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

[H-2] `SnowmanAirdrop::claimSnowman` does not check `s_hasClaimedSnowman`, allowing users to claim multiple NFTs

jessica

Unchecked claim status leads to repeated NFT claims

Description

The SnowmanAirdrop contract is designed to allow each eligible user to claim their Snowman NFT exactly once. It tracks claim status via the s_hasClaimedSnowman mapping, which is set to true after a successful claim.

However, claimSnowman never reads s_hasClaimedSnowman before processing a claim. A user who earns or buys more Snow tokens after their first claim can call claimSnowman again and receive additional NFTs.

function claimSnowman(address receiver, bytes32[] calldata merkleProof, uint8 v, bytes32 r, bytes32 s)
external
nonReentrant
{
// @> s_hasClaimedSnowman is never checked here
if (receiver == address(0)) {
revert SA__ZeroAddress();
}
// ...
i_snow.safeTransferFrom(receiver, address(this), amount);
s_hasClaimedSnowman[receiver] = true; // @> set but never read
i_snowman.mintSnowman(receiver, amount);
}

Risk

Likelihood:

  • Any user who has already claimed can earn or buy 1 more Snow token and call claimSnowman again

  • The merkle proof and signature remain valid as long as the user's balance matches the expected amount

Impact:

  • Users can claim Snowman NFTs repeatedly, breaking the one-claim-per-user airdrop design

  • Unlimited NFT minting is possible through the airdrop contract

Proof of Concept

function testSameUserCanClaimTwice() public {
// Get alice's digest
bytes32 alDigest = airdrop.getMessageHash(alice);
​
// alice signs a message
(uint8 alV, bytes32 alR, bytes32 alS) = vm.sign(alKey, alDigest);
​
vm.warp(block.timestamp + 1 weeks);
vm.startPrank(alice);
snow.approve(address(airdrop), 1);
airdrop.claimSnowman(alice, AL_PROOF, alV, alR, alS);
vm.stopPrank();
​
vm.warp(block.timestamp + 1 weeks);
​
vm.startPrank(alice);
snow.earnSnow();
snow.approve(address(airdrop), 1);
airdrop.claimSnowman(alice, AL_PROOF, alV, alR, alS);
vm.stopPrank();
}

Recommended Mitigation

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
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge 12 days 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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