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

Replay attack

The Seraphs

Root + Impact

Description

  • Each user should be able to claim the Snowman NFT only once.\

  • However, the contract does not check if a user has already claimed, allowing users to repeatedly claim multiple NFTs as long as they have the tokens and valid addresses

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();
}
​
// @> Missing check here:
// if (s_hasClaimedSnowman[receiver]) {
// revert SA__AlreadyClaimed();
// }
​
i_snow.safeTransferFrom(receiver, address(this), amount);
​
s_hasClaimedSnowman[receiver] = true;
​
emit SnowmanClaimedSuccessfully(receiver, amount);
​
i_snowman.mintSnowman(receiver, amount);
}’’’
​

Risk

Likelihood:

  • Users with a valid Merkle proof and signature can repeatedly claim the snowmen as long as they hold tokens.\

  • The contract allows multiple claims without stopping receivers that have already claimed.

Impact:

  • Allows receivers to claim multiple Snowman NFTs.

  • Could claim all the snowmen so there isn’t any for anyone else

  • Could lead to loss of user trust and unfair distribution.

Proof of Concept


Step 1: Meet Requirements

  • Receiver fulfills all necessary conditions to claim a snowman.

  • Step 2: Claim Execution

    • The following code runs:

      i_snow.safeTransferFrom(receiver, address(this), amount); s_hasClaimedSnowman[receiver] = true; emit SnowmanClaimedSuccessfully(receiver, amount); i_snowman.mintSnowman(receiver, amount);

    • The snowman is successfully claimed.

  • Step 3: Replay Attack

    • Since there's no check preventing multiple claims:

      • The receiver can call the same function again.

      • They keep receiving snowmen with no limit.

  • Impact

    • One user can drain the entire snowman supply.

    • Others are blocked from claiming.

// A user calls claimSnowman() multiple times, each time sending their tokens to the contract
// and receiving new NFTs repeatedly.
​
for (uint i = 0; i < 10; i++) {
snowmanAirdrop.claimSnowman(userAddress, merkleProof, v, r, s);
}
​

Recommended Mitigation

📌 Key Mitigation Points
✅ Prevents multiple claims by the same user.

🔐 Stops replay attacks by tracking and checking claim history.

⛔ Blocks abuse where users try to drain snowman supply.

💡 Simple, effective fix by leveraging the existing s_hasClaimedSnowman mapping.

🧱 Can be extended with further enhancements (e.g., cooldown timers, claim limits).

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();
}
​
// ✅ Mitigation: prevent multiple claims
if (s_hasClaimedSnowman[receiver]) {
revert SA__AlreadyClaimed();
}
​
i_snow.safeTransferFrom(receiver, address(this), amount);
​
s_hasClaimedSnowman[receiver] = true;
​
emit SnowmanClaimedSuccessfully(receiver, amount);
​
i_snowman.mintSnowman(receiver, amount);
}
​
​
Updates

Lead Judging Commences

yeahchibyke Lead Judge 5 months 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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