Learn blockchain by breaking things.

Write real smart contracts in your browser. No wallet, no install, no crypto required.

Start with lesson 13 lessons, no signup, about 10 minutes.
…

Change one letter. Watch the whole thing change.

Where this goes

10 tracks, 84 lessons. Nothing costs anything, ever.

  1. 0

    Genesis

    Why a chain can't be quietly edited.

  2. 1

    Mechanics

    Follow a transaction end to end.

  3. 4

    Solidity

    Write and deploy your first contract.

  4. 7

    Breach Lab

    Drain a vulnerable vault, then patch it.

  5. 8

    Capstones

    Ship seven real projects.

Track 7

The Breach Lab

You don’t read about reentrancy. You’re handed a vault with 100 ETH and told to empty it.

Then you patch it, and run your own exploit against the patch until it fails.

Attacker.sol
receive() external payable {
  if (address(vault).balance >= 1 ether) {
    vault.withdraw();
  }
}
Drained. Victim balance: 0 ETH