AirDropper
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Submission Details
Severity: high
Valid

Eligible users can replay the same Merkle proof to drain the airdrop

hongphat19

Root + Impact

Description

The normal behavior should allow each eligible address to claim its airdrop allocation once. A Merkle proof proves that the (account, amount) pair is included in the distribution tree.

The claim function verifies the proof and transfers tokens, but it never records that the account or leaf has already claimed. The same valid (account, amount, proof) can be submitted repeatedly until the airdrop contract runs out of tokens.

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))));
if (!MerkleProof.verify(merkleProof, i_merkleRoot, leaf)) {
revert MerkleAirdrop__InvalidProof();
}
​
// @> No claimed/account/leaf state is checked or updated before transfer
emit Claimed(account, amount);
i_airdropToken.safeTransfer(account, amount);
}

Risk

Likelihood:

  • This occurs whenever an eligible user or any caller has a valid proof for an allocation and the airdrop contract still holds tokens.

  • This occurs with the exact same proof because the Merkle root remains valid and no claim state is mutated.

Impact:

  • One eligible allocation can be claimed multiple times, draining tokens intended for other eligible users.

  • Later legitimate users can be unable to claim because the airdrop token balance has already been exhausted.

Proof of Concept

function testSameProofCanBeReusedToDrainAirdrop() public {
AirdropToken token = new AirdropToken();
MerkleAirdrop airdrop = new MerkleAirdrop(merkleRoot, token);
​
uint256 amountToCollect = 25 * 1e6;
uint256 amountToSend = amountToCollect * 4;
token.mint(address(this), amountToSend);
token.transfer(address(airdrop), amountToSend);
​
vm.deal(collectorOne, airdrop.getFee() * 4);
​
vm.startPrank(collectorOne);
airdrop.claim{value: airdrop.getFee()}(collectorOne, amountToCollect, proof);
airdrop.claim{value: airdrop.getFee()}(collectorOne, amountToCollect, proof);
airdrop.claim{value: airdrop.getFee()}(collectorOne, amountToCollect, proof);
airdrop.claim{value: airdrop.getFee()}(collectorOne, amountToCollect, proof);
vm.stopPrank();
​
assertEq(token.balanceOf(collectorOne), amountToSend);
assertEq(token.balanceOf(address(airdrop)), 0);
}

Recommended Mitigation

Record claims before transferring tokens. Tracking by leaf is flexible because it prevents the exact Merkle allocation from being reused.

+mapping(bytes32 leaf => bool claimed) public hasClaimed;
+
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))));
if (!MerkleProof.verify(merkleProof, i_merkleRoot, leaf)) {
revert MerkleAirdrop__InvalidProof();
}
+ require(!hasClaimed[leaf], "Already claimed");
+ hasClaimed[leaf] = true;
+
emit Claimed(account, amount);
i_airdropToken.safeTransfer(account, amount);
}
​
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 14 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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