Microsoft 365 Security Best Practices for Professional Services Firms

Microsoft 365 is the backbone of most professional services firms in Toronto. Exchange for email. Teams for collaboration. SharePoint for document management. OneDrive for file storage. It's an incredibly powerful platform — but out of the box, it's not configured for security.

We audit Microsoft 365 environments every week. The same misconfigurations show up again and again — and any one of them could be the entry point for a breach. Here are the settings and practices every firm should implement.

1. Enforce multi-factor authentication (MFA) on every account

This is non-negotiable. MFA blocks over 99% of account compromise attacks. Yet we still find firms where MFA is only enabled for partners, or only on certain accounts, or not at all. Every account — from the managing partner to the summer intern — needs MFA enabled. Use the Microsoft Authenticator app or FIDO2 security keys. SMS-based MFA is better than nothing, but it's vulnerable to SIM-swapping attacks.

2. Implement Conditional Access policies

Conditional Access is one of the most powerful security features in Microsoft 365 — and one of the most underused. It lets you define rules like: "Only allow login from managed devices," "Block access from high-risk countries," or "Require MFA when accessing SharePoint from outside the office." For law firms handling sensitive client data, Conditional Access policies should be mandatory. They give you granular control over who can access what, from where, and under what conditions.

3. Lock down SharePoint and OneDrive sharing

By default, Microsoft 365 allows users to share files externally with anyone. For a professional services firm, this is a data leak waiting to happen. Configure external sharing policies to restrict who can share, what they can share, and with whom. Implement sensitivity labels to classify documents (e.g., "Client Confidential," "Internal Only") and enforce protection based on classification. Review sharing permissions quarterly — you'd be surprised how many "Anyone with the link" shares accumulate over time.

4. Enable Microsoft Defender for Office 365

If your firm is on Business Premium or E5 licensing, you have access to Microsoft Defender for Office 365 — and you should be using it. Defender provides advanced threat protection for email (Safe Links, Safe Attachments), anti-phishing policies that detect impersonation attempts, and automated investigation and response for detected threats. For firms still on Business Basic or Standard, upgrading to Business Premium is one of the highest-ROI security investments you can make.

5. Configure email authentication (DMARC, DKIM, SPF)

Email authentication protocols prevent attackers from spoofing your domain — sending emails that appear to come from your firm. SPF, DKIM, and DMARC should be configured for every domain your firm uses. Without them, an attacker can send an email that looks like it's from your managing partner to a client, requesting a wire transfer. It happens more often than you'd think.

6. Audit admin accounts and privileged access

Global Admin accounts are the keys to the kingdom. If compromised, an attacker has full control of your entire Microsoft 365 environment. Limit Global Admin accounts to 2-3 maximum. Use dedicated admin accounts (not the same account used for daily email). Enable Privileged Identity Management (PIM) for just-in-time admin access. Monitor admin activity with unified audit logs.

The bigger picture

Microsoft 365 security isn't a one-time configuration — it's an ongoing practice. New features are released monthly. New threats emerge weekly. Your security posture needs to evolve continuously. That's why many Toronto professional services firms partner with a managed IT provider who specializes in Microsoft 365 governance. It's not just about setting it up right — it's about keeping it right.