A reentrancy attack occurs when a smart contract calls another contract, allowing the called contract to make recursive calls back to the original contract before its initial execution completes. This can exploit vulnerabilities in the way funds or state changes are handled.The most infamous instance of this attack was the DAO hack in 2016, where an attacker repeatedly withdrew funds from a contract by exploiting this flaw. The attacker initiated a withdrawal process which called back into the original contract, allowing them to drain funds before the contract updated its balances to reflect the withdrawal.To mitigate reentrancy attacks, developers often use techniques like the checks-effects-interactions pattern, which ensures all internal state changes are completed before any external calls. Additionally, employing mutexes or using the “pull over push” payment model can help prevent such vulnerabilities. By understanding and addressing reentrancy risks, developers can enhance the security of their smart contracts significantly.