Trick or Treat

Summary

The SpookySwap smart contract contains a vulnerability in its refund mechanism within the trickOrTreat and resolveTrick functions. The contract attempts to refund excess ETH sent by users using a low-level call. If the refund fails (e.g., when the recipient is a malicious contract that reverts upon receiving ETH), the entire transaction is reverted due to the require(refundSuccess, "Refund failed") statement. This allows an attacker to deploy contracts that consistently cause refund failures, leading to transaction reverts. By orchestrating multiple such failed refund attempts, an attacker can exhaust the contract's gas resources and approach block gas limits, effectively causing a Denial of Service (DoS) condition that locks down the contract's purchasing functionality.

Vulnerability Details

The vulnerability stems from how the SpookySwap contract handles refunds when users overpay for NFTs. Specifically, in both the trickOrTreat and resolveTrick functions, the contract uses a low-level call to refund excess ETH sent by the user:

This refund mechanism has the following issues:

1. Dependence on Recipient's Ability to Receive ETH: If the msg.sender is a smart contract that does not implement a payable receive or fallback function, or deliberately reverts upon receiving ETH, the refund will fail.

2. Transaction Reversion on Refund Failure: The use of require(refundSuccess, "Refund failed") means that if the refund fails, the entire transaction reverts. This not only prevents the user from purchasing the NFT but also ensures that no state changes occur.

3. Potential for Repeated Failures: An attacker can deploy multiple malicious contracts that always revert on receiving ETH. By repeatedly attempting to purchase NFTs using these contracts, the attacker can cause numerous transaction reverts.

Proof of Concept:
The following Forge test demonstrates how a malicious contract can exploit the refund mechanism to cause transaction reverts, leading to a DoS condition.

Impact

Exploiting this vulnerability can lead to a Denial of Service (DoS) condition where legitimate users are unable to successfully purchase NFTs. By deploying multiple malicious contracts that cause refund failures, an attacker can consume significant gas resources and potentially approach block gas limits. This results in:

• Contract Lockdown: Preventing any new transactions from being processed, effectively locking down the contract.

• Service Disruption: Legitimate users attempting to purchase NFTs will experience transaction failures.

• Economic Impact: Undermines the contract's revenue model and token distribution mechanisms.

• Reputational Damage: Erodes user trust and confidence in the platform, potentially deterring future participation and investment.

Proof of Concept:

The following Forge test demonstrates how a malicious contract can exploit the refund mechanism to cause transaction reverts, leading to a DoS condition.

Explanation:

1. MaliciousRefundContract:

   • Purpose: Simulates a malicious contract that always reverts when attempting to receive ETH, thereby causing refund failures.

   • Functionality: The receive function is overridden to always revert, ensuring that any ETH sent to it (as a refund) will fail.

2. SpookySwapTest:

   • Setup:

     • Deploy SpookySwap: Initializes the SpookySwap contract with a single treat named "Candy" costing 1 ether.

   • Attack Execution (testDenialOfServiceInRefund):

     • Impersonation: Begins impersonating the maliciousUser address to simulate malicious behavior.

     • Refund Failures: The malicious contract attempts to purchase "Candy" by sending 2 ether, expecting a refund of 1 ether. However, since the refund recipient (MaliciousRefundContract) cannot receive ETH, the refund attempt fails, causing the entire transaction to revert.

     • Assertions:

       • No NFTs Minted: Confirms that no NFTs were minted as all purchase attempts reverted.

       • Zero Contract Balance: Ensures that the contract's balance remains zero, indicating that no ETH was retained from failed refunds.

Tools Used

Manual Review

Recommendations

To mitigate the identified Denial of Service (DoS) vulnerability in the SpookySwap contract's refund mechanism, the following measures are strongly recommended:

1. Adopt the Pull Over Push Pattern for Refunds

Description:
Instead of automatically sending refunds to users (push), allow users to withdraw their refunds themselves (pull). This approach eliminates dependencies on the recipient's ability to receive ETH, thereby preventing refund-induced transaction reverts.

Implementation Steps:

1. Track Pending Refunds:

   • Introduce a mapping to record pending refunds for each user.

     mapping(address => uint256) public pendingRefunds;

2. Modify trickOrTreat Function to Record Refunds:

   • Instead of sending the refund directly, record it in the pendingRefunds mapping.

     if (msg.value > requiredCost) {

     uint256 refund = msg.value - requiredCost;

     pendingRefunds[msg.sender] += refund;

     emit RefundRecorded(msg.sender, refund);

     }

3. Modify resolveTrick Function to Record Refunds:

   • Instead of sending the refund directly, record it in the pendingRefunds mapping.

   if (totalPaid > requiredCost) {

   uint256 refund = totalPaid - requiredCost;

   pendingRefunds[msg.sender] += refund;

   emit RefundRecorded(msg.sender, refund);

   }

4. Implement withdrawRefund Function:

   • Allow users to withdraw their pending refunds at their convenience.

     // @notice Allows users to withdraw their pending refunds.

     function withdrawRefund() external nonReentrant {

     uint256 refundAmount = pendingRefunds[msg.sender];

     require(refundAmount > 0, "No refunds available");

     ​

     // Reset the refund before transferring to prevent reentrancy

     pendingRefunds[msg.sender] = 0;

     ​

     (bool success, ) = msg.sender.call{value: refundAmount}("");

     require(success, "Refund withdrawal failed");

     ​

     emit RefundWithdrawn(msg.sender, refundAmount);

     }

5. Define Relevant Events:

   • Emit events to track refund records and withdrawals.

     // @notice Emitted when a refund is recorded.

     event RefundRecorded(address indexed user, uint256 amount);

     ​

     // @notice Emitted when a refund is withdrawn.

     event RefundWithdrawn(address indexed user, uint256 amount);

     ​

Conclusion

The identified Denial of Service (DoS) vulnerability in the SpookySwap contract's refund mechanism poses a significant threat by allowing malicious actors to prevent legitimate users from purchasing NFTs, effectively locking down the contract's functionality. By implementing the recommended mitigation strategies—Adopting the Pull Over Push pattern—the contract can be fortified against such exploits. Addressing this vulnerability is crucial to ensure the contract's operational integrity, economic viability, and user trust.