Microsoft 365 Data Protection for Law Firms: DLP, Labels, and Sensitive Data

Blog post – By Securing Your Law Firm

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Most law firms pay for Microsoft 365 and still leave client data exposed.

Out of the box, Microsoft 365 cannot tell the difference between a routine file and a confidential matter file. It has no idea what your matter numbers, engagement templates, or client names look like. That means data can leave in email, Teams, SharePoint, or OneDrive without any alert.

The fix is Microsoft Purview. With sensitive information types, DLP policies, and sensitivity labels, you can teach Microsoft 365 what matters and enforce it before a leak becomes a client problem.

In this article:

Why default Microsoft 365 settings leave firms exposed
How to define sensitive data in Purview
How DLP and labels work together
What to test before rollout
What to check in your firm now

Why default settings leave firms exposed

A law firm's crown jewels are not the same as a retailer's. Most firms do not need to block every credit card number. They need to catch matter numbers, engagement template IDs, and confidential client names that are easy to miss.

When that data is shared externally, the cost is not just inconvenience. It can mean a client trust issue, a regulatory question, or a failed insurance renewal.

Step 1: Define the data Microsoft should protect

In Purview, start with sensitive information types.

For a law firm, the most useful custom types are usually:

Matter or case numbers
Contract and template identifiers
Restricted client names
PII already covered by Microsoft's built-in templates

Do not use broad rules that create noise. A pattern that matches everything creates false positives. A pattern that is too narrow misses obvious leaks.

Step 2: Turn those definitions into DLP rules

A sensitive information type alone does nothing. DLP policies do the enforcement.

Start with one practical rule: block external sharing of content that contains a matter number. Then add rules for client names, template IDs, and PII.

At minimum, define:

the condition
the action
the notification to users
the reporting path

One important limitation matters: email and Teams are usually blocked in real time, but SharePoint and OneDrive enforcement can lag. That is why DLP should be paired with labels.

Step 3: Add sensitivity labels

Sensitivity labels protect data at the file level. They travel with the document and keep encryption and access restrictions in place even after the file moves.

For most firms, four to six labels are enough:

Public
Internal
Client Confidential
Highly Restricted

Labels and DLP work best together. DLP stops the leak. Labels protect the document if it is already outside the intended path.

Step 4: Test before rollout

Before firm-wide rollout:

test a real document against the rule
send a sample email that matches the pattern
confirm the block and warning appear
upload a matching file to SharePoint or OneDrive and check actual enforcement timing

Do not assume the controls work just because they were created.

What to check in your firm now

Ask these questions:

Do we know what a matter number or confidential client name looks like?
Would an outbound email with a matter number be stopped?
Do confidential files keep protection after they leave OneDrive or SharePoint?
Could we show a client or insurer proof of these controls?

If the answers are vague, you likely have a visibility gap.

Make the controls real

This is not a theory exercise. It is a practical security control set that firms should be able to document and defend.

If you want a tighter, more defensible Microsoft 365 setup, our Microsoft 365 & Workstation Security service helps firms configure the controls, document the evidence, and fix the gaps.

Request the full service catalog or get your free Zero-Access Exposure Review™.