![](https://hackernoon.com/hn-images/1*CxWtmnF_eTGhr5CTGsTexA.png)

Make a smart contract do things it didn’t want to…
--------------------------------------------------

This is a [in-depth series](https://medium.com/@nicolezhu) around [Zeppelin](https://openzeppelin.org/)’s smart contract [security puzzles](https://ethernaut.zeppelin.solutions/). I’ll give you the direct resources and key concepts you’ll need to solve the puzzles 100% on your own.

Over the next weeks, we’ll reproduce how some serious hacks were induced, notably:

*   [$50M DAO hack](http://hackingdistributed.com/2016/06/18/analysis-of-the-dao-exploit/): resulting from a re-entrancy problem
*   [$30M Parity hack](https://blog.zeppelin.solutions/on-the-parity-wallet-multisig-hack-405a8c12e8f7): induced by a delegatecall() exploitation

**_Required_**_: Basic knowledge of smart contract development_

### How Ethernaut works

All smart contract source code are compiled into two formats, by the Ethereum Virtual Machine (EVM):

*   **Application Binary Interface (ABI)**: a communication layer between solidity and Javascript, in JSON format
*   **Bytecode**: the low level machine language that gets executed by the the EVM

When you request `get a new instance` for each level, Ethernaut deploys the compiled bytecode to a new address on the Ropsten test network:

![](https://hackernoon.com/hn-images/1*Sz1a7G2pQ62UnkHoieve4w.jpeg)

Once this new instance is created on the blockchain, its address is returned to your web client through an event, as seen in the game’s main contract, `Ethernaut.sol`:

[https://github.com/OpenZeppelin/ethernaut/blob/master/contracts/Ethernaut.sol](https://github.com/OpenZeppelin/ethernaut/blob/master/contracts/Ethernaut.sol)

Finally, Web3 wraps an ABI around this new contract instance, and allows you to interact with the contract through your web console.

![](https://hackernoon.com/hn-images/1*-N9MYrh3OceDaHkV4ObpeQ.png)

Notice that all \``` public` `` functions are available in the web client

### Detailed Walkthrough

_This level requires you to guess a secret password in order to “get cleared” to move on._

1.  Notice that Ethernaut passed a secret `_password` into the constructor, when it created your contract instance:

function Instance(string \_password) public {   
    password = \_password;   
}

2\. This password is stored as a `public` `string` variable

string public password;

3\. All public, basic variable types in Solidity have an auto-generated getter function. This means you can directly _read_ this not-so-secret password by typing into the console:

await contract.password()

_You can use async/await to work with Web3 promises with more ease_

4\. To pass this level, simply call the final `authenticate` function and pass in the retrieved password, via the console:

await contract.authenticate("\[password here\]");

_You’ll be modifying storage in the authenticate function, so expect to pay some gas when calling this transaction._

5\. Finally, you should be able to double check if you’ve passed this level:

await contract.getCleared();

### Key Security Takeaways

*   All functions and variables stored on the blockchain are viewable by the public
*   Never store passwords directly inside a smart contract, (not even as `private` variables, as we’ll learn shortly)

### More Levels

[**Ethernaut Lvl 1 Walkthrough: how to abuse the Fallback function**  
_This is a in-depth series around Zeppelin team’s smart contract security puzzles. I’ll give you the direct resources…_hackernoon.com](https://hackernoon.com/ethernaut-lvl-1-walkthrough-how-to-abuse-the-fallback-function-118057b68b56 "https://hackernoon.com/ethernaut-lvl-1-walkthrough-how-to-abuse-the-fallback-function-118057b68b56")[](https://hackernoon.com/ethernaut-lvl-1-walkthrough-how-to-abuse-the-fallback-function-118057b68b56)

[**Ethernaut Lvl 2 Fallout Walkthrough: how simple developer errors become big mistakes**  
_This is a in-depth series around Zeppelin team’s smart contract security puzzles. I’ll give you the direct resources…_medium.com](https://medium.com/@nicolezhu/ethernaut-lvl-2-walkthrough-how-simple-developer-errors-become-big-mistakes-b705ff00a62f "https://medium.com/@nicolezhu/ethernaut-lvl-2-walkthrough-how-simple-developer-errors-become-big-mistakes-b705ff00a62f")[](https://medium.com/@nicolezhu/ethernaut-lvl-2-walkthrough-how-simple-developer-errors-become-big-mistakes-b705ff00a62f)

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