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

claim has no double-claim protection: one eligible user can drain the entire 100 USDC pot

hatchworker

Root + Impact

Description

  • Normal behavior: each of the 4 eligible addresses should be able to claim its 25 USDC exactly once, so the 100 USDC pot is distributed evenly and cannot be reclaimed.

  • The issue: claim() verifies the merkle proof and transfers tokens, but never records that account already claimed. There is no s_hasClaimed mapping (or nonce) anywhere in the contract:

    function claim(address account, uint256 amount, bytes32[] calldata merkleProof)
    external
    payable
    {
    // @> no check: has `account` already claimed?
    if (msg.value != CLAIM_FEE) { revert MerkleAirdrop__InvalidClaimFee(); }
    bytes32 leaf = keccak256(bytes.concat(keccak256(abi.encode(account, amount))));
    if (!MerkleProof.verify(merkleProof, i_merkleRoot, leaf)) {
    revert MerkleAirdrop__InvalidProof();
    }
    // @> no write: never records the claim
    i_airdropToken.safeTransfer(account, amount);
    }

Risk

  • Reason 1 // Any of the 4 eligible addresses can call claim repeatedly with the same proof — each call costs only the 1e9 wei (1 gwei) fee.

  • Reason 2 // Nothing in the contract state changes after a successful claim; the proof stays valid forever.

  • Impact 1 // One eligible user can drain the entire 100 USDC pot; the other three get nothing.

  • Impact 2 // The airdrop distribution model is completely broken.

Proof of Concept

PoC verified by a passing Foundry test (test_PoC_H1_SingleUserDrainsWholePot): collectorOne claims 4x with the same proof and ends with the full 100e6; contract balance is 0.

Recommended Mitigation

Add an s_hasClaimed mapping and check-and-set it in claim() before the transfer:

+ error MerkleAirdrop__AlreadyClaimed();
+ mapping(address => bool) private s_hasClaimed;
​
function claim(address account, uint256 amount, bytes32[] calldata merkleProof) external payable {
+ if (s_hasClaimed[account]) { revert MerkleAirdrop__AlreadyClaimed(); }
+ s_hasClaimed[account] = true;
...
i_airdropToken.safeTransfer(account, amount);
}
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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