Microsoft 365 Security Checklist: Essential Steps to Protect Your Business
Microsoft 365 has become a core platform for business communication, document sharing, collaboration, and remote work. Organisations use services such as Exchange Online, SharePoint, OneDrive, and Microsoft Teams to manage emails, store business data, and collaborate with employees, customers, and external partners.
However, deploying Microsoft 365 does not automatically make an organisation secure. Misconfigured identity settings, excessive user permissions, weak authentication policies, and insufficient monitoring can expose business accounts and sensitive information to cyber threats.
For example, an employee’s compromised account could allow an attacker to access confidential emails, create malicious inbox rules, download shared documents, or send phishing emails from a legitimate business address. If the account has administrative privileges, the potential impact can extend across the organisation’s Microsoft 365 environment.
A structured Microsoft 365 security checklist helps businesses identify these weaknesses, verify existing security controls, and reduce the risk of unauthorised access.
This guide covers the essential Microsoft 365 security checks that IT administrators and security teams should perform regularly.
1. Review Identity and Access Management Settings
Identity security is one of the most important parts of Microsoft 365 security. Most users access business resources through their Microsoft Entra ID identities, making user accounts a critical security boundary.
If an attacker obtains valid credentials, they may be able to access corporate resources without exploiting a traditional software vulnerability.
Essential security checks
- Review user accounts: Identify inactive accounts, duplicate identities, and accounts belonging to employees who have left the organisation. Disable or remove access according to your retention and offboarding procedures.
- Enforce multi-factor authentication (MFA): Require MFA for users, particularly administrators and accounts with access to sensitive business information. Where possible, prefer phishing-resistant methods such as passkeys or FIDO2 security keys.
- Review Conditional Access policies: Use Microsoft Entra Conditional Access to evaluate sign-in context and enforce appropriate access requirements. Depending on licensing and configuration, policies can require MFA, restrict access from certain locations, or require compliant devices.
- Apply least privilege: Grant users only the permissions necessary to perform their roles. Avoid assigning administrative roles when standard user access is sufficient.
- Monitor risky sign-ins: Review available sign-in logs and identity risk detections for unfamiliar locations, unusual authentication patterns, and suspicious access attempts.
Organisations should also review legacy authentication protocols that may bypass modern authentication controls. Disable them where they are no longer required and verify that legitimate business applications continue to function.
Why it matters: Strong identity controls reduce the likelihood that stolen passwords or compromised accounts will lead to wider access across the Microsoft 365 environment.2. Secure Administrator Accounts and Privileged Access
Administrator accounts require additional protection because they can modify security policies, manage users, configure applications, and access sensitive resources.
A compromised administrator account can have a much greater impact than a compromised standard user account.
What should administrators check?
Separate administrative and daily-use accounts. Administrators should use dedicated accounts for privileged tasks rather than routinely checking email or browsing the internet with accounts that hold elevated permissions.
Review Microsoft Entra directory roles. Examine assignments such as Global Administrator, Exchange Administrator, SharePoint Administrator, and Security Administrator. Remove unnecessary assignments and investigate any unexpected privilege escalation.
Implement Privileged Identity Management (PIM), where licensed. Microsoft Entra Privileged Identity Management supports just-in-time, time-limited privileged access, with approval and authentication requirements available depending on configuration.
Protect emergency access accounts. Maintain appropriately secured emergency accounts for recovery if normal administrator access becomes unavailable. Monitor their use and test the recovery process periodically.
Audit application permissions. Review enterprise applications, service principals, and application consent grants. An application with excessive permissions may provide a route to sensitive data even when user authentication is properly secured.
Security teams should document why each privileged role or application permission is required and assign an owner responsible for reviewing it.3. Strengthen Exchange Online Email Security
Email remains a common route for phishing, business email compromise, malware delivery, and account takeover. Microsoft 365 email security should therefore include both technical controls and ongoing monitoring.
Exchange Online Protection provides baseline email filtering, while Microsoft Defender for Office 365 offers additional capabilities depending on the organisation’s subscription.
Essential email security checks
- Configure SPF, DKIM, and DMARC: These email authentication mechanisms help receiving mail systems evaluate whether messages are authorised to use your domain. Configure them correctly and monitor DMARC reports before moving to a strict enforcement policy.
- Review anti-phishing policies: Configure impersonation protection and mailbox intelligence features where available. Pay particular attention to executives, finance teams, and employees who handle payments or sensitive information.
- Inspect suspicious inbox rules: Attackers may create rules that hide security notifications, redirect messages, or move incoming emails to obscure folders. Investigate unexpected forwarding and rule changes.
- Restrict automatic external forwarding: Review outbound spam policies and remote-domain settings. Allow automatic forwarding only when there is a documented business requirement.
- Review Safe Links and Safe Attachments: Where Microsoft Defender for Office 365 licensing supports them, configure these features to help protect users against malicious URLs and attachments.
- Audit mailbox delegation: Review full-access permissions, Send As permissions, and Send on Behalf permissions. Remove access that is no longer required.
Verify email authentication
For a business domain such as company.com, administrators should verify that SPF authorises the intended sending services, DKIM signing is enabled for the domain, and DMARC evaluates messages using the correct alignment settings.
A common mistake is publishing an SPF record without accounting for all legitimate sending services. Another is creating a DMARC record but never reviewing the reports or progressing towards an appropriate enforcement policy.
Why it matters: Proper email security configuration helps reduce domain spoofing, suspicious message delivery, and opportunities for attackers to misuse compromised mailboxes.
4. Review SharePoint and OneDrive Sharing Permissions
Microsoft 365 makes it easy to share files internally and externally. However, sharing settings can gradually become too permissive as teams collaborate with vendors, clients, contractors, and temporary employees.
A file that was originally shared for a short-term project may remain accessible long after the project ends.
Key checks for SharePoint and OneDrive
Review external sharing settings. Examine organisation-wide and site-level sharing policies. Set the permitted sharing level according to business requirements, and use more restrictive settings for sites containing sensitive data.
Audit anonymous sharing links. Links that allow anyone with the link to access a resource can be forwarded beyond the intended recipients. Restrict their use where they are not necessary and review existing links regularly.
Set appropriate link permissions. Prefer specific-people links when access needs to be limited to named recipients. Use view-only permissions where editing is unnecessary.
Review guest accounts. Identify external users who no longer require access. Remove or disable obsolete guest accounts in accordance with your access and retention policies.
Apply sensitivity labels and data loss prevention (DLP). Where supported by the organisation’s Microsoft Purview licensing and configuration, use sensitivity labels to classify documents and DLP policies to detect or restrict inappropriate sharing of sensitive information.
Check site ownership. Ensure that business-critical SharePoint sites have appropriate owners who can review membership, permissions, and sharing activity.
Example security scenario
An employee shares a spreadsheet containing customer details with an external consultant using an anonymous link. The project ends, but the link remains active. Anyone who obtains that link may still be able to access the spreadsheet, depending on its permissions and configuration.
Regular reviews of external sharing links, guest access, and sensitive documents can help prevent this type of exposure.5. Secure Microsoft Teams and Collaboration Settings
Microsoft Teams connects employees through chats, meetings, file sharing, applications, and external collaboration. Its integration with other Microsoft 365 services means that a Teams configuration issue can also affect access to files and business information.
Administrators should review Teams policies alongside SharePoint, OneDrive, and identity controls.
Microsoft Teams security checklist
- Control external access: Review whether users can communicate with people from external organisations and whether any domains should be blocked or explicitly allowed.
- Review guest access: Determine what guests can do within teams, channels, meetings, and shared files. Disable capabilities that are not required.
- Audit team membership: Identify teams containing former employees, obsolete guest accounts, or users who no longer need access to confidential discussions.
- Restrict app permissions: Review third-party and custom applications permitted in Teams. Investigate applications requesting access to messages, files, or other sensitive resources.
- Configure meeting controls: Review lobby settings, presenter permissions, anonymous participation, and recording access according to the sensitivity of each meeting.
- Monitor file-sharing behaviour: Remember that files shared in Teams are generally stored in SharePoint or OneDrive. Their underlying permissions and sharing policies must also be reviewed.
6. Enable Audit Logging and Monitor Security Events
Preventive controls are important, but they cannot guarantee that every attack will be blocked. Organisations also need sufficient visibility to investigate suspicious activity and respond quickly.
Microsoft Purview Audit provides auditing capabilities for supported Microsoft 365 activities, subject to the organisation’s licensing, configuration, and retention settings. Microsoft Entra sign-in and audit logs provide additional identity-related visibility.
What should security teams monitor?
Security event | Why it matters |
Unusual sign-in activity | May indicate stolen credentials or an attempted account takeover |
New administrator role assignments | Could indicate unauthorised privilege escalation |
Suspicious inbox rule creation | May indicate an attempt to hide emails or redirect communications |
Unexpected external file sharing | May expose sensitive business information |
Unusual application consent | May allow an application to access data or act on behalf of a user |
Changes to security policies | May weaken existing controls or disable protection |
Mass file downloads or deletions | May indicate data theft or destructive activity |
Administrators should confirm that required audit sources are available and that logs are retained for a sufficient period. Retention varies by subscription and configuration, so do not assume that every event will remain available indefinitely.
For larger environments, security teams can integrate relevant alerts and logs with Microsoft Sentinel or another SIEM platform. Correlating identity, email, endpoint, and cloud activity can help analysts investigate incidents that would otherwise appear unrelated.
For example, a suspicious sign-in followed by the creation of an inbox forwarding rule and unusual file downloads may provide stronger evidence of account compromise than any one event viewed independently.7. Review Microsoft Defender Security and Device Protection
Microsoft 365 security also depends on the devices employees use to access corporate resources. An account may have strong authentication, but an unmanaged or compromised endpoint can still introduce risk.
The available controls depend on the Microsoft 365 subscription and the additional security products deployed by the organisation.
Essential endpoint and threat protection checks
- Review Microsoft Defender alerts: Investigate relevant detections involving malware, suspicious processes, credential theft, and potentially compromised endpoints.
- Check endpoint onboarding: Confirm that supported corporate devices are onboarded to Microsoft Defender for Endpoint where the organisation uses it.
- Verify security updates: Ensure operating systems, browsers, and business applications receive timely security patches.
- Enforce device compliance: Where supported, use Microsoft Intune and Conditional Access to restrict access from devices that do not meet defined security requirements.
- Review security baselines: Validate endpoint configurations against approved standards, including firewall settings, endpoint protection, disk encryption, and local administrator access.
- Investigate alerts across services: Correlate relevant email, identity, and endpoint alerts rather than treating each detection in isolation.
Administrators should also verify that security alerts reach the appropriate team and that escalation procedures are tested. A detection that is never reviewed may not provide meaningful protection.
8. Protect Business Data with Microsoft Purview
Access control helps determine who can reach information, but organisations also need to understand what data they hold and how it is being used.
Microsoft Purview provides capabilities for information protection, data loss prevention, auditing, and compliance. Available features depend on licensing.
Recommended data security checks
Identify sensitive information. Determine where customer records, financial documents, employee information, intellectual property, and other confidential data are stored.
Configure sensitivity labels. Establish appropriate classifications and protection requirements. For example, a confidential document may require restricted access or encryption, depending on the chosen label configuration.
Review DLP policies. Configure policies for relevant data types and locations, such as Exchange Online, SharePoint, and OneDrive. Start with testing or simulation where appropriate to reduce disruption to legitimate business activity.
Investigate policy matches. Review DLP alerts and incidents to determine whether sensitive information is being shared or transmitted in ways that violate organisational policy.
Validate retention and deletion rules. Ensure that retention settings align with legal, regulatory, and business requirements. Retention and security are related but distinct: retaining data longer than necessary can also create risk.
Test policy effectiveness. Use controlled test cases to confirm that the expected alerts, restrictions, and user notifications actually occur.
Data protection policies should reflect the organisation’s actual information flows, not just a generic set of default rules.9. Establish a Regular Microsoft 365 Security Review Process
Microsoft 365 is a continuously changing environment. Employees join and leave, new applications are connected, collaboration settings change, and Microsoft introduces new security features.
A one-time configuration review cannot account for these changes.
A recurring Microsoft 365 security assessment helps organisations identify configuration drift, review permissions, and confirm that security controls continue to work as intended.
Suggested review schedule
Frequency | Recommended activities |
Daily or automated | Review high-priority security alerts and investigate critical detections |
Weekly | Review suspicious sign-ins, email threats, and unresolved security incidents |
Monthly | Review privileged access, external sharing, application permissions, and security policy changes |
Quarterly | Conduct a broader Microsoft 365 configuration review, including identity, email, collaboration, and data protection controls |
After major changes | Validate security following tenant migrations, new integrations, major policy changes, or significant incidents |
The frequency should be adjusted according to business risk, regulatory requirements, tenant size, and the organisation’s security operations capabilities.
For higher-risk environments, automate recurring checks through supported Microsoft security tools, reporting capabilities, or approved APIs. Automation can identify configuration changes quickly, but results still need validation and remediation.Final Thoughts
Microsoft 365 security requires more than enabling MFA or relying on default protection settings. Effective security depends on how identities are managed, administrator privileges are controlled, email and collaboration tools are configured, and suspicious activity is detected and investigated.
Regularly reviewing these controls helps businesses identify weaknesses before they develop into serious incidents. It also provides a clearer understanding of whether existing security policies are effective in day-to-day operations.
For organisations with complex Microsoft 365 environments, a structured Microsoft 365 security assessment can help identify misconfigurations, excessive permissions, gaps in monitoring, and opportunities to strengthen the overall security posture.
SecuriGlobe provides cybersecurity services, including security assessments and VAPT, to help organisations evaluate their security controls, identify technical weaknesses, and prioritise remediation based on risk.
If your organisation needs to review its wider security posture, explore SecuriGlobe’s cybersecurity services or contact the team to discuss your requirements.