Snowman Merkle Airdrop
AI First Flight #10
Beginner FriendlyFoundrySolidityNFT
EXP
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Submission Details
Severity: low
Valid

Claims can be replayed because claimed status is never enforced

mythdd

Root + Impact

Description

  • Normal behavior: each whitelisted receiver should be able to claim their Snowman NFT allocation only once. The s_hasClaimedSnowman mapping is intended to track that state.

  • The issue is that claimSnowman() sets s_hasClaimedSnowman[receiver] = true but never checks it before processing a claim. A receiver or relayer can reuse the same Merkle proof and the same EIP-712 signature after the receiver refills their Snow balance to the same whitelisted amount.

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();
}
​
// @> Missing: if (s_hasClaimedSnowman[receiver]) revert;
​
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);
​
// @> Status is written but never read.
s_hasClaimedSnowman[receiver] = true;
​
emit SnowmanClaimedSuccessfully(receiver, amount);
i_snowman.mintSnowman(receiver, amount);
}

Risk

Likelihood:

  • Receivers can earn or buy Snow again during the farming period after their first claim.

  • A signature for the same receiver and same amount remains valid forever because no nonce, deadline, or claimed check is enforced.

Impact:

  • A whitelisted receiver can claim more NFTs than their allocation.

  • A relayer holding a user's signature and allowance can replay the claim when the user's balance returns to the signed amount.

Proof of Concept

The test performs a valid claim for Alice, then refills Alice's Snow balance back to the same amount committed in the Merkle tree. Because the contract never rejects an address that has already claimed, the same proof and signature can be submitted again and Alice receives a second NFT.

function testClaimCanBeReplayedAfterReceiverRefillsSameBalance() public {
(SnowmanAirdrop airdrop, Snow snow, Snowman nft) = _deployAirdrop();
(address alice, uint256 aliceKey) = makeAddrAndKey("alice");
address relayer = makeAddr("relayer");
​
vm.prank(alice);
snow.approve(address(airdrop), type(uint256).max);
bytes32 digest = airdrop.getMessageHash(alice);
(uint8 v, bytes32 r, bytes32 s) = vm.sign(aliceKey, digest);
​
vm.prank(relayer);
airdrop.claimSnowman(alice, AL_PROOF, v, r, s);
assertEq(nft.balanceOf(alice), 1);
​
vm.warp(block.timestamp + 1 weeks);
vm.prank(alice);
snow.earnSnow();
​
vm.prank(relayer);
airdrop.claimSnowman(alice, AL_PROOF, v, r, s);
assertEq(nft.balanceOf(alice), 2);
assertTrue(airdrop.getClaimStatus(alice));
}

Recommended Mitigation

Check and set the claimed flag before external token/NFT interactions.

+ error SA__AlreadyClaimed();
​
function claimSnowman(address receiver, bytes32[] calldata merkleProof, uint8 v, bytes32 r, bytes32 s)
external
nonReentrant
{
if (receiver == address(0)) {
revert SA__ZeroAddress();
}
+ if (s_hasClaimedSnowman[receiver]) {
+ revert SA__AlreadyClaimed();
+ }
if (i_snow.balanceOf(receiver) == 0) {
revert SA__ZeroAmount();
}
​
// verify signature and proof
​
+ s_hasClaimedSnowman[receiver] = true;
i_snow.safeTransferFrom(receiver, address(this), amount);
- s_hasClaimedSnowman[receiver] = true;
i_snowman.mintSnowman(receiver, amount);
}

Also consider adding a nonce and deadline to the signed claim.

Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 2 hours ago
Submission Judgement Published
Validated
Assigned finding tags:

[L-01] Missing Claim Status Check Allows Multiple Claims in SnowmanAirdrop.sol::claimSnowman

# Root + Impact   **Root:** The [`claimSnowman`](https://github.com/CodeHawks-Contests/2025-06-snowman-merkle-airdrop/blob/b63f391444e69240f176a14a577c78cb85e4cf71/src/SnowmanAirdrop.sol#L44) function updates `s_hasClaimedSnowman[receiver] = true` but never checks if the user has already claimed before processing the claim, allowing users to claim multiple times if they acquire more Snow tokens. **Impact:** Users can bypass the intended one-time airdrop limit by claiming, acquiring more Snow tokens, and claiming again, breaking the airdrop distribution model and allowing unlimited NFT minting for eligible users. ## Description * **Normal Behavior:** Airdrop mechanisms should enforce one claim per eligible user to ensure fair distribution and prevent abuse of the reward system. * **Specific Issue:** The function sets the claim status to true after processing but never validates if `s_hasClaimedSnowman[receiver]` is already true at the beginning, allowing users to claim multiple times as long as they have Snow tokens and valid proofs. ## Risk **Likelihood**: Medium * Users need to acquire additional Snow tokens between claims, which requires time and effort * Users must maintain their merkle proof validity across multiple claims * Attack requires understanding of the missing validation check **Impact**: High * **Airdrop Abuse**: Users can claim far more NFTs than intended by the distribution mechanism * **Unfair Distribution**: Some users receive multiple rewards while others may receive none * **Economic Manipulation**: Breaks the intended scarcity and distribution model of the NFT collection ## Proof of Concept Add the following test to TestSnowMan.t.sol  ```Solidity function testMultipleClaimsAllowed() public { // Alice claims her first NFT vm.prank(alice); snow.approve(address(airdrop), 1); bytes32 aliceDigest = airdrop.getMessageHash(alice); (uint8 v, bytes32 r, bytes32 s) = vm.sign(alKey, aliceDigest); vm.prank(alice); airdrop.claimSnowman(alice, AL_PROOF, v, r, s); assert(nft.balanceOf(alice) == 1); assert(airdrop.getClaimStatus(alice) == true); // Alice acquires more Snow tokens (wait for timer and earn again) vm.warp(block.timestamp + 1 weeks); vm.prank(alice); snow.earnSnow(); // Alice can claim AGAIN with new Snow tokens! vm.prank(alice); snow.approve(address(airdrop), 1); bytes32 aliceDigest2 = airdrop.getMessageHash(alice); (uint8 v2, bytes32 r2, bytes32 s2) = vm.sign(alKey, aliceDigest2); vm.prank(alice); airdrop.claimSnowman(alice, AL_PROOF, v2, r2, s2); // Second claim succeeds! assert(nft.balanceOf(alice) == 2); // Alice now has 2 NFTs } ``` ## Recommended Mitigation **Add a claim status check at the beginning of the function** to prevent users from claiming multiple times. ```diff // Add new error + error 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(); } // Rest of function logic... s_hasClaimedSnowman[receiver] = true; } ```

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