Microsoft 365 Security Hardening: A Complete CIS Benchmark Guide

Microsoft 365 is the backbone of most Northern Virginia businesses. It handles your email, documents, communications, and increasingly your identity and access management. But here is the problem: Microsoft 365 is not secure out of the box.

The default configuration is optimized for ease of onboarding, not security. Legacy authentication is enabled, MFA is optional, and powerful security features are buried in admin consoles waiting to be configured. Attackers know this. They actively target poorly configured M365 tenants.

This guide walks through the essential hardening steps based on the CIS Benchmark for Microsoft 365, starting with the highest-impact controls and building to an advanced security posture. Our endpoint protection services include Microsoft 365 hardening as a core component.

This is non-negotiable. MFA blocks 99.9% of automated attacks and is the single most impactful security control you can implement. Do not stop at conditional MFA for external users. Enforce MFA for all users, including internal access.

Implementation: Create a Conditional Access policy requiring MFA for all cloud apps for all users. Exclude break-glass emergency accounts only (and protect those accounts with physical FIDO2 keys or strict location-based policies).

Legacy authentication protocols (POP, IMAP, SMTP, ActiveSync without modern auth) do not support MFA. Attackers use these protocols to bypass MFA entirely. Over 80% of M365 compromise attempts use legacy auth.

Implementation: Create a Conditional Access policy that blocks all legacy authentication. Most organizations can do this without impact. If you have legacy devices or applications that require it, isolate and modernize them as a priority project.

Conditional Access (Azure AD P1 or higher) is your primary policy enforcement mechanism. Build policies for these scenarios:

• Require MFA for all cloud apps (baseline)

• Block access from untrusted locations or countries you do not operate in

• Require compliant or domain-joined devices for sensitive apps

• Block access from devices out of compliance (missing patches, unmanaged)

• Require approved client apps (block unmanaged email clients)

• Require terms of use acceptance for external partner access

• Session controls for web access (restrict copy/paste, download, print)

Report-only mode is your friend during rollout. Deploy policies in report-only first, monitor impact, then enable gradually.

Enabled with Azure AD P2 or M365 E5, ID Protection uses machine learning to detect and automatically respond to identity risks including:

• Leaked credentials detected on the dark web

• Impossible travel (logins from geographically impossible locations)

• Anonymous IP address access (Tor, VPN with known malicious profiles)

• Atypical login patterns (new device, new location, unusual time)

• Credential stuffing and password spray attacks

Configure automated responses: require MFA change on medium risk, require password change on high risk. This provides real-time identity protection without manual SOC intervention.

Email is the primary attack vector. Microsoft Defender for Office 365 (formerly ATP) provides critical protections that the base Exchange Online Protection (EOP) lacks.

Configure anti-phishing policies with:

• Impersonation protection for your domain, executives, and sensitive roles

• Mailbox intelligence for unusual forwarding rules

• Spoof intelligence to detect forged sender addresses

• User-reported phishing analysis

• Safe Links and Safe Attachments with time-of-click verification

Proper email authentication prevents attackers from spoofing your domain. Ensure all three records are configured:

• SPF: Authorize Microsoft’s sending IPs and your legitimate third-party senders

• DKIM: Enable for your custom domain (use Microsoft’s 1024-bit key or your own 2048-bit key)

• DMARC: Start with p=none and monitoring, then move to p=quarantine, and eventually p=reject

DLP policies prevent sensitive data from being shared inappropriately. Configure policies for:

• Credit card numbers, SSNs, and other PII

• CUI (Controlled Unclassified Information) for government contractors

• Financial data (routing numbers, account numbers)

• Health information (HIPAA-protected data)

• Intellectual property and trade secrets

Start in test mode with policy tips to educate users, then enforce after monitoring impact.

Enable unified audit logging to capture all administrative actions, mailbox access, and content changes. Store logs for at least 90 days (365 days recommended). Forward critical security events to your SIEM for correlation.

For organizations using Microsoft Intune (included in Business Premium), enforce device compliance policies requiring:

• Device encryption (BitLocker for Windows, FileVault for Mac)

• Minimum OS versions

• Firewall enabled and configured

• Antivirus active and updated (Defender or third-party)

• Patch compliance within your defined window

• Device password/PIN compliance

Conditional Access policies then require compliant devices for access to corporate resources, closing the loop between device health and access control.

Microsoft 365 security hardening is not a one-time project. Microsoft introduces dozens of new security features per year. Your configuration should evolve with them. Follow this cadence:

• Monthly: Review Microsoft Secure Score, address top recommendations

• Quarterly: Full security configuration review, test new features in the Microsoft 365 admin center

• Annually: External security assessment with penetration testing of your M365 configuration

SecureMe247 offers Microsoft 365 security assessments and hardening services for Northern Virginia businesses. Our engineers configure CIS Benchmark controls, implement Conditional Access policies, and deploy Defender for Office 365. Contact us for a free M365 security assessment.

Is Microsoft 365 secure out of the box?

No. Microsoft 365’s default security configuration is designed for ease of onboarding, not maximum security. Most organizations start with Security Defaults, which provide basic protection, but these are far from sufficient for businesses handling sensitive data. Proper hardening requires CIS benchmarks, Conditional Access policies, and additional security features available in E3/E5 or Business Premium licenses.

What is a Conditional Access policy and do I need it?

Conditional Access is Microsoft’s policy engine that enforces security controls based on signals like user identity, device health, location, and risk level. For example, you can require MFA for all external access but skip MFA on trusted corporate devices. Yes, every organization needs Conditional Access policies. They are available in Azure AD P1, included with Microsoft 365 Business Premium.

What are the most important Microsoft 365 security settings?

The top priorities are: enable MFA for all users, block legacy authentication protocols, enable unified audit logging, configure anti-phishing policies in Defender for Office 365, enable mailbox auditing, implement Conditional Access policies, enable Microsoft Entra ID Protection, and configure Data Loss Prevention (DLP) policies. These controls cover the most common attack vectors.

What is CIS Benchmark for Microsoft 365?

The CIS (Center for Internet Security) Benchmark for Microsoft 365 is a set of prescriptive configuration guidelines designed to harden an M365 tenant against common attacks. It covers Identity, Exchange Online, SharePoint, Teams, and Defender. Following the benchmark provides a clear path to compliance with NIST, CMMC, and other frameworks.

How often should Microsoft 365 security configurations be reviewed?

At minimum quarterly, and after any major Microsoft release or security incident. Microsoft introduces dozens of security features per year, and your tenant configuration should evolve with them. A common finding in security assessments is that organizations have enabled new Microsoft features but never configured them properly.

Does Microsoft 365 Business Premium provide adequate security?

Microsoft 365 Business Premium is the recommended minimum for security-conscious organizations under 300 users. It includes Exchange Online Archiving, Azure AD P1, Microsoft Defender for Office 365 (Plan 1), Defender for Business, DLP, and Intune. For organizations over 300 users requiring more advanced features, E5 is the target suite.