Home Underhanded Cairo, pt. 2. Solution

Underhanded Cairo, pt. 2. Solution

The results are in. Congratulations to wong_ssh and 0xkaliber, the only two participants who found the attack. Job well done guys! Thank you as well to all the others who gave it a try 🙏 So what’s going on then? Where’s the vulnerability?

It’s a supply chain attack. There is a backdoor in the project’s dependency 💥

Let’s dive into the details.

The project declares a single dependency in its Scarb.toml, the auth lib. Notice that it’s using the v0.1.0 version, but the latest version in the repo is tagged v0.2.0. I’ve done that to throw any negligent auditor off track. Even if you would inspect auth but forgot to checkout the commit pointed to by the v0.1.0 tag, you’d never find the vulnerability.

The backdoor was introduced in this v0.1.0 commit. The assert_owner check will always pass when called from a contract with the address 0x61747461636b6572 (i.e. felt-encoded string “attacker”). Here’s a test case to prove it:

fn test_hack() {
    let token: IERC20LinearDispatcher = deploy_token();
    let attacker = as_addr('attacker');
    assert(token.balance_of(attacker).amount == 0, 'pre hack bal');
    let mint_amount: u128 = 1000000000000000;
    token.mint_to(attacker, mint_amount.into());
    assert(token.balance_of(attacker).amount == mint_amount, 'post hack bal');

Obviously, the test shouldn’t pass because attacker is not the owner. Yet it does, thanks to the backdoor. Simple as that.

As mentioned, this is just a proof of concept to highlight the threat of supply chain attacks. Ultimately, as builders, we are responsible for any code we ship into production. We should be deeply familiar with the libraries we use in our code.

Currently, the situation in the Cairo ecosystem is even more dangerous because Scarb doesn’t yet support a lockfile. In theory, if an attacker gains access to a repo of a popular library, they could introduce a backdoor, simply change git tags and just wait until contracts with this infected library are deployed on mainnet. Thankfully, the team behind Scarb is already working on lockfile support ♥️

In the meantime, as developers, we can either not use any dependencies or use tested and trusted libraries like the one from OpenZeppelin. As auditors, always remember to check the dependencies as well - hopefully they are in scope 🤞

Together, we can #keepStarknetSafe.

This post is licensed under CC BY 4.0 by the author.