ERC-777

ERC-777 is an Ethereum token standard for fungible tokens, proposed in 2017 and later given final status, that set out to improve on ERC-20 while remaining backward compatible with it. Its main innovations were hooks, functions that notify the sending and receiving addresses when tokens move so that contracts can react or refuse, and operators, trusted addresses allowed to send tokens on a holder's behalf. Hooks are registered through a separate on-chain registry defined in ERC-1820. The same hooks proved dangerous: they let a receiving contract run code in the middle of a transfer, opening the door to reentrancy attacks on applications that assumed ERC-20 behaviour. In April 2020, attackers exploited this pattern against the imBTC pool on Uniswap and the Lendf.me lending protocol. Developers widely avoid the standard, and the OpenZeppelin contracts library removed its implementation in its 2023 major release. Also see ERC-223 and smart contract.