Despite mainframes processing $10.4 trillion in U.S. credit and debit card transactions annually, most banks treat them as security islands invisible to modern monitoring platforms—a gap that contributes to the $4.88 million average breach cost. From unmonitored ESM administrative access to invisible CICS transactions and the inability to correlate mainframe events with network threats, these seven critical blind spots leave financial institutions vulnerable to fraud, insider threats, and multi-million dollar compliance failures.