Bernhard Mueller, the creator of MythX, shows how to detect vulnerabilities in Ethereum smart contracts.
On November 6th, a user playing with the Parity multisig wallet library contract “accidentally” triggered its kill() function, effectively freezing the funds on all Parity multisig wallets linked to the library’s code. According to early estimates this might have made more than $100 million worth of Ether inaccessible (update: in the meantime, that number has gone up to $280 million).
As it happens, just a couple of weeks ago I wrote about using symbolic analysis to detect unprotected SUICIDE instructions. The newest Parity accident was caused by an instance of this vulnerability. In this blog article, I’ll examine what happened in the latest attack and introduce a new Mythril analysis module that can be used detect similar issues in Solidity code and on-chain contracts.
Ethereum rendition by Shed Designs for EventChain.io
First, let’s have a brief look at what happened. User devops199 (for the sake of simplicity, I’ll just call him “the attacker”) managed to cause havoc by sending only two transactions. The first transaction called the initWallet function to set the owner state variable to devops199’s address.
Function: initWallet(address[] _owners, uint256 _required, uint256 _daylimit)MethodID: 0xe46dcfeb[0]:0000000000000000000000000000000000000000000000000000000000000060[1]:0000000000000000000000000000000000000000000000000000000000000000[2]:0000000000000000000000000000000000000000000000000000000000000000[3]:0000000000000000000000000000000000000000000000000000000000000001[4]:000000000000000000000000ae7168deb525862f4fee37d987a971b385b96952
You might be wondering why this worked so easily. Weren’t there checks in place to prevent random senders from calling initWallet? As you can see in the source code, access to this function is restricted with the only_uninitialized modifier:
/ throw unless the contract is not yet initialized.modifier only_uninitialized { if (m_numOwners > 0) throw; _; }// constructor — just pass on the owner array to the multiowned and// the limit to daylimit
function initWallet(address[] _owners, uint _required, uint _daylimit) only_uninitialized
This modifier allows the wallet to be initialized once. Normally, when a new wallet is set up, this library function is called immediately during creation.
However, WalletLibrary itself was never to be supposed to be used as a wallet. It’s only purpose was to provide code to other contracts calling it via DELEGATECALL. Therefore, and very unfortunately, the library contract instance itself had never been initialized. This was an oversight by the developers, and can be partly attributed to Ethereum’s confusing library/CALL semantics. This topic would warrant its own blog post at some point, but for now let’s focus on the particular attack.
Being the contract owner, the attacker now called the kill function in the second transaction:
Function: kill(address _to)MethodID: 0xcbf0b0c0[0]:000000000000000000000000ae7168deb525862f4fee37d987a971b385b96952
This terminated the library contract, causing all wallets that depend on the library code to be unusable.
Detecting the Bug
Mythril’s “unchecked suicide” module attempts to detect exposed kill functions like the one in WalletLibrary. The module is executed automatically when running mythril with the -x option.
To try it out, you can run the analysis against the Parity wallet. Unfortunately, Mythril’s on-chain scanning feature can’t be used on the dead contract instance, and the WalletLibrary code only compiles with solc 4.10. The best way to get it to work is to first compile the code in remix, then pass the runtime bytecode to Mythril with the -c option. You should get the following output:
./myth -x -c "60606040(…)0029"=== Unchecked SUICIDE ===
Type: Warning
The function kill(address) executes the SUICIDE instruction.
The remaining Ether is sent to an address provided as a function argument.
There is a check on storage index keccac_1461501637330902918203684832716283019655932542975_&_caller. This storage index can be written to by calling the function 'initMultiowned(address[],uint256)'.
There is a check on storage index 0. This storage index can be written to by calling the function 'initMultiowned(address[],uint256)'.
There is a check on storage index 1. This storage index can be written to by calling the function 'initMultiowned(address[],uint256)'.
The algorithm works as follows:
- Make an inventory of all
SSTOREintructions that are not constrained by msg.sender (i.e. callable by anyone); - Look for
SUICIDEinstructions and check the constraints on each. - If a ‘tainted’
SSTOREfor every relevant storage index found, attempt to solve the constraints on the basic block containing theSUICIDEblock to make sure it is reachable.
It’s a pretty simple check, but sufficient to detect trivial flaws like the Parity one (although it isn’t false-positive-proof).
About Mythril and MythX
Mythril is a free and open-source smart contract security analyzer. It uses symbolic execution to detect a variety of security vulnerabilities.
MythX is a cloud-based smart contract security service that seamlessly integrates into smart contract development environments and build pipelines. It bundles multiple bleeding-edge security analysis processes into an easy-to-use API that allows anyone to create purpose-built smart contract security tools. MythX is compatible with Ethereum, Tron, Vechain, Quorum, Roostock and other EVM-based platforms.