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

Missing Double-Claim Prevention Allows Infinite Claims

muzammilmohammed678

Root + Impact

Description

  • The contract has no mechanism to track which addresses have already claimed their airdrop. Users can call the `claim` function multiple times with the same merkle proof, draining the contract of all tokens.

    ### Root + Impact

    The `claim` function verifies the merkle proof but doesn't check if the account has already claimed. There's no mapping or state variable to track claimed addresses.

    ```solidity

    // src/MerkleAirdrop.sol:30-40

    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);

    }

    ```

    Once a user has a valid merkle proof, they can call this function repeatedly to claim the same amount multiple times until the contract is drained.


Risk

Likelihood:

  • * Users receive their merkle proofs which remain valid indefinitely

    * There's no state tracking to prevent repeated claims

    * Users can call the function multiple times with the same proof

    * This can be exploited immediately after deployment

Impact:

  • * A single user can drain the entire contract by claiming repeatedly

    * Legitimate users may find the contract empty when they try to claim

    * The airdrop distribution becomes unfair and potentially completely drained

    * Financial loss equal to the total airdrop amount

Proof of Concept

1. User receives valid merkle proof for 25 USDC
2. User calls `claim(account, 25000000, proof)` with `msg.value = 1e9`
3. Transaction succeeds, user receives 25 USDC
4. User calls `claim(account, 25000000, proof)` again with `msg.value = 1e9`
5. Transaction succeeds again, user receives another 25 USDC
6. User repeats until contract is drained

Recommended Mitigation

```diff
// src/MerkleAirdrop.sol:17
+ mapping(address => bool) public hasClaimed;
​
// src/MerkleAirdrop.sol:30-40
function claim(address account, uint256 amount, bytes32[] calldata merkleProof) external payable {
if (msg.value != FEE) {
revert MerkleAirdrop__InvalidFeeAmount();
}
+ if (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();
}
+ hasClaimed[account] = true;
emit Claimed(account, amount);
i_airdropToken.safeTransfer(account, amount);
}
```
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge 16 days 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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