🛡️ Mitigating Corporate Fraud: Check Washing, Reverse Positive Pay, and ACH Blocks

📌 Introduction

Corporate payment fraud has evolved from simple check theft into sophisticated schemes that target businesses of every size. Fraudsters exploit vulnerabilities in paper checks, ACH transactions, and treasury management processes to divert funds, alter payment instructions, and drain corporate accounts.

According to industry fraud surveys, checks remain one of the most targeted payment methods despite the growth of digital payments. As a result, treasury teams, CFOs, controllers, and business owners are increasingly implementing tools such as Reverse Positive Pay, ACH Blocks, and check fraud controls to reduce risk.

This guide explains how these fraud prevention tools work, when to implement them, and how to build a layered defense against payment fraud.

🎯 Understanding Check Washing

What Is Check Washing?

Check washing occurs when criminals:

1. Steal a mailed check
2. Use chemicals to remove the original payee and amount
3. Rewrite the check to themselves or an accomplice
4. Deposit the altered check

Example

Original Check:

Pay To: ABC Supplies
Amount: $1,250.00

After Fraud:

Pay To: John Smith
Amount: $9,850.00

The criminal deposits the altered check before the business discovers the fraud.

🚩 Common Warning Signs

• Missing mailed checks
• Vendors reporting non-payment
• Duplicate check numbers
• Unexpected account withdrawals
• Altered check images in bank statements
• Checks clearing for unusual amounts

🔍 Step 1: Implement Reverse Positive Pay

What Is Reverse Positive Pay?

Reverse Positive Pay shifts responsibility for check verification to the business.

Instead of the bank automatically matching issued checks, the bank sends a list of presented checks for review.

The company must approve or reject each item before payment.

Process Flow

Business Issues Check
         ↓
Check Presented for Payment
         ↓
Bank Sends Exception Report
         ↓
Treasury Team Reviews
         ↓
Approve or Reject
         ↓
Bank Pays or Returns Check

Benefits

✅ Detects Altered Checks

Amounts and payees can be verified before payment.

✅ Stops Stolen Checks

Unknown checks can be rejected.

✅ Improves Treasury Visibility

Finance teams review suspicious transactions daily.

✅ Reduces Financial Losses

Fraudulent checks can be stopped before settlement.

Example

A criminal attempts to cash:

Check #10455
Amount Presented: $8,700

Internal records show:

Check #10455
Issued Amount: $870

Treasury rejects the item.

Potential loss avoided:

💰 $7,830

🔒 Step 2: Upgrade to Positive Pay (Best Practice)

Many banks offer both:

How Positive Pay Works

The company uploads an issued-check file:

Check Number
Payee
Amount
Issue Date

The bank automatically compares incoming checks against the file.

Any mismatch creates an exception.

Example Exception

Uploaded:
Check #5001
Amount: $1,000

Presented:
Check #5001
Amount: $10,000

Result:

🚨 Exception Generated

Payment withheld pending review.

🚫 Step 3: Deploy ACH Blocks

ACH fraud has become one of the fastest-growing forms of corporate payment fraud.

Criminals may:

• Obtain account numbers
• Submit unauthorized debits
• Create fake vendor withdrawals
• Use payroll diversion schemes

What Is an ACH Block?

An ACH Block completely prevents ACH withdrawals from an account.

Only approved transaction types can occur.

Typical Use Case

Treasury Account:

Purpose:
Cash Reserves
Investments
Emergency Liquidity

Because the account should never receive ACH debits:

ACH Debits = Blocked

Any attempt is automatically rejected.

Benefits

✅ Stops unauthorized ACH withdrawals

✅ Protects reserve accounts

✅ Reduces manual monitoring

✅ Provides automatic fraud prevention

🔑 Step 4: Use ACH Filters for Operating Accounts

Operating accounts often require ACH activity.

Instead of a full block, use ACH Filters.

How ACH Filters Work

Allow transactions only from approved companies.

Example:

Example

Attempted Debit:

Originator:
XYZ Marketing LLC

Amount:
$17,500

Not on whitelist.

Result:

🚫 Rejected Automatically

📬 Step 5: Reduce Check Exposure

The easiest fraud to stop is fraud that never has an opportunity to occur.

Best Practices

📧 Use Electronic Payments

Replace checks with:

• ACH Credit
• RTP (Real-Time Payments)
• Wire Transfers
• Virtual Cards

📦 Avoid Unsecured Mailboxes

Use:

• Post office drop locations
• Secure mailrooms
• Internal courier systems

✍️ Use Fraud-Resistant Ink

Gel ink is more difficult to chemically alter.

🔢 Minimize Check Inventory

Keep unused check stock secured.

🏦 Step 6: Segregate Bank Accounts

Modern treasury departments often use multiple accounts.

Example Structure

Operating Account

Vendor Payments
Payroll
Receivables

Treasury Account

Cash Reserves
Investments

Tax Account

Quarterly Taxes
Sales Tax

Payroll Account

Employee Payroll
Benefits

If one account is compromised:

✅ Entire company liquidity remains protected.

📊 Example Treasury Control Architecture

Primary Operating Account
        │
        ├─ Positive Pay
        ├─ ACH Filters
        ├─ Daily Reconciliation
        │
Payroll Account
        │
        ├─ ACH Whitelist
        ├─ Limited Balance
        │
Treasury Reserve Account
        │
        ├─ ACH Block
        ├─ No Checks
        ├─ Dual Approval

This layered structure dramatically reduces fraud exposure.

🖥️ Example Daily Treasury Fraud Review

Morning Checklist

☐ Review Positive Pay exceptions

☐ Review Reverse Positive Pay items

☐ Review ACH exceptions

☐ Verify wire activity

☐ Confirm large vendor payments

☐ Review returned transactions

☐ Investigate unusual account activity

Time required:

⏱️ Approximately 10–20 minutes per day

Potential savings:

💰 Tens of thousands of dollars in avoided losses

⚠️ Common Mistakes

❌ Keeping All Cash in One Account

Creates a single point of failure.

❌ No ACH Controls

Leaves accounts vulnerable to unauthorized debits.

❌ Mailing Large-Dollar Checks

Increases exposure to check washing.

❌ Ignoring Daily Exception Reports

Fraud losses often escalate because warnings are missed.

❌ Relying Solely on Bank Detection

Fraud prevention works best when both bank and company participate.

📋 Corporate Fraud Prevention Checklist

Check Controls

☐ Positive Pay enabled

☐ Reverse Positive Pay reviewed daily

☐ Secure check stock storage

☐ Fraud-resistant check printing

☐ Limited check usage

ACH Controls

☐ ACH Block on reserve accounts

☐ ACH Filters on operating accounts

☐ Approved originator whitelist

☐ Daily ACH monitoring

☐ Dual approval process

Treasury Controls

☐ Multi-account banking structure

☐ Daily reconciliations

☐ Segregation of duties

☐ Payment approval workflows

☐ Fraud response procedures documented

🎓 Key Takeaway

Corporate fraud prevention is no longer a single-control exercise. The strongest organizations use a layered treasury defense strategy:

🛡️ Check Washing Protection → Positive Pay & Reverse Positive Pay

🛡️ ACH Fraud Protection → ACH Blocks & ACH Filters

🛡️ Operational Protection → Account Segregation & Daily Monitoring

🛡️ Strategic Protection → Reduced check usage and stronger payment governance

Businesses that combine these controls significantly reduce exposure to payment fraud while improving treasury visibility, compliance, and operational resilience.

📚 Sources

📖 Association for Financial Professionals — Payments Fraud and Control Surveys

📖 National Automated Clearing House Association — ACH Risk Management and Fraud Prevention Guidance

📖 Federal Deposit Insurance Corporation — Business Banking Security Recommendations

📖 Federal Reserve System — Payment Risk Management Resources

📖 American Bankers Association — Check Fraud and Treasury Management Best Practices

📖 Treasury management guidance published by major U.S. commercial banks regarding Positive Pay, Reverse Positive Pay, ACH Blocks, and ACH Filters.