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

`MerkleAirdrop::claim` has no replay protection; a single valid proof can be reused to drain the entire airdrop

hopeh

Description


claimverifies a Merkle proof for (account, amount)and transfers the tokens, but it records no claim state — there is no hasClaimedmapping or nullifier. The same valid proof can therefore be replayed any number of times:


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); // @> no per-account claimed flag set or checked
}
```
​
`claim` is permissionless and takes `account`/`amount`/`proof` as arguments, so anyone can call it repeatedly with a single eligible (account, amount, proof) tuple.
​
## Risk
​
**Likelihood: High**
​
- A single valid proof (any of the 4 airdrop recipients, which are public in the Merkle tree) is enough; no special access is required, and the only cost per call is the tiny `FEE` (1e9 wei = 1 gwei).
​
**Impact: High**
​
- An attacker repeatedly calls `claim` for one eligible account, transferring `amount` each time until the contract's entire token balance is drained to that account. The whole airdrop (all recipients' funds) is stolen/misdirected for the cost of a few gwei of fees. Total loss of the airdropped funds.
​
## Proof of Concept
​
```solidity
// With a valid (account, amount, proof) for recipient A:
// for (uint i; i `amount` sent to A on each call until the contract's balance is exhausted.
```
​
## Recommended Mitigation
​
Record and check a per-account (or per-leaf) claimed flag before transferring:
​
```solidity
mapping(address => bool) private s_hasClaimed;
function claim(...) external payable {
...
if (s_hasClaimed[account]) revert MerkleAirdrop__AlreadyClaimed();
s_hasClaimed[account] = true; // effect before interaction
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) ```

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