We worked together over the course of a week on ten exciting, intensive development projects, and ended with a bang on the last day. The victims were three ICO projects: In the following we will give an in-depth technical explanation of the hack, describe the white-hat response, and draw some lessons about how such breaches might be prevented in the future. While it is true that the addition of the right modifiers would have prevented the attack, the attack is a little more clever than this would suggest.
The vulnerable MultiSig wallet was split into two contracts to reduce the ethereum wallet setup yahoo mail of each wallet and save gas: Here is a toy version of WalletLibrary:. WalletLibrary looks pretty boring: Beyond some initialization code that will be called in the constructor of Wallet, WalletLibrary provides the basic functionality you would expect from a wallet: Anybody can deposit money into the wallet, but only the owner can withdraw her funds or change the owner of the wallet.
This time, the ethereum wallet setup yahoo mail looks more complex. Notice the use of delegatecall throughout the contract. This means that the library code will run, but will directly modify the data of the client calling the library. Ethereum wallet setup yahoo mail essentially is as if the code of the library had been pasted into the client issuing the delegatecall.
Any storage writes inside the delegatecall will be made to the storage of the client, not the storage of the library. At the EVM level, a contract is just a single program that takes a variable-length binary blob of data as input and produces a variable-length binary blob of data as its output. A transaction in EVM provides an address and some data. Above, the function selector for calling a function called initWallet that takes an address as its argument is the mysterious looking bytes4 sha3 "initWallet address ".
Now that we understand delegatecall and how function selectors work, we can read and understand the Wallet contract. This is followed by a withdraw function ethereum wallet setup yahoo mail, once again, delegates its task to WalletLibrary. Finally, we want our wallet to be able to receive funds. This is commonly handled with a Solidity construct called a fallback function. Since we might want all sorts of logic to be triggered upon the receipt of funds e.
You might have noticed that the initWallet function of WalletLibrary changes the owner of the Wallet. Whoever owns the Wallet can then withdraw whatever funds are contained in the contract.
So, the attacker cannot call Wallet. As it turns out, the implementation of the fallback function means that we are not.
In the Parity contract, it is likely that forwarding data was only done to save gas costs, as the contract is not upgradeable and all forwards could have been made explicit at compile time. Instead, this implicit default forwarding was used over explicit forwarding to expose certain functions like revoke. So, when an attacker calls Wallet. In reality, the initWallet function was more complicated and took more parameters, but the principle of the attack is exactly the one described above.
Not bad for two function calls and 60 cents in gas cost! If initWallet had been marked as internal, there would have been no corresponding entry in the jump table, making it impossible to call initWallet from outside the contract. If initWallet had checked for double initialization, there would also have been no problem.
A white-hat recovery team MEH-WH developers identified and drained all remaining vulnerable wallets into this wallet. This is all well and good for the recovered funds, but the stolen funds are in all likelihood unrecoverable. The Ethereum community cannot, for instance, easily execute a hard fork as they did in the case of The DAO.
The DAO had a built-in day delay, during which the stolen funds were locked into the contract and subject ethereum wallet setup yahoo mail recovery. The MEH only needs to identify compliant exchanges to cash out or convert to ZEC to retain the stolen funds with full anonymity. Unless the hacker trips up, the community will have to resign itself to the loss of the moneymore than in any U.
Since the contract was originally added as one big blob, it was likely copied from somewhere else, making its provenance in development unclear. Note that the first version of the contract already contained the vulnerable code. It is hard to believe that such an large and valuable! At ethereum wallet setup yahoo mail one other multisig contract had an analogous bug that stemmed from the lack of a function modifier. First of all, we recommend that Solidity adopt a default-private level of visibility for contract functions.
This change would have likely prevented this exploit and others like it. This may be an opportunity to batch a number of other safe usability related changes, much needed additional types, and solutions to common gripes into Solidity. It's also an opportune time to think about versioning at the source language level, ethereum wallet setup yahoo mail be able to easily introduce new features into the language without having to worry about backwards compatibility.
Ethereum wallet setup yahoo mail seems likely that the missing function modifiers would have been discovered by the developers if Wallet had just been a single contract instead of delegatecalling out to WalletLibrary.
Nevertheless, we do, of course, strongly recommend that smart contract authors thoroughly test their contracts, and a test policy that included testing for function visibility on every function would have exposed the issue.
The creation of a rigorously ethereum wallet setup yahoo mail best practices guide for testing and smart contract review that requires that visibility assumptions be made explicit and tested is thus, we believe, one of the strong lessons from this attack.
Today, it is not uncommon to see large contracts deployed with only a handful of unit tests, with little to no reasoning about interaction between contracts, and with unit tests that do not even accomplish full statement or decision coverage.
Beyond these glaring omissions of basic software quality techniques standard in the space, it remains apparent that there is still work ethereum wallet setup yahoo mail be done in understanding best practices for high level tooling and language design for smart contracts. The Parity project has released a post-mortem giving a high level overview of the ethereum wallet setup yahoo mail and discussing the steps Parity will take in the future to ensure that such an attack will not happen again.
Many of their conclusions agree with the ones we made here. We would like to thank Everett Hildenbrandt of the KEVM project for his feedback and helpful suggestions on explaining the attack. His research interests include applied cryptography, security, and machine learning. Phil Daian is a first year Ph. Hacker and professor at Cornell, with interests that span distributed systems, OSes and networking.
Multiple signatures are better than ethereum wallet setup yahoo mail. Here is a toy version of WalletLibrary: The response A white-hat recovery team MEH-WH developers identified and drained all remaining vulnerable wallets into this wallet.
If you hold a multisig contract that was drained, please be patient. We believe that there are multiple levels on which lessons should be drawn from this attack: Acknowledgements Ethereum wallet setup yahoo mail would like to thank Everett Hildenbrandt of the KEVM project for his feedback and helpful suggestions on explaining the attack.
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