Migrating Your Law Firm to Microsoft 365: The Security Checklist Most Firms Skip

Blog post - By Securing Your Law Firm

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When small to mid-sized law firms move from on-premises servers or scattered consumer apps like Dropbox and basic webmail to Microsoft 365, the promise is usually simple: fewer servers, easier collaboration, and better security.

That promise can be real. It is also easy to overstate.

Assuming Microsoft 365 is secure out of the box for privileged legal work is one of the most expensive mistakes a firm can make. Microsoft secures the cloud platform itself, but your firm is still responsible for identity, access, sharing, device control, and data governance.

In this article:

• Why the shared responsibility model matters for law firms
• Which Microsoft 365 plans create the biggest security gaps
• The 6 hardening controls every firm should configure
• How third-party vendors and BAAs fit into the migration
• A 5-point audit managing partners can run today

Why default cloud settings fail legal ethics tests

Under ABA Model Rule 1.6(c), lawyers must make reasonable efforts to prevent unauthorized disclosure of client information. That standard applies whether data lives on a file server, in email, or in Microsoft 365.

Fresh Microsoft 365 tenants are usually optimized for convenience and collaboration. That means broad sharing, permissive sign-in behavior, and easy third-party app consent unless someone intentionally changes the defaults.

For a law firm, that can translate into anonymous links, unreviewed device access, and client files syncing onto personal laptops with little friction. If a matter file leaks because the tenant was never hardened, "the cloud did it" will not satisfy a bar committee, a client, or an insurer.

The licensing trap

Not every Microsoft 365 plan gives you the controls a law firm actually needs.

Business Basic and Business Standard can cover mail and productivity, but they do not provide the same security baseline as Business Premium. If your firm needs Conditional Access, Intune device management, Microsoft Defender for Business, and Purview information protection, the cheapest tier is usually the wrong foundation.

Business Premium is the practical baseline for most small firms because it brings together the identity, endpoint, and data controls that make cloud migration defensible. Buying storage without the controls to govern it is not a security strategy.

The 6 non-negotiable controls

1. Enforce Conditional Access and MFA

Multi-factor authentication should be mandatory for every account, including administrators. Conditional Access should also block risky sign-ins from foreign locations, unmanaged devices, and suspicious authentication patterns.

2. Lock down external sharing

SharePoint and OneDrive should not allow casual "Anyone with the link" sharing by default. Set sharing to specific people only, require expiration dates and passwords for external links, and disable re-sharing where possible.

3. Harden email authentication

SPF, DKIM, and DMARC should be configured together so outside mail systems can verify your domain. For law firms, a DMARC policy that reaches p=reject is the goal because it reduces spoofing and business email compromise risk.

4. Turn on Defender for Office 365

Attackers routinely hide payloads inside fake settlement documents, court notices, and invoices. Safe Links and Safe Attachments help inspect content before it reaches an attorney's inbox.

5. Control devices and data sync

If a user signs in from an unmanaged device, the firm should know about it and decide whether access is allowed. Intune, device compliance policies, and data loss prevention reduce the odds that firm content lives unencrypted on personal hardware.

6. Restrict third-party app consent

Employees should not be able to click "Allow" on an app that wants access to mail, calendar, or files without review. Limit consent so only approved administrators can authorize integrations.

Third-party vendors and BAAs

Microsoft is not the only vendor to review during migration. Every cloud app that will touch client data needs to be checked for contract terms, security posture, and retention behavior.

If your firm handles PHI, you also need a signed BAA where required. Microsoft offers a BAA within its Service Trust Portal, but your tenant still has to be configured in a way that supports HIPAA administrative and technical safeguards.

That means vendor review is not a checkbox. It is part of the migration design.

A 5-point audit managing partners can run now

If your firm has already moved, or is halfway there, ask these five questions:

1. Are anonymous sharing links disabled across SharePoint and OneDrive?
2. Is MFA enforced through Conditional Access, not just recommended?
3. Is DMARC set to p=reject for the firm domain?
4. Do we have a current BAA for any workflow that handles PHI?
5. Can we wipe firm data from a lost phone or laptop right now?

If any answer is unclear, the tenant probably needs a hardening pass before the firm treats the migration as complete.

The right way to think about cloud migration

Microsoft 365 can be a strong platform for a law firm, but only after the tenant is configured for legal work instead of generic collaboration. The goal is not to "move to the cloud" and hope for the best.

The goal is to reduce risk, protect client confidences, and create a tenant your partners can explain to clients, insurers, and regulators with confidence. That takes deliberate setup, not default settings.

If you want an outside view of what your firm already exposes publicly, start with the free Proprietary Exposure Review. If you are planning a migration or need to harden an existing tenant, the Law Firm Security Baseline is built for that work.

This article is for informational purposes only and does not constitute legal or compliance advice.