Can a Reentrancy Attack Be Performed without a Fallback Function?
Yes, a reentrancy attack can be performed without a fallback function. Any external call from one contract to another can be a vector for reentrancy.
For example, if a contract calls a function on an external contract that the attacker controls, that function can be programmed to call back into the original contract. The vulnerability is not the fallback function itself, but the act of making an external call before updating state.
The fallback function is just a common and convenient way to trigger the re-entry when Ether is sent.
Glossar
Reentrancy Attack
Exploit ⎊ The reentrancy attack, a critical vulnerability in smart contracts and financial systems, leverages recursive function calls to circumvent intended security protocols.
Fallback Function
Contingency ⎊ A fallback function within cryptocurrency derivatives serves as a pre-defined operational protocol activated when primary system components, such as oracles or clearinghouses, experience disruptions or fail to deliver expected data or execution.
OnERC721Received Hook
Trigger ⎊ The OnERC721Received Hook functions as an event trigger within smart contracts adhering to the ERC721 non-fungible token standard, initiating a predefined function call upon receipt of an NFT.