Snowman Merkle Airdrop
First Flight #42
Beginner FriendlyFoundrySolidityNFT
100 EXP
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Submission Details
Severity: low
Valid

No Check for Double Claim

smexor

Root + Impact

Description

Normal Behavior:

When a user is eligible for a Snowman NFT via an airdrop, they should be able to claim it exactly once by providing a valid Merkle proof and EIP-712 signature. The contract uses s_hasClaimedSnowman to track if a user has already claimed.


Issue:

The claimSnowman() function does not check whether a user has already claimed their NFT. This allows a user to call the function and receive unlimited NFTs repeatedly, provided they still hold the token amount and can replay the same proof and signature.

function claimSnowman(address receiver, bytes32[] calldata merkleProof, uint8 v, bytes32 r, bytes32 s)
external
nonReentrant
{
if (receiver == address(0)) {
revert SA__ZeroAddress();
}
if (i_snow.balanceOf(receiver) == 0) {
revert SA__ZeroAmount();
}
​
if (!_isValidSignature(receiver, getMessageHash(receiver), v, r, s)) {
revert SA__InvalidSignature();
}
​
uint256 amount = i_snow.balanceOf(receiver);
​
bytes32 leaf = keccak256(bytes.concat(keccak256(abi.encode(receiver, amount))));
​
if (!MerkleProof.verify(merkleProof, i_merkleRoot, leaf)) {
revert SA__InvalidProof();
}
​
i_snow.safeTransferFrom(receiver, address(this), amount);
​
// @> s_hasClaimedSnowman[receiver] = true;
​
emit SnowmanClaimedSuccessfully(receiver, amount);
​
i_snowman.mintSnowman(receiver, amount);
}
​

Risk

Likelihood:

  • Any user with a valid Merkle proof and EIP-712 signature can execute the claimSnowman() function multiple times.

  • No verification exists to block repeat claims; the s_hasClaimedSnowman[receiver] flag is written to after the transfer and mint, but never checked.

Impact:

  • Unlimited NFT mints to the same user, potentially devaluing the collection.

  • Infinite draining of Snow tokens from the user to the contract with no rate limiting, increasing risk if tokens can be recovered later or used maliciously.

Proof of Concept

// Assume this is called once successfully
airdrop.claimSnowman(receiver, validProof, v, r, s);
​
// Can be repeated with the same parameters
airdrop.claimSnowman(receiver, validProof, v, r, s); // Second NFT received
airdrop.claimSnowman(receiver, validProof, v, r, s); // Third NFT received
// ... until balance runs out
​

Recommended Mitigation

Add a check at the start of the claimSnowman() function and define SA__AlreadyClaimed():
​
function claimSnowman(address receiver, bytes32[] calldata merkleProof, uint8 v, bytes32 r, bytes32 s)
external
nonReentrant
{
+ if (s_hasClaimedSnowman[receiver]) {
+ revert SA__AlreadyClaimed();
+ }
​
if (receiver == address(0)) {
revert SA__ZeroAddress();
}
Updates

Lead Judging Commences

yeahchibyke Lead Judge 4 days ago
Submission Judgement Published
Validated
Assigned finding tags:

Lack of claim check

The claim function of the Snowman Airdrop contract doesn't check that a recipient has already claimed a Snowman. This poses no significant risk as is as farming period must have been long concluded before snapshot, creation of merkle script, and finally claiming.

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