This series will provide answers to the list of Solidity interview questions that were published by RareSkills.
The difference between tx.origin and msg.sender is that tx.origin is the address that initiated the transaction while msg.sender is the immediate address that sent the current function call that’s being processed.
Another difference is that tx.origin is always an EOA (Externally Owned Account), while msg.sender can be an EOA or a contract.
For example,
if an EOA calls a contract’s function and that function calls a function on another contract via call, from the perspective of the second contract, the tx.origin would be the EOA while the msg.sender would be the address of the first contract.
Demonstration:
// SPDX-License-Identifier: MIT
pragma solidity 0.8.24;
/**
* This is the target contract that uses `tx.origin` for authentication.
* This is problematic because `tx.origin` represents the original
* sender of the transaction and can be exploited by an attacker.
*/
contract Authenticator {
address public owner;
constructor() {
// Set the owner to be the account that deploys the contract.
owner = msg.sender;
}
function changeOwner() external {
/**
* This check is vulnerable! If a different contract calls this
* function, tx.origin would still refer to the original user who
* initiated the transaction, not the contract that's currently calling
* this function.
*/
require(tx.origin == owner, "Only the owner can change ownership");
// This could be set to the attacker's contract.
owner = msg.sender;
}
}
contract AttackerContract {
Authenticator authenticator;
constructor(address _authenticator) {
authenticator = Authenticator(_authenticator);
}
/**
* This function will be used to trick the owner of Authenticator to call
* it. When this function is called, it then calls the changeOwner()
* function on Authenticator. Because Authenticator's changeOwner function
* checks tx.origin and not msg.sender, it will see the original
* transaction initiator (the unsuspecting user) as the sender, and not
* this contract, thereby changing the ownership to this attacker contract.
*/
function trickAuthenticatorContractOwnerToCallThis() external {
authenticator.changeOwner();
}
}
It’s important to note that relying on tx.origin for authentication purposes is discouraged because it can make contracts vulnerable to phishing attacks. This is because the tx.origin does not change during the entire transaction execution, regardless of how many contracts call each other.
Therefore, if an attacker was able to get a user to call a malicious contract, the attacker could authenticate as tx.origin on the authenticating contract.
For this reason, it is better to use msg.sender for authentication purposes to help ensure the immediate caller is the address that is undergoing authentication.