Breaking Ethereum 😈

Lessons learnt from broken contracts

10 September 2016

Thomson Reuters Hackethon

PΓ©ter SzilΓ‘gyi

Ethereum Core Developer

If you don't know the chain dynamics...

Ether faucet – 0x793ae8c1b1a160bfc07bfb0d04f85eab1a71f4f2

contract Faucet {
    uint amount = 0.01 ether;
    uint freq   = 5760;

    mapping (address => uint) prev;

    function request() {
        if (address(this).balance < amount) {
        if(block.number < prev[msg.sender] + freq) {
        prev[msg.sender] = block.number;

Give away 0.01 Ether to anyone, once per 24 hours... what could go wrong? πŸ˜‡

Ether faucet – pwned πŸ™ƒ

Payout (0.01 Ether) is a nice amount

Faucet security ⇔ Account uniqueness

Lesson: Accounts are free, instantaneous and infinite!

Roulette – 0x5fe5b7546d1628f7348b023a0393de1fc825a4fd

Sizeable implementation of a roulette game

contract Roulette {
    uint seed = 1;

    function rand() private returns (uint) {
        seed = ((seed*3 + 1)/2 % 10**9);
        return (seed + block.number + block.difficulty + block.timestamp + block.gaslimit) % 37;

Uses an onchain random number generator... what could go wrong? πŸ˜‡

Roulette – pwned πŸ™ƒ

Miners make the chain

Transactions are aware of the chain

Lesson: Blockchain state is free for all to use and abuse!

Etherdice – 0x2faa316fc4624ec39adc2ef7b5301124cfb68777

Fairly involved dice game

contract Dice {
    function evaluate(bytes32 seed) {
        // [...] verify the seed
        for (uint i = 0; i < bets.length; i++) {
            // [...] evaluate bets and pay winners

Iterate over all accumulated bets in one go... what could go wrong? πŸ˜‡

Etherdice – self pwned πŸ™ƒ

Blocks have limited gas allowances

Etherdice iterated all bets when closing a round

Lesson: Operations above O(1) will eventually exceed the gas limit!*

If you don't know the language dynamics...

GovernMental – 0xf45717552f12ef7cb65e95476f217ea008167ae3

Twisted Ponzi scheme with smart contracts

contract GovernMental {
    address[] creditors; uint[] credited;

    function invest() {
        if (block.timestamp - lastInvested < TWELVE_HOURS) { ... } else {
            creditors = new address[](0);
            credited  = new uint[](0);

Casually reset the contract at round end... what could go wrong? πŸ˜‡

GovernMental – self pwned πŸ™ƒ

Contract storage in EVM is a single hash map

Freeing up a field ⇔ zeroing out a storage entry

Lesson: Understand and avoid magical constructs!

King of the Ether – 0xb336a86e2feb1e87a328fcb7dd4d04de3df254d0

Game of Thrones pyramid contract

contract KingOfTheEtherThrone {
    address monarch;

    function claim() {
        // [...] calculate the ruler's compensation

        monarch = msg.sender;

Send blindly to compensate the previous ruler... what could go wrong? πŸ˜‡

King of the Ether – broken πŸ™ƒ

Sending funds is an external CALL operation

But what if the ether transfer failed?

Lesson: Anything that can go wrong, will go wrong!

If you don't know the EVM dyamics...

MakerDAO – 0xe02640be68df835aa3327ea6473c02c8f6c3815a

Contracts and frameworks for an on-chain crypto exchange

contract MakerEthToken {
    function withdraw(uint amount) {
        if (balances[msg.sender] >= amount) {
            if ( {
                balances[msg.sender] -= amount;

Send funds with full gas allowance... what could go wrong? πŸ˜‡

MakerDAO – preventive pwned πŸ™ƒ

Calling another contract relinquishes execution

Recipient may have enough gas to call further

Lesson: External calls will eventually loop back in!

Pre-homestead multisig wallet

Wallet contract requiring multiple authorizations

contract MultiOwned {
    mapping(uint => uint) owners;

    modifier onlyowner {
        if (owners[tx.origin] > 0) {

Simply use tx.origin for authentication... what could go wrong? πŸ˜‡

Pre-homestead multisig wallet – swapped before pwn πŸ™ƒ

Pre-homestead, libraries used CALLCODE

Internal transactions retain the same tx.origin

Lesson: Authorization forwarding is exceptionally risky!

TheDAO – 0xbb9bc244d798123fde783fcc1c72d3bb8c189413

Lesson: Don't! Just don't!

Beer keg challenge 🍺

Beer keg – 0x629469c8db3a4d7bcc3a823effcf8900119ba7e7

Untappable beer contract

contract BeerKeg {
    bytes20 prev; // Nickname of the previous tap attempt

    function tap(bytes20 nickname) {
        prev = nickname;
        if (prev != nickname) {
Lesson: You tell me! 😈

Legacy of the fallen ones... ଘ(ΰ©­*ΛŠα΅•Λ‹)ΰ©­

Thank you

PΓ©ter SzilΓ‘gyi

Ethereum Core Developer

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