Hardening Microsoft 365: Security Best Practices Checklist – IntegriCom

Introduction

Safeguarding sensitive information and critical data is non-negotiable, especially for organizations relying on Microsoft 365 (formerly Office 365). At IntegriCom, we often see businesses focus heavily on traditional infrastructure (servers, workstations, and network devices) while overlooking where most of their organization’s data actually resides within cloud services.

This gap is commonly exploited by modern cybersecurity threats. Many successful attacks target weak identity protection, a lack of multi-factor authentication, and limited visibility into cloud activity rather than traditional systems.

As phishing attacks, ransomware attacks, and evolving threats continue to increase, strengthening your Microsoft 365 security with the right security measures and security settings is essential. This article is a practical guide to implementing proven practices for Microsoft 365 hardening. Whether you’re an IT professional or supporting business needs, this checklist will help you protect your environment, reduce security risks, and stay ahead of emerging threats and the latest threats.

What is Microsoft 365 Hardening?

Microsoft 365 hardening is the process of implementing advanced security controls, protection settings, and best practices to secure your cloud environment. This includes protecting user identities, enforcing multi-factor authentication, applying conditional access policies, and ensuring strong data protection across Microsoft Office apps and cloud services.

A strong hardening strategy also incorporates data loss prevention, data retention, retention policies, and sensitivity labels to safeguard sensitive data and meet data protection standards and industry regulations through tools like the compliance center.

Effective Microsoft 365 hardening focuses on identifying real-world attack paths, reducing potential vulnerabilities, and strengthening your overall security posture to mitigate risk from evolving threats.

Why is Hardening Microsoft 365 Important?

With the rise of phishing attempts, malicious links, and cybersecurity threats, attackers are increasingly targeting Microsoft 365 environments because identity-based attacks are faster and more effective than traditional methods.

Many attacks follow a predictable pattern: a user interacts with a phishing email, credentials are captured, unauthorized access is gained, and data is exposed or manipulated.

Without proper hardening, organizations face:

• Unauthorized access to sensitive information
• Increased risk of data breaches and costly breaches
• Exposure to ransomware attacks and persistent account compromise

By implementing strong security features and security controls, organizations can reduce risks, address vulnerabilities, and maintain a consistent level of security across users, employees, and systems.

What’s Your Microsoft Secure Score?

Before delving into the checklist, it’s essential to understand your Microsoft Secure Score, a metric provided by Microsoft that measures your organization’s security posture within Microsoft 365. It provides insights into your security configuration and recommends actions to improve it.

The key thing to note is that this is a Microsoft-provided score and should not be treated as a complete measure of your environment. While it recommends valuable actions such as enabling multi-factor authentication, achieving a perfect score often requires additional licensing.

Some features are worth the investment, while others may not align with your business needs. Additionally, the score does not cover all potential vulnerabilities, making it important to go beyond its recommendations.

How do I Improve my Office 365 Security Score?

To improve your Microsoft 365 security:

• Regularly review your Secure Score
• Implement recommended security measures and practices
• Continuously monitor for suspicious activities
• Do not rely solely on the score to measure your environment

How do You Harden Microsoft 365?

Part 1: Secure Identities & Access (Entra ID formerly Azure AD)

Entra ID Basic Set Up Best Practices

1. AD Connect – Cleanup: Regularly review and clean up AD Connect configurations to reduce vulnerabilities and strengthen your security posture
2. Custom Landing Page: Enhance security by customizing the Entra ID landing page with your branding. This helps users identify phishing attempts and avoid malicious links
3. Manage Local PC Admins in AAD: Control local PC administrators through Entra ID using role-based access control and granular access control
4. Pre-Approve Enterprise Applications: Pre-approve essential enterprise applications to reduce security risks and prevent unauthorized access to your organization’s data

User & Admin Access Restriction

5. Do not allow users to grant consent to unmanaged applications: This is a common attack vector used in phishing attacks, where bad actors gain access to sensitive data
6. Do not allow users to register applications: Limit application registrations to administrators to reduce potential vulnerabilities
7. Restrict Access to the Entra ID Admin Portal: Secure admin portals by limiting access and enforcing strict security controls

Multi-Factor Authentication

8. Enable multi-factor authentication for all users: This is one of the most effective ways to prevent unauthorized access. App-based authentication methods are more secure than SMS-based methods
9. Enable self-service password reset: Allow users to securely reset passwords while maintaining security settings
10. Reconfirming user authentication methods: Require users to regularly validate MFA methods to maintain strong protection

Set Conditional Access Policies

11. Force MFA on All Administrators: Strengthen admin accounts and reduce security risks
12. Force MFA on All Accounts: Extend protection across all users and employees
13. Block Legacy Authentication: This is one of the most important security measures to reduce exposure to phishing attempts and emerging threats. Legacy authentication can bypass MFA and is frequently targeted
14. Geo-Blocking: Restrict access based on geographical location to mitigate potential threats
15. Lock down Accounts programs used to login to Program IP’s: Restrict access to approved IP ranges for better security controls

Set Up Security Alerts & Logs

16. Notify all admins when other admins reset passwords: Improve visibility into admin activity
17. Entra ID Identity Protection Alerts enabled: Stay informed on suspicious activities and security risks
18. Setup MFA Notifications: Monitor authentication events for unusual behavior
19. Enable Unified Audit Log (UAL): Centralize logs for better risk assessments and monitoring

Manually Review the Following Regularly

20. Review Risky Users and Sign-ins: Identify compromised accounts and reduce risk
21. Review the list of Enterprise Applications for suspicious apps: Detect unauthorized access to sensitive information

Additional Security Protocols in Entra ID (Machine learning capabilities enabled with higher level of levels of M365 licenses)

22. Turn on sign-in risk policy: Proactively address potential vulnerabilities
23. Turn on user risk policy: Monitor users for suspicious activities
24. Harden External Collaboration Settings: Protect shared data and external access

Part 2: Secure M365 Admin Center (MSOnline)

1. Use limited administrative roles: Apply least privilege access to reduce security risks
2. Creating DA Accounts (Device Admin Account): Designate Device Admin Accounts for device-level administration without granting full global admin privileges, reducing exposure if credentials are compromised
3. Designate more than one global admin: Ensure redundancy for business continuity
4. Designate fewer than 5 global admins: Reduce attack surface and risk
5. Customize the helpdesk link in AAD: Improve support while maintaining secure processes
6. Password Policy: Align password policies with your company’s security settings and business needs

Part 3: Secure Microsoft Exchange

1. Turn on audit data recording: Capture activity for better risk assessments
2. Turn on mailbox auditing for all users: Improve visibility into user activity
3. Block External Email Forwarding Globally & Check Mail Forwarding Rules: Prevent data exfiltration and data breaches
4. Set outbound spam notifications: Detect compromised accounts and phishing attempts
5. Review mailbox delegation: Ensure secure access to mailboxes
6. No transport rules to external domains: Prevent unauthorized data transfer
7. Do not use mail forwarding rules to external domains: Protect sensitive information
8. Do not use mail flow rules that bypass anti-spam protection: Maintain full protection against malicious links and threats

Email Protection Best Practices

1. Implement anti-phishing policies: Protect against phishing attacks, impersonation, and domain spoofing
2. Use safe links and safe attachments: Scan URLs and files in real time to prevent malicious content delivery
3. Prevent rules that bypass spam filtering: Ensure all messages are scanned for threats and monitored for suspicious behavior

These steps help defend against cybersecurity threats, ransomware attacks, and phishing attempts targeting Exchange Online.

Part 4: Additional Best Practices for Hardening Microsoft 365

SharePoint/OneDrive

1. Configure Expiration Time for External Sharing Links (SharePoint Online): Control access to shared data
2. Protect sensitive data with access controls: Maintain strong data protection across collaboration tools

Endpoint Manager (Intune)

1. Use Microsoft Intune for mobile device management: Secure devices accessing company data
2. Basic MAM Policy: Implement policies to protect business data on mobile devices and support modern work environments

Final Points

Keep in mind that one-time hardening is not enough. You will want to audit these changes at regular intervals. Choose an interval that is appropriate for your organization. At IntegriCom, we audit our clients biannually. Another very crucial point is that this list is an ever-changing and ever-evolving list as Microsoft continues to rapidly update and change these cloud services. You must keep researching ways to harden your tenant and keep up to date with the changes Microsoft makes to Office 365 / M365.

Secure Office 365 / Microsoft 365 with IntegriCom Managed IT Services

In conclusion, adopting these best practices for hardening Microsoft 365 is paramount in safeguarding your organization’s digital assets. Ensure a robust security posture, including these periodic hardening actions, with IntegriCom’s Managed IT Services. Schedule a consultation today to fortify your defenses against evolving cyber threats.

Calvin, an Atlanta native, is a Senior Engineer at IntegriCom® located in Suwanee, GA, and Gainesville, GA. As an advocate of security and sound processes, Calvin makes sure our internal technology, as well as the technology of our clients, is sound and robust. He helps our clients breathe easier about their technology.