What Is the Fundamental Security Mechanism That Prevents Flash Loan Abuse in a General Sense?

The core security mechanism is the atomic nature of the transaction. A flash loan requires the borrowed funds to be repaid within the same blockchain transaction block.

If the loan is not repaid, the entire transaction is automatically reverted, as if it never happened. This all-or-nothing principle ensures that the lender is never at risk of loss, though it does not prevent the manipulation of other protocols within that single transaction.