AirDropper
AI First Flight #5
Beginner FriendlyDeFiFoundry
EXP
View results
Previous Next
Submission Details
Severity: high
Valid

Missing claim tracking allows repeated claims draining entire airdrop balance

ra9huvansh

Root + Impact

Description

  • The claim() function contains no mechanism to track whether an address has already claimed their airdrop.

  • Any eligible address can call claim() unlimited times, receiving tokens on every call.

// No mapping like s_hasClaimed[account] exists anywhere in the contract
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(); }
emit Claimed(account, amount);
i_airdropToken.safeTransfer(account, amount); // transfers every single time
}

Risk

Likelihood:

  • Any eligible address can immediately exploit this with zero technical skill, just call claim() again

  • No on-chain protection exists to prevent it

Impact:

  • A single eligible address can drain the full 100 USDC contract balance

All other eligible users receive nothing

Protocol's core purpose (fair distribution to 4 addresses) is completely broken


Proof of Concept

function testDoubleClaimDrainsContract() public {
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();
// collectorOne drained all 100 USDC, other 3 users get nothing
assertEq(token.balanceOf(collectorOne), amountToCollect * 4);
}

Recommended Mitigation

+ mapping(address => bool) private s_hasClaimed;
+ error MerkleAirdrop__AlreadyClaimed();
​
function claim(address account, uint256 amount, bytes32[] calldata merkleProof) external payable {
if (msg.value != FEE) { revert MerkleAirdrop__InvalidFeeAmount(); }
+ if (s_hasClaimed[account]) revert MerkleAirdrop__AlreadyClaimed();
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 about 2 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) ```

Support

FAQs

Can't find an answer? Chat with us on Discord, Twitter or Linkedin.

Give us feedback!