This rule identifies merchants with abnormally high transaction decline rates that may indicate fraud testing, security issues, or operational problems.

System alerts when:
1. Merchant decline rate exceeds 20% of authorization attempts
2. Measured over rolling time window
3. Significantly above normal authorization failure rates

>20% decline rate indicates potential issues:
- Card testing/validation fraud in progress
- Compromised merchant account being used for fraud
- Payment system configuration problems
- Expired/compromised card credential testing
- Customer fraud targeting (stolen cards being attempted)

High declines can result from:
- Credit card fraud rings testing stolen cards
- Technical issues with payment gateway
- Incorrect merchant category configuration
- Customers using invalid payment methods
- Merchant accepting high-risk transactions

Alert enables operations team to:
- Merchant contact for technical assistance
- Payment system configuration review
- Fraud attack investigation and mitigation
- Customer communication improvement
- Security posture assessment

Investigation focuses on:
- Decline reason codes analysis
- Transaction pattern examination
- Comparison to merchant historical rates
- Industry benchmark comparison
- Fraud vs. technical issue determination

May require:
- Enhanced fraud screening activation
- Technical support intervention
- Merchant education on payment acceptance
- Security review if fraud attack suspected