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

s_hasClaimedSnowman Is Set But Never Enforced — Repeated Claims Possible

harmonygemini8

Description

claimSnowman() records that a receiver has claimed by setting s_hasClaimedSnowman[receiver] = true, but this mapping is never read anywhere. There is no require(!s_hasClaimedSnowman[receiver]) at the start of the function, so the same receiver can claim repeatedly as long as they again hold the snapshot amount of Snow (1 in the provided setup) and provide a valid signature.

Current claim flow:

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

Since the Merkle leaf is built from (receiver, live balance), a receiver whose balance is exactly 1 again (e.g. after receiving a transfer or earning 1 Snow weekly) produces a leaf that verifies against the tree, and can claim another NFT.

Risk

Impact: Low — a user can obtain more Snowman NFTs than intended (one per re-acquired balance). No funds are stolen, but the one-claim-per-user invariant of the airdrop is broken.

Likelihood: Medium — requires the receiver to re-acquire the snapshot amount of Snow and sign a new message; Snow is earnable weekly for free, so this is practical.

Proof of Concept

The test below shows Alice claiming twice: the first claim burns her 1 Snow and mints NFT #0; Bob sends her 1 Snow; she claims again and receives NFT #1:

function testPoc4_DoubleClaim() public {
uint256 amount = snow.balanceOf(alice);
vm.prank(alice);
snow.approve(address(airdrop), amount);
bytes32 d1 = airdrop.getMessageHash(alice);
(uint8 v1, bytes32 r1, bytes32 s1) = vm.sign(alKey, d1);
vm.prank(bob);
airdrop.claimSnowman(alice, AL_PROOF, v1, r1, s1);
assertEq(nft.balanceOf(alice), 1);
​
vm.prank(bob);
snow.transfer(alice, 1);
​
vm.prank(alice);
snow.approve(address(airdrop), 1);
bytes32 d2 = airdrop.getMessageHash(alice);
(uint8 v2, bytes32 r2, bytes32 s2) = vm.sign(alKey, d2);
vm.prank(bob);
airdrop.claimSnowman(alice, AL_PROOF, v2, r2, s2);
​
assertEq(nft.balanceOf(alice), 2);
assertEq(nft.ownerOf(1), alice);
}

Test result: [PASS] testPoc4_DoubleClaim — nft.balanceOf(alice) == 2 after two claims.

Recommended Mitigation

Enforce the claim flag at the beginning of claimSnowman():

error AlreadyClaimed();
​
function claimSnowman(...) external nonReentrant {
if (s_hasClaimedSnowman[receiver]) revert AlreadyClaimed();
...
}
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge 1 day 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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