ERC-827

ERC-827 was a proposal, discussed in 2017 and 2018, to extend the ERC-20 token standard with functions that transfer or approve tokens and then immediately call a function on another contract, passing along arbitrary data, all in a single transaction. The aim was to simplify interactions such as paying a contract and using its service at once, avoiding ERC-20's two-step approve-then-transfer process. Security researchers soon showed that letting a token contract make arbitrary calls on a user's behalf was dangerous: an attacker could use the token's own contract to call other contracts, including making the token approve or move funds it should not. Because of these flaws, the main reference implementation was withdrawn and the proposal was never finalized as a standard. It is cited as a lesson in smart-contract design. Later approaches to the same problem include ERC-777 and account abstraction.