 environment, leading to data breaches, service disruptions, and severe damage. Therefore, securing them is paramount.

Microsoft 365 security best practices for admin accounts include:

Dedicated admin accounts, separate from regular user accounts.
Strongest MFA for all admin actions.
Emergency “break-glass” accounts for recovery.
Least privilege: Grant only necessary permissions.
Just-in-time (JIT) access with Azure AD Privileged Identity Management (PIM).
Comprehensive logging and auditing of admin activity.
Regularly review and remove unnecessary privileges.

3. Deploy Advanced Threat Protection

Email is a primary attack vector. Microsoft Defender for Office 365 provides essential multi-layered defense. Email remains the most common way for malware and phishing attacks to enter an organization. Advanced threat protection helps to proactively defend against these sophisticated attacks.

Essential Features:

Safe Attachments: Sandboxes attachments to block malicious content.
Safe Links: Real-time URL checking to prevent phishing.
Anti-phishing: AI-powered detection of impersonation and spoofing.
Impersonation settings: Enhanced scrutiny for suspicious executive/partner emails.

Broader Protection:

Extend protection to SharePoint, OneDrive, and Teams.

Maintenance:

Keep Exchange Online Protection (EOP) and Defender for Office 365 settings up-to-date.

4. Implement Comprehensive Conditional Access Policies

Conditional Access is a real-time policy engine that grants access based on identity, location, device health, and risk. It allows you to enforce granular controls over who can access what resources and under what conditions. This is crucial in today’s world of hybrid work and diverse access scenarios.

Baseline Policies:

Block legacy authentication.
Require device compliance.
Block access from untrusted locations.
Implement risk-based access.
Enforce session controls (time limits, reauthentication).
Mandate MFA for admin portals.

Best Practices:

Start in report-only mode for testing.
Use clear naming conventions.
Apply policies broadly, with exceptions as needed.
Build resilient policies.
Regularly review and test.

5. Enable Comprehensive Logging and Monitoring

Visibility is key to detecting and responding to threats. You can’t defend against what you can’t see. Comprehensive logging and monitoring provide the crucial data and insights needed to identify suspicious activity, investigate potential breaches, and understand the security posture of your Microsoft 365 environment.

Core Components:

Microsoft 365 audit log search.
Microsoft Secure Score.
Azure Identity Protection.
Alert policies for high-risk activities.
Appropriate log retention.

Proactive Response:

Regular log reviews.
Automated responses to common events.
Incident response plan.

6. Implement Data Loss Prevention (DLP) and Information Protection

Protect sensitive data from unauthorized access and leaks. DLP and information protection tools help you classify, label, and protect your most valuable data, preventing it from falling into the wrong hands, whether through accidental exposure or malicious intent.

Key Measures:

DLP policies for Microsoft 365 apps and Teams.
Sensitivity labels for data classification.
Auto-labeling.
Encryption for data in transit and at rest.

Structured Approach:

Identify sensitive data.
Define data handling policies.
User education.
Regular policy review.

7. Secure Endpoints and Manage Devices

Secure all devices accessing Microsoft 365, especially in hybrid work environments. With employees working from various locations and using a mix of personal and company-owned devices, securing these endpoints is critical. Compromised devices can provide attackers with a foothold into your Microsoft 365 environment.

Microsoft Intune:

Device compliance policies.
App protection policies.
Endpoint security settings (antivirus, firewall, EDR).
BYOD management.
Microsoft Defender for Endpoint integration.

Device Security Best Practices:

Clear security policies for all devices.
Encryption and strong authentication.
Automatic updates.
Remote wipe capability.

8. Apply System Updates and Security Baselines

Keep systems patched and configured securely. Unpatched vulnerabilities are a major security risk. Attackers often exploit known weaknesses in software to gain access. Regular updates and security baselines help to minimize these vulnerabilities.

Microsoft Security Baselines:

Microsoft 365 security baselines.
Exchange email management baselines.
Windows device security baselines.
Attack surface reduction rules.

Update Management:

Azure Update Manager.
Regular maintenance windows.
Testing before deployment.
Update compliance monitoring.

Ongoing Action:

Apply the latest Microsoft security recommendations.

9. Block Legacy Authentication and Control Network Access

Eliminate insecure authentication methods and limit network exposure. Legacy authentication protocols don’t support modern security controls like MFA, making them a prime target for attackers. Controlling network access adds another layer of defense by restricting where access to your Microsoft 365 environment can originate.

Access Restrictions:

Block legacy authentication (POP, SMTP, IMAP).
Adaptive network hardening.
Geofencing.
Azure Private Link.
Azure Firewall.

Implementation Notes:

Identify affected applications.
Minimize exceptions with compensating controls.
Monitor for legacy authentication attempts.

10. Adopt Zero Trust Security Principles

Assume a breach and verify every access request. In today’s complex and distributed environments, you can’t rely on traditional perimeter-based security. Zero Trust treats every user and device as potentially compromised and requires strict verification for every access attempt.

Zero Trust Principles:

Explicitly verify all access requests.
Enforce least privilege with JIT/JEA.
Assume a breach and minimize the blast radius.

Zero Trust Implementation:

Identity-centric approach.
Extend protection to all resources.
Continuous monitoring and validation.

By implementing these 10 Microsoft 365 security best practices, organizations can significantly improve their security posture and mitigate evolving cyber threats.

How Quisitive Can Help Improve Your Microsoft 365 Security Posture

Security is an ongoing process that requires continuous assessment, adaptation, and vigilance. Not sure where to start? Quisitive offers a Microsoft 365 Security Assessment to help you:

Discover gaps in your security strategy
Learn about the security tools available to you with Microsoft 365
Uncover risky activities
Create an improvement plan unique to your business and organization

Learn more >