Ethernaut: Fallback

CTFs constitute a crucial part in the process of becoming a successful security researcher as they require from you an eye for details, a good understanding of solidity and great technical skills to perform the attack and capture the flag. The most talented security researchers are great at CTFs, at solving them of course and sometimes at designing them. CTFs will not turn you into a great web3 security researcher over night, but it will surely arm you with good enough technical skills to be able to write decent coded PoCs to have your finding validated and maybe get selected for report.

Ethernaut is a CTF(Capture The Flag) developed by the openzeppelin team that you most propbably already heard of. If you haven't, consider taking a look at this roadmap https://www.0xjarix.com/if-i-had-to-start-again/
This CTF gathers 31 challenges for the moment, this number keeps increasing so check their website every now and then: https://ethernaut.openzeppelin.com/
Maybe you'll design your challenge one day and send it to the openzeppelin team.

Of all the kinds of articles I publish, CTF writeups are those I wish you read the least. I am a big advocate of giving everything the time it needs, if you cannot solve a challenge that you know for a contains an intentional bug in such a small codebase, do not expect to do really well in the contests. There are 2 reasons why someone can fail at solving a challenge, and when I say 'fail' I mean giving up and looking at the writeups, knowing damn well these CTFs are not time-bounded. So if you failed you either:

• aren't ready for this challenge yet and that is most probably due to the fact that you skipped some steps in the roadmap
• are lazy, you read the challenge, read the codebase, maybe not enough times, you had some assumptions maybe, you might have identified some entrypoints or some conditions to bypass or break, but you did not give it enough time, you did not allow yourself to succeed and that's a shame

Look carefully at the contract's code below.

You will beat this level if

1. you claim ownership of the contract
2. you reduce its balance to 0

Things that might help

• How to send ether when interacting with an ABI
• How to send ether outside of the ABI
• Converting to and from wei/ether units (see help() command)
• Fallback methods

1. you claim ownership of the contract
2. you reduce its balance to 0

When 1 is achieved 2 becomes trivial as you may have realised

In order to claim ownership we have to set owner to our address, by skimming the contract we realise that owner is set to msg.sender at exactly 3 occurences:

1. In the constructor(): the contract is already deployed and has an owner
2. In contribute(): We would have to have 1000 ether for that to happen, and that' the intended behavior of the contract, if a user has more contributions than the owner than that user deserves ownership...
3. In receive(): We should send 1 wei or more directly to the contract and have prior contributions worth 1 wei or more. 2 wei in exchange of ownership and 1000 ether, that's a bargain!