How to Set Browser DLP Rules for SSNs, EINs and Bank Numbers

What does a browser DLP rule do for a CPA firm?

A browser DLP (data loss prevention) rule watches what staff paste, type or upload into websites on firm devices and stops taxpayer identifiers from reaching sites the firm has not approved. For a CPA firm, that means Social Security numbers, EINs and bank details stay out of public AI tools, personal email and file-sharing sites.

The everyday risk looks ordinary. A preparer pastes a client's prior-year return into a free AI tool to summarize it, or uploads a K-1 to a personal drive to work from home. A browser DLP rule catches that moment, shows the user a short message and records the event, which is evidence your security program works.

This guide covers the setup in Microsoft Purview, which many firms on Microsoft 365 already use. The same logic applies to other DLP products.

Which rules require a CPA firm to control this data?

Three sets of federal rules apply to tax return information at most CPA firms: Section 7216, the FTC Safeguards Rule and IRS security guidance. DLP is one of the clearest ways to show you follow all three.

• Section 7216. 26 U.S.C. 7216 makes knowing or reckless disclosure of tax return information by a preparer a misdemeanor, and Section 6713 adds a civil penalty of $250 per disclosure, up to $10,000 a year. Our guide to keeping client tax documents out of public AI explains how that applies to AI tools.
• FTC Safeguards Rule. Tax preparers are financial institutions under the rule, and 16 CFR 314.4(c) requires safeguards including access controls and monitoring and logging of authorized users' activity to detect unauthorized access or use of customer information. Our FTC Safeguards Rule checklist for tax preparers covers the other required safeguards.
• IRS guidance. Publication 4557 and Publication 5708 set out how the IRS expects tax professionals to protect taxpayer data and write a Written Information Security Plan (WISP). A DLP rule is a concrete control your WISP can name.

None of these rules names a specific product. They describe outcomes, and a tested DLP policy with logs is a practical way to produce them.

Which sensitive information types should browser DLP rules detect?

Build the rules on four built-in Microsoft Purview sensitive information types and one custom type for EINs. Purview has no built-in EIN detector, so firms that skip the custom type leave business clients' identifiers unprotected.

Purview's built-in sensitive information types include:

• U.S. social security number (SSN)
• U.S. individual taxpayer identification number (ITIN)
• U.S. bank account number
• ABA Routing Number

For EINs, create a custom sensitive information type. The IRS describes the EIN in Publication 1635 as a nine-digit number in the format XX-XXXXXXX. A pattern of two digits, a hyphen and seven digits will also match other numbers, so require a nearby keyword such as "EIN," "employer identification number," "FEIN" or "tax ID" to raise confidence. Test it against a sample of real workpapers before you enforce it.

Set thresholds by count. One SSN in a pasted paragraph might be a single client question; twenty SSNs is a client list. A common pattern is to audit or warn on low counts and block on high counts, so staff are not stopped for routine work in approved systems.

Which browser activities should DLP rules cover?

Cover two activities on firm devices: pasting into websites and uploading files to websites. Then add inline protection for AI apps if your licensing includes it.

For paste, Microsoft documents a rule activity called Paste to supported browsers. It works in Microsoft Edge, in Chrome and Firefox on Windows with the Microsoft Purview extension, and in Safari on macOS. The activity requires a sensitive service domain group, which is a named list of sites the rule applies to. Build two groups: one for unapproved AI and file-sharing sites, and one for approved destinations such as your tax software, portal and firm-managed AI tool.

For uploads, use the endpoint DLP activity that restricts uploads to cloud service domains, configured with your blocked and approved site lists. That stops a client file from reaching a personal storage site even when nobody pastes anything.

For AI specifically, Microsoft also offers DLP for unmanaged AI apps in Edge for Business, which inspects text sent to and files uploaded to AI apps in the generative AI category. It applies to Windows devices managed by Microsoft Intune and can use Purview pay-as-you-go billing, so check costs before you plan on it.

Should the rule audit, warn or block?

Start in audit or simulation mode for two to four weeks, then move to block with override for approved-but-sensitive sites and full block for unapproved sites. Jumping straight to block during filing season creates help desk tickets and workarounds.

• Audit: logs the event with no interruption. Use it first to see real traffic.
• Block with override: shows a message and lets the user continue with a business justification, which is logged. Use it where legitimate work happens.
• Block: stops the action. Use it for public AI tools, personal email and personal storage.

Microsoft documents two layouts: set the overall action to audit and list target sites as block exceptions, or set it to block and list trusted sites as audit exceptions. Pick the one that leaves the shorter exception list. Review the Activity explorer weekly for the first month and adjust.

What licensing and setup does this need?

Endpoint DLP is not listed for Microsoft 365 Business Premium on its own, so most small firms need a license change before they can build these rules. Microsoft's Purview service description lists Endpoint DLP under Microsoft 365 E5 and the Microsoft Purview Suite, among others, while Business Premium is listed for DLP in Exchange, SharePoint and OneDrive.

Before you build the rules, confirm:

• Every firm workstation is onboarded to Purview and running a current antimalware client. Our workstation checklist for tax season 2027 covers the rest of the hardening.
• Chrome and Firefox have the Microsoft Purview extension, or are blocked on firm devices.
• Unmanaged browsers and personal devices cannot reach client data at all, through conditional access or another control.
• Someone owns the weekly review of DLP alerts.

What should a CPA firm do before filing season?

Get the rules into audit mode now so you have tuned, enforced rules by the time returns start arriving. Plan for several weeks, including two to four weeks in audit mode.

1. Confirm licensing and onboard every firm workstation.
2. Build the EIN custom type and test all five detectors against sample files.
3. Create the two sensitive service domain groups: blocked sites and approved sites.
4. Create paste and upload rules in audit mode and review the results for two to four weeks.
5. Move to block with override and block, and tell staff what they will see and why.
6. Add the DLP rules, the review owner and the alert process to your WISP.

How s90 helps

s90 designs, tests and monitors DLP and AI guardrails for CPA firms, so client identifiers stay inside approved systems and the WISP describes controls that actually run. If you want to know where your current setup stands before the 2027 filing season, we can start with a short review.