AirDropper
AI First Flight #5
Beginner FriendlyDeFiFoundry
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Submission Details
Severity: high
Valid

Airdrop can be claimed several times by the same claimer leading to unfair airdrop distribution

valya

Root + Impact

Description

  • The MerkleAirdrop contract was designed to distribute 100 USDC to 4 addresses, however it is possible to claim airdrop several times by the only one address and receive all funds from the protocol because there is no record whether the user has already claimed the airdrop or not.

function claim(address account, uint256 amount, bytes32[] calldata merkleProof) external payable {
@> // no check whether the airdrop has already been claimed
if (msg.value != FEE) {
revert MerkleAirdrop__InvalidFeeAmount();
}
bytes32 leaf = keccak256(bytes.concat(keccak256(abi.encode(account, amount))));
if (!MerkleProof.verify(merkleProof, i_merkleRoot, leaf)) {
revert MerkleAirdrop__InvalidProof();
}
emit Claimed(account, amount);
i_airdropToken.safeTransfer(account, amount);
}

Risk

Likelihood:

  • The issue occurs when the user claims the airdrop repeadetly using MerkleAirdrop::claim() till the MerkleAirdrop's USDC balance is not completely exhausted.

Impact:

  • USDC are distributed not fairly among eligible users.

Proof of Concept

Please, add the following test test_canClaimTwice to the test suite MerkleAirdropTest.t.sol. The same user claims the airdrop twice and receives more USDC than it was intended.

function test_canClaimTwice() public {
uint256 startingBalance = token.balanceOf(collectorOne);
vm.deal(collectorOne, airdrop.getFee() * 2);
​
vm.startPrank(collectorOne);
airdrop.claim{ value: airdrop.getFee() }(collectorOne, amountToCollect, proof);
vm.stopPrank();
​
vm.warp(block.timestamp + 60);
vm.roll(block.number + 1);
​
vm.startPrank(collectorOne);
airdrop.claim{ value: airdrop.getFee() }(collectorOne, amountToCollect, proof);
vm.stopPrank();
uint256 endingBalance = token.balanceOf(collectorOne);
assertEq(endingBalance - startingBalance, amountToCollect * 2);
}

Recommended Mitigation

Add a mapping to record whether the user has already claimed his airdrop and check this once the user claims his money:

+ error MerkleAirdrop__AlreadyClaimed();
+ mapping(address claimer => bool claimed) private s_hasClaimed;
​
function claim(address account, uint256 amount, bytes32[] calldata merkleProof) external payable {
+ if(s_hasClaimed[account]){
+ revert MerkleAirdrop__AlreadyClaimed();
+ }
​
if (msg.value != FEE) {
revert MerkleAirdrop__InvalidFeeAmount();
}
bytes32 leaf = keccak256(bytes.concat(keccak256(abi.encode(account, amount))));
if (!MerkleProof.verify(merkleProof, i_merkleRoot, leaf)) {
revert MerkleAirdrop__InvalidProof();
}
+ s_hasClaimed[account] = true;
emit Claimed(account, amount);
i_airdropToken.safeTransfer(account, amount);
}
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge 1 day ago
Submission Judgement Published
Validated
Assigned finding tags:

[H-02] Eligible users can claim their airdrop amounts over and over again, draining the contract

## Description A user eligible for the airdrop can verify themselves as being part of the merkle tree and claim their airdrop amount. However, there is no mechanism enabled to track the users who have already claimed their airdrop, and the merkle tree is still composed of the same user. This allows users to drain the `MerkleAirdrop` contract by calling the `MerkleAirdrop::claim()` function over and over again. ## Impact **Severity: High**<br/>**Likelihood: High** A malicious user can call the `MerkleAirdrop::claim()` function over and over again until the contract is drained of all its funds. This also means that other users won't be able to claim their airdrop amounts. ## Proof of Code Add the following test to `./test/MerkleAirdrop.t.sol`, ```javascript function testClaimAirdropOverAndOverAgain() public { vm.deal(collectorOne, airdrop.getFee() * 4); for (uint8 i = 0; i < 4; i++) { vm.prank(collectorOne); airdrop.claim{ value: airdrop.getFee() }(collectorOne, amountToCollect, proof); } assertEq(token.balanceOf(collectorOne), 100e6); } ``` The test passes, and the malicious user has drained the contract of all its funds. ## Recommended Mitigation Use a mapping to store the addresses that have claimed their airdrop amounts. Check and update this mapping each time a user tries to claim their airdrop amount. ```diff contract MerkleAirdrop is Ownable { using SafeERC20 for IERC20; error MerkleAirdrop__InvalidFeeAmount(); error MerkleAirdrop__InvalidProof(); error MerkleAirdrop__TransferFailed(); + error MerkleAirdrop__AlreadyClaimed(); uint256 private constant FEE = 1e9; IERC20 private immutable i_airdropToken; bytes32 private immutable i_merkleRoot; + mapping(address user => bool claimed) private s_hasClaimed; ... function claim(address account, uint256 amount, bytes32[] calldata merkleProof) external payable { + if (s_hasClaimed[account]) revert MerkleAirdrop__AlreadyClaimed(); if (msg.value != FEE) { revert MerkleAirdrop__InvalidFeeAmount(); } bytes32 leaf = keccak256(bytes.concat(keccak256(abi.encode(account, amount)))); if (!MerkleProof.verify(merkleProof, i_merkleRoot, leaf)) { revert MerkleAirdrop__InvalidProof(); } + s_hasClaimed[account] = true; emit Claimed(account, amount); i_airdropToken.safeTransfer(account, amount); } ``` Now, let's unit test the changes, ```javascript function testCannotClaimAirdropMoreThanOnceAnymore() public { vm.deal(collectorOne, airdrop.getFee() * 2); vm.prank(collectorOne); airdrop.claim{ value: airdrop.getFee() }(collectorOne, amountToCollect, proof); vm.prank(collectorOne); airdrop.claim{ value: airdrop.getFee() }(collectorOne, amountToCollect, proof); } ``` The test correctly fails, with the following logs, ```shell Failing tests: Encountered 1 failing test in test/MerkleAirdropTest.t.sol:MerkleAirdropTest [FAIL. Reason: MerkleAirdrop__AlreadyClaimed()] testCannotClaimAirdropMoreThanOnceAnymore() (gas: 96751) ```

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