Introduction
Microsoft 365 has become the backbone of modern business, providing email, collaboration, document storage and increasingly AI-powered productivity tools. However, many organisations assume that because they use Microsoft 365, they are automatically secure.
Unfortunately, this is not the case.
Cyber criminals actively target Microsoft 365 tenants through phishing, credential theft, business email compromise and ransomware attacks. The good news is that many of the most effective security measures are straightforward to implement.
This checklist highlights the key controls every UK business should review in 2026.
1. Enable Multi-Factor Authentication (MFA)
If you only implement a single security control, make it MFA.
Multi-factor authentication significantly reduces the likelihood of compromised accounts resulting from stolen passwords.
Best practice includes:
Enforcing MFA for all users
Requiring phishing-resistant methods where possible
Disabling legacy authentication protocols
Reviewing MFA registration regularly
2. Secure Administrator Accounts
Administrative accounts are prime targets for attackers.
Consider:
Separate administrator and user accounts
Dedicated privileged accounts
Conditional Access policies
Just-in-time administrative access
Never use a global administrator account for everyday email and web browsing.
3. Review Conditional Access Policies
Conditional Access allows organisations to control how users access Microsoft 365 resources.
Examples include:
Blocking access from high-risk countries
Requiring compliant devices
Enforcing MFA for sensitive applications
Restricting access from unknown locations
When properly configured, Conditional Access provides a significant improvement in security posture.
4. Implement Microsoft Defender
Many organisations already own Microsoft Defender licensing but are not using it fully.
Key areas include:
Defender for Endpoint
Defender for Office 365
Defender for Identity
Defender for Cloud Apps
These tools provide advanced threat detection and response capabilities that go far beyond traditional antivirus.
5. Protect SharePoint and OneDrive Data
Sensitive information often resides in SharePoint and OneDrive.
Review:
External sharing settings
Anonymous sharing links
Data Loss Prevention (DLP) policies
Retention policies
Sensitivity labels
Good governance helps prevent accidental data exposure.
6. Monitor Sign-In Activity
Regularly reviewing sign-in logs can identify suspicious behaviour before it becomes a major incident.
Look for:
Impossible travel events
Repeated failed logons
High-risk sign-ins
Legacy authentication attempts
Early detection is often the difference between a minor issue and a major breach.
7. Conduct Regular Security Reviews
Security is not a one-time project.
Businesses should schedule regular assessments covering:
Microsoft Secure Score
User permissions
Device compliance
Configuration reviews
Vulnerability management
Conclusion
Microsoft 365 can be an extremely secure platform when configured correctly. The challenge is that many organisations only use a fraction of the security controls available to them.
A structured security assessment can identify weaknesses, improve resilience and help organisations meet Cyber Essentials, ISO27001 and other compliance obligations.