Category: Blockchain

author: aman

Blog IV - Part I - Day 14

Let us get some dirty hands on Solidity, to exploit some very dangerous Ethereum - Solidity bugs.

2 Bugs/vulenrabilities in this very micro-blog. Covering bugs like, Denial of Service with Block Gas Limit, where the attacker exploits the bug by taking benefit from limited GAS available for each transaction, and unchecked_send() bug, which when made by mistake, could be a disaster to the host contract holder, and users.

Let us do it...

In this micro-blog

• delegatecall (the proxy calls) (SWC-112) (Inclusion of Functionality from Untrusted Control Sphere)
• DoS With Block Gas Limit (SWC - 128)
• Integer Overflow (SWC - 101)
• Reentrancy Bug(DAO attack) (Improper Enforcement of Behavioral Workflow) (SWC-107)
• uncheckedSend() (SWC - 113)
• tx.origin bug
• Variable Shadowing (SWC-119)

1. dos_gas.sol() [check out the exploitation of the bug at this gist])(https://gist.github.com/johnsoncarl/480aee528f35b8579c7dcf87c61c59d2)

DOS with Block Gas limit is A denial of service attack, where a host contract denies to perform its duties due to limited amount of gas provided for each transaction (about 3 million).

    for(uint i=0;i<500;i++) {
        listAddresses.push(msg.sender);
    }

Here to make the contract to always true change the upper bound of i to some lesser value, say i<100. Increase the value to fail it at a certain point.

uncheckedSend() [check out the exploitation of the bug at this gist])()

Whenever a contract, say sender, transfers the ether to another contract,say receiver, the payable function of the receiver is triggered, and this can be misused. For eg. payable function of the receiver contains some computationally heavy instructions, it can cause transfer() to fail and send() function to return false. Thus if the send() is not checked, it may cause a bug called uncheckedSend.

Also, since send() doesn't propogate the exception, its harmful of the users to use it.

contract attacker{
    bool public flag=false;
    function change() public{
		if(!flag) 	flag=true;
		else    	flag=false;
	}
	function() external payable {if(flag)	revert();}
}
contract Test{
	attacker a = new attacker();
	bool private flag0 = true;
	bool private status;
	function set0(int val) public returns (bool){
    		if (val % 10 == 0) {a.change();}
    		else flag0=false;
  	}
    function echidna_send() public payable returns(bool){
			address(this).transfer(msg.value);
            return address(a).send(0);
		}
	function() external payable{}
}

Here, echidna_send() will be the main function whose bool value will be checked by the tool.

• payable functions : payable functions are necessary for the contract to accept the ether. Whenever a contract, say sender, transfers the ether to another contract,say receiver, the payable function of the receiver is triggered.

• echidna_send() : contains address(this).transfer(msg.value); which is responsible for transferring ether to the Test contract. Which will then be transferred to the the instance of the contract attacker, a. Note: we are transferring 0 ethers to the contract address and then to the instance a. As address.send() doesn't revert state whenever the payment fails. So we try to return its bool value, which is then catched by echidna_send(), and thus by the tool. This is the value that the tool mainly checks for, and thus will be able to tell whether the contract payment through send was completed or not.

• set0(int val) : random value is provided to set0(int val) as argument. Which then waits for the no. satisfy the condition if (val % 10 == 0). As soon as this value is catched, it triggeres change() function of the contract.

• change() : This is responsible for flipping the flag value. So as soon as this function is triggered, flag=false changes to true, and now revert state in the payable will be activated. Now, the contract attacker, will be reverting each transaction made to it.

So this is how it works: [a is the instance if contract attacker] We first start running the contract with a.flag == false, and wait for a value in set(int val), to flip the flag of contract a to true, and thus activating the revert in payable. This will fail everytime the payment is made. And since, the send() doesn't revery any exception, it shall revert true of false. Which is catched by echidna_send(), and will be returned to the tool, to state that the payment could not be completed.

View this thread for more about address.send and address.transfer

I took it exactly from the exploitation repo I made earlier. Please email directly, in case of any doubts:

aman0902pandey(@)gmail.com