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

Multiple Claims Allowed Due to Missing Claim Status Check in SnowmanAirdrop

shahparv1310

Description

Normal Behavior

The SnowmanAirdrop contract should enforce one-time claims per address, setting s_hasClaimedSnowman[receiver] = true after successful claims and preventing subsequent claims.

Issue Description

The claimSnowman() function sets the claim status flag but never checks it at the beginning, allowing users to claim multiple times by acquiring additional Snow tokens.

Root + Impact

Description

The contract implements claim status tracking but fails to use it as a guard condition, rendering the one-time claim mechanism ineffective.

Root Cause

// @> claimSnowman() function missing claim status check
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: require(!s_hasClaimedSnowman[receiver], "Already claimed");
// ... verification logic ...
s_hasClaimedSnowman[receiver] = true; // @> Sets flag but never checks it
emit SnowmanClaimedSuccessfully(receiver, amount);
i_snowman.mintSnowman(receiver, amount);
}

The getClaimStatus() function exists to check claim status 2 , but it's not used to prevent re-claims.

Risk

Likelihood:

  • Reason 1: Users can acquire additional Snow tokens through earnSnow() or buySnow() after initial claim

  • Reason 2: No claim status validation allows repeated execution of the claim function

Impact:

  • Impact 1: Users can claim unlimited NFTs by repeatedly acquiring Snow tokens, breaking the airdrop's economic model

  • Impact 2: Inflation of Snowman NFT supply devaluing legitimate claims and compromising the airdrop mechanism

Proof of Concept

function testMultipleClaims() public {
// Setup: User gets initial Snow tokens and claims NFTs
vm.startPrank(user);
snow.earnSnow(); // Get 1 Snow token
// First claim succeeds
vm.warp(block.timestamp + 1 weeks);
airdrop.claimSnowman(user, validProof, v, r, s);
assert(snowman.balanceOf(user) == 1);
assert(airdrop.getClaimStatus(user) == true);
// User gets more Snow tokens
snow.earnSnow(); // Get another Snow token after waiting
vm.warp(block.timestamp + 1 weeks);
// Second claim also succeeds - VULNERABILITY!
airdrop.claimSnowman(user, validProof, v, r, s);
assert(snowman.balanceOf(user) == 2); // User got double NFTs
}

Recommended Mitigation

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();
}
+ require(!s_hasClaimedSnowman[receiver], "Already claimed");
// ... rest of function ...
}
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 3 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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