Why Do 'On-Chain Approvals' Often Lead to Trouble? Underlying Logic and Security Boundaries of Approve (Upgraded)

Explains risks of on-chain approvals, how to identify dangerous ones, and how to revoke them to prevent asset theft.

{{{##anchor=preface}}} Preface If you use DeFi, you've likely seen warnings about approving contracts. Most on-chain thefts aren't hackers brute-forcing private keys; they are users actively signing an approval , allowing an attacker to legally move their assets. This article clarifies why ERC20 needs approvals, the risks involved, and how to stay safe. {{{##anchor=part-1.2}}} 1. What is Approve? It’s a 'Power of Attorney' for Tokens ERC20 tokens (USDT, USDC, etc.) require user authorization before a contract can move them. Think of it as an "automatic payment limit" on your card. The function is: `approve(spender, amount)` Spender: The authorized contract address. Amount: The maximum allowance the spender can move. {{{##anchor=part-1.3}}} 2. Why Must ERC20 Rely on Approvals? {{{##anchor=part-1.3-因为链上智能合约没有办法主动从你地址扣代币}}} Because Smart Contracts Cannot Proactively Deduct Tokens From Your Address Contracts cannot "log in" to your account or call your private key. They can only deduct tokens if you have authorized them to do so. This design prevents unauthorized misuse of tokens. {{{##anchor=part-1.4}}} 3. The Real Danger is 'Allowance Size,' Not the Act of Approving {{{##anchor=part-