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

Unbounded loop in `PuppyRaffle::enterRaffle` function is a potential denial of service (DoS) attack

lukamm

Unbounded loop in PuppyRaffle::enterRaffle function is a potential denial of service (DoS) attack

Description

The PuppyRaffle::enterRaffle function loops through the players array to check for duplicates. However, the longer the PuppyRaffle::players array is, the more checks a new player will have to make. This means the gas costs for the players who enter right away when the raffle starts will be dramatically lower than those who enter later. Every additional address in the players array is an additional check the loop will have to make.

@> for (uint256 i = 0; i < players.length - 1; i++) {
for (uint256 j = i + 1; j < players.length; j++) {
require(players[i] != players[j], "PuppyRaffle::Duplicate player");
}
}

Risk

Likelihood:

The increased cost of entering will occur continuously for each raffle as more users enter.
It will eventually reach block gas limits and the function will completely break once the amount of users who have entered is large enough

Impact:

The gas cost for raffle entrance will greatly increase as more players enter the raffle. Discouraging later users from entering, causing a rush at the start of a raffle to be one of the first entrants in the queue.

An attacker might make the PuppyRaffle::players array so big that no one else enters, or the PuppyRaffle::enterRaffle function reaches block gas limits and will always revert. This would guarantee them the win

Proof of Concept

If we have 2 sets of 100 players enter, the gas costs will be as such:

  • 1st 100 players: ~6252048 gas

  • 2nd 100 players: ~18068138 gas

This is more than 3x more expensive for the second 100 players.

Place the following test into PuppyRaffleTest.t.sol

function testEnterRaffleDenialOfService() public {
uint256 playersNum = 100;
address[] memory players = new address[](playersNum);
for (uint256 i = 0; i < playersNum; i++) {
players[i] = address(i);
}
​
uint256 gasStart = gasleft();
puppyRaffle.enterRaffle{value: entranceFee * playersNum}(players);
uint256 gasEnd = gasleft();
uint256 gasUsed = gasStart - gasEnd;
console.log("Gas used for first enterRaffle call:", gasUsed);
​
address[] memory playersTwo = new address[](playersNum);
for (uint256 i = 0; i < playersNum; i++) {
playersTwo[i] = address(i + playersNum);
}
​
uint256 gasStartTwo = gasleft();
puppyRaffle.enterRaffle{value: entranceFee * playersNum}(playersTwo);
uint256 gasEndTwo = gasleft();
uint256 gasUsedTwo = gasStartTwo - gasEndTwo;
console.log("Gas used for second enterRaffle call:", gasUsedTwo);
​
assertGt(gasUsedTwo, gasUsed);
}

Recommended Mitigation

There are a few recommendations.

  1. Consider allowing duplicates. Users can make new wallet addresses and enter multiple times anyway.

  2. Consider using mapping to check for duplicates. This would allow constant time lookup of whether a user has already entered.

Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge 3 days ago
Submission Judgement Published
Validated
Assigned finding tags:

[M-01] `PuppyRaffle: enterRaffle` Use of gas extensive duplicate check leads to Denial of Service, making subsequent participants to spend much more gas than prev ones to enter

## Description `enterRaffle` function uses gas inefficient duplicate check that causes leads to Denial of Service, making subsequent participants to spend much more gas than previous users to enter. ## Vulnerability Details In the `enterRaffle` function, to check duplicates, it loops through the `players` array. As the `player` array grows, it will make more checks, which leads the later user to pay more gas than the earlier one. More users in the Raffle, more checks a user have to make leads to pay more gas. ## Impact As the arrays grows significantly over time, it will make the function unusable due to block gas limit. This is not a fair approach and lead to bad user experience. ## POC In existing test suit, add this test to see the difference b/w gas for users. once added run `forge test --match-test testEnterRaffleIsGasInefficient -vvvvv` in terminal. you will be able to see logs in terminal. ```solidity function testEnterRaffleIsGasInefficient() public { vm.startPrank(owner); vm.txGasPrice(1); /// First we enter 100 participants uint256 firstBatch = 100; address[] memory firstBatchPlayers = new address[](firstBatch); for(uint256 i = 0; i < firstBatchPlayers; i++) { firstBatch[i] = address(i); } uint256 gasStart = gasleft(); puppyRaffle.enterRaffle{value: entranceFee * firstBatch}(firstBatchPlayers); uint256 gasEnd = gasleft(); uint256 gasUsedForFirstBatch = (gasStart - gasEnd) * txPrice; console.log("Gas cost of the first 100 partipants is:", gasUsedForFirstBatch); /// Now we enter 100 more participants uint256 secondBatch = 200; address[] memory secondBatchPlayers = new address[](secondBatch); for(uint256 i = 100; i < secondBatchPlayers; i++) { secondBatch[i] = address(i); } gasStart = gasleft(); puppyRaffle.enterRaffle{value: entranceFee * secondBatch}(secondBatchPlayers); gasEnd = gasleft(); uint256 gasUsedForSecondBatch = (gasStart - gasEnd) * txPrice; console.log("Gas cost of the next 100 participant is:", gasUsedForSecondBatch); vm.stopPrank(owner); } ``` ## Recommendations Here are some of recommendations, any one of that can be used to mitigate this risk. 1. User a mapping to check duplicates. For this approach you to declare a variable `uint256 raffleID`, that way each raffle will have unique id. Add a mapping from player address to raffle id to keep of users for particular round. ```diff + uint256 public raffleID; + mapping (address => uint256) public usersToRaffleId; . . function enterRaffle(address[] memory newPlayers) public payable { require(msg.value == entranceFee * newPlayers.length, "PuppyRaffle: Must send enough to enter raffle"); for (uint256 i = 0; i < newPlayers.length; i++) { players.push(newPlayers[i]); + usersToRaffleId[newPlayers[i]] = true; } // Check for duplicates + for (uint256 i = 0; i < newPlayers.length; i++){ + require(usersToRaffleId[i] != raffleID, "PuppyRaffle: Already a participant"); - for (uint256 i = 0; i < players.length - 1; i++) { - for (uint256 j = i + 1; j < players.length; j++) { - require(players[i] != players[j], "PuppyRaffle: Duplicate player"); - } } emit RaffleEnter(newPlayers); } . . . function selectWinner() external { //Existing code + raffleID = raffleID + 1; } ``` 2. Allow duplicates participants, As technically you can't stop people participants more than once. As players can use new address to enter. ```solidity function enterRaffle(address[] memory newPlayers) public payable { require(msg.value == entranceFee * newPlayers.length, "PuppyRaffle: Must send enough to enter raffle"); for (uint256 i = 0; i < newPlayers.length; i++) { players.push(newPlayers[i]); } emit RaffleEnter(newPlayers); } ```

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