Microsoft 365 Security Best Practices: Harden Your Tenant in 2026

Posted: December 31, 1969 to Cybersecurity.

Why Microsoft 365 Security Demands Attention in 2026

Microsoft 365 is the backbone of productivity for millions of organizations worldwide. It handles email, file storage, collaboration, identity management, and communication. That ubiquity also makes it one of the most targeted platforms by cybercriminals. Business email compromise attacks, credential theft, data exfiltration through SharePoint and OneDrive, and lateral movement through compromised tenants are daily realities for organizations that fail to harden their Microsoft 365 environments.

The challenge is that Microsoft 365 ships with a configuration that prioritizes ease of use over security. Default settings allow external sharing, permit legacy authentication protocols, and leave advanced threat protection features disabled. Securing your tenant requires deliberate, systematic configuration changes across identity, email, data loss prevention, and audit controls.

At Petronella Technology Group, we have spent more than 23 years helping businesses in Raleigh, NC and nationwide implement robust security configurations. This guide walks through the essential Microsoft 365 security best practices every organization should implement in 2026.

Enforce Multi-Factor Authentication for All Users

Multi-factor authentication (MFA) remains the single most effective control for preventing unauthorized access to Microsoft 365 accounts. Microsoft's own research indicates that MFA blocks more than 99.9 percent of automated credential attacks. Despite this, many organizations still have users authenticating with passwords alone.

Implementing MFA across your entire tenant should be the first step in any Microsoft 365 security hardening project. Security defaults, which Microsoft provides at no additional cost, enable MFA for all users and block legacy authentication protocols. For organizations that need more granular control, Conditional Access policies in Azure AD (now Microsoft Entra ID) provide the flexibility to tailor MFA requirements based on user risk, location, device compliance, and application sensitivity.

Best practices for MFA deployment include requiring phishing-resistant methods such as FIDO2 security keys or Microsoft Authenticator with number matching, disabling SMS and voice call as MFA methods due to SIM swapping risks, and creating break-glass accounts with hardware security keys stored securely for emergency access.

Configure Conditional Access Policies

Conditional Access is the policy engine at the heart of Microsoft Entra ID security. It evaluates signals such as user identity, device platform, location, application being accessed, and real-time risk level to make access decisions. Properly configured Conditional Access policies dramatically reduce your attack surface.

Essential Conditional Access policies for 2026 include:

Block Legacy Authentication: Legacy protocols like POP3, IMAP, and SMTP AUTH do not support MFA and are frequently exploited in password spray attacks. Create a policy that blocks all authentication attempts using legacy protocols across your entire tenant.

Require Compliant Devices: For organizations using Microsoft Intune for device management, require that devices meet compliance policies before accessing corporate resources. This ensures that only managed, patched, and encrypted devices can access your data.

Restrict Access by Location: Define named locations for your office networks and trusted IP ranges, then create policies that require additional verification or block access entirely from unexpected locations. This is particularly effective for privileged accounts.

Risk-Based Policies: Microsoft Entra ID Protection evaluates sign-in risk in real time. Configure policies that require MFA for medium-risk sign-ins and block high-risk sign-ins outright, forcing password resets before access is restored.

Session Controls: Limit session duration for sensitive applications, enforce sign-in frequency policies, and require reauthentication for privileged operations. Session controls prevent attackers from maintaining persistent access through stolen session tokens.

Tenant Security Defaults and Beyond

Microsoft provides security defaults as a baseline for tenants that do not have Azure AD Premium licensing. Security defaults enforce MFA registration for all users, block legacy authentication, and require MFA for administrative actions. For small organizations without the resources to manage Conditional Access policies, security defaults provide a solid foundation.

However, security defaults are intentionally simple and lack the granularity most organizations need. Once you implement Conditional Access policies, you should disable security defaults to avoid conflicts. Your Conditional Access policies should replicate and exceed the protections that security defaults provide.

Beyond Conditional Access, tenant-level settings that require attention include disabling user consent for third-party applications (requiring administrator approval for all app registrations), restricting the ability to create Azure AD groups and Teams, configuring password protection to block commonly used passwords, and enabling self-service password reset with appropriate verification methods.

Email Security: Anti-Phishing, Safe Links, and Safe Attachments

Email remains the primary attack vector for most organizations, and Microsoft 365's email security features have matured significantly. Microsoft Defender for Office 365 (formerly Advanced Threat Protection) provides multiple layers of email protection that should be configured for maximum effectiveness.

Anti-Phishing Policies: Configure anti-phishing policies to protect against impersonation attacks targeting your executives, partners, and domains. Enable mailbox intelligence, which uses machine learning to understand each user's communication patterns and detect anomalies. Set the impersonation safety tips to help users identify suspicious messages.

Safe Links: Safe Links rewrites URLs in email messages and Office documents, scanning them at the time of click rather than only at delivery. This provides protection against time-delayed attacks where a URL is benign at delivery but redirects to a malicious site hours later. Enable Safe Links for email, Teams, and Office applications, and configure it to scan URLs even when they point to your own organization's domains.

Safe Attachments: Safe Attachments opens email attachments in a sandboxed environment before delivering them to recipients, detecting malicious content that signature-based scanning might miss. Enable Safe Attachments for SharePoint, OneDrive, and Teams in addition to email to prevent malicious files from being shared through collaboration channels.

DMARC, DKIM, and SPF: These email authentication protocols prevent attackers from sending emails that appear to come from your domain. Ensure SPF records are configured for all sending sources, DKIM signing is enabled, and DMARC is set to a quarantine or reject policy. Monitor DMARC reports regularly to identify unauthorized senders and legitimate services that need to be added to your SPF record.

Data Loss Prevention Policies

Data Loss Prevention (DLP) policies prevent sensitive information from leaving your organization through email, Teams messages, SharePoint sharing, and other channels. Microsoft 365 DLP can detect sensitive data types including Social Security numbers, credit card numbers, health records, and custom patterns specific to your business.

Effective DLP implementation starts with identifying your sensitive data types and understanding where that data flows within your organization. Create policies that initially run in test mode, generating alerts without blocking content, to understand the volume and nature of matches. Once you have tuned the policies to minimize false positives, switch to enforcement mode with appropriate actions such as blocking transmission, requiring justification, or notifying compliance officers.

For organizations subject to regulatory requirements such as HIPAA or CMMC, DLP policies provide a critical technical control for demonstrating that you are actively preventing unauthorized disclosure of protected information.

SharePoint and OneDrive Permissions

SharePoint and OneDrive sharing settings are among the most commonly misconfigured elements of Microsoft 365. Default settings often allow users to share files and folders with anyone, including external users, without requiring authentication. This creates significant data leakage risk.

Recommended sharing configurations include setting the default sharing link type to "Specific people" rather than "Anyone" or "People in your organization," requiring authentication for external sharing, disabling the ability to share with "Anyone" links at the tenant level unless there is a documented business requirement, setting expiration dates on sharing links, and blocking downloads for external users when view-only access is sufficient.

Regularly review sharing reports to identify files and folders that have been shared externally. Microsoft 365's built-in sharing reports and third-party tools can surface content that has been shared broadly, allowing you to remediate oversharing before it results in a data breach.

Audit Logging and Investigation

Microsoft 365's Unified Audit Log captures user and administrator activity across Exchange, SharePoint, OneDrive, Azure AD, Teams, and other services. This logging is essential for security investigations, compliance documentation, and detecting suspicious behavior.

Verify that unified audit logging is enabled for your tenant. While Microsoft enables it by default for most tenants, some older tenants may have it disabled. Configure audit log retention to meet your compliance requirements. Standard retention is 180 days with E3 licensing, but organizations with E5 or compliance add-on licensing can retain logs for up to 10 years.

Beyond enabling logging, establish processes for regular log review. Set up alerts for high-risk activities such as mailbox forwarding rule creation, mass file downloads, administrative privilege escalation, and sign-ins from unusual locations. Microsoft Sentinel or a third-party SIEM can aggregate these logs with other security data for comprehensive monitoring.

Microsoft Secure Score

Microsoft Secure Score provides a quantified measurement of your organization's security posture within Microsoft 365. It evaluates your configurations against Microsoft's recommendations and assigns a numerical score, with specific improvement actions ranked by impact and implementation effort.

While Secure Score should not be your only guide for security decisions, it provides an excellent starting point and a useful benchmark for tracking improvement over time. Focus on the high-impact actions first, particularly those that address identity security and email protection. Many organizations can improve their Secure Score by 30 to 50 points within the first few weeks of a dedicated hardening effort.

Review your Secure Score monthly and investigate any decreases, which may indicate configuration drift or the introduction of new services that have not been properly secured.

Backup Strategy: Protecting Against Data Loss

Microsoft's shared responsibility model explicitly states that data protection is the customer's responsibility. Microsoft protects the infrastructure, but accidental deletion, malicious insider activity, ransomware, and retention policy gaps can all result in permanent data loss if you rely solely on Microsoft's native capabilities.

Implement a third-party backup solution that covers Exchange Online mailboxes, SharePoint sites, OneDrive accounts, Teams data, and Entra ID configurations. Ensure backups run at least daily, that backup data is stored outside your Microsoft 365 tenant, and that you regularly test restoration procedures. Many organizations discover during an incident that their backup solution cannot restore data as expected because they never tested the process.

Building a Hardened Microsoft 365 Tenant

Securing Microsoft 365 is not a one-time project. It requires ongoing attention as Microsoft releases new features, your organization's needs evolve, and the threat landscape changes. Establish a quarterly review cycle to evaluate your Conditional Access policies, DLP rules, sharing settings, and security configurations against current best practices.

Our managed IT services include ongoing Microsoft 365 security management, ensuring your tenant configurations stay aligned with best practices and compliance requirements as both the platform and threat landscape evolve.

Petronella Technology Group has more than 23 years of experience helping organizations in Raleigh, NC and across the country secure their IT infrastructure. Whether you need a comprehensive Microsoft 365 security assessment, help implementing specific controls, or ongoing security management, our team has the expertise to strengthen your defenses.

Contact Petronella Technology Group to schedule a Microsoft 365 security review and start hardening your tenant today.

PTG developed ComplianceArmor, a proprietary compliance documentation platform that automates policy generation, risk assessment documentation, and audit preparation across CMMC, HIPAA, SOC 2, and NIST frameworks. This platform reduces compliance preparation time by up to 60 percent compared to manual approaches.