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

No Check for Already Claimed Status Allows Multiple Claims

vikramgoutham289

Root + Impact

The s_hasClaimedSnowman mapping is set but never checked before processing claims, allowing users to potentially claim multiple times if they acquire tokens matching new Merkle proofs.

Description

The claimSnowman function sets s_hasClaimedSnowman[receiver] = true after a successful claim to prevent users from claiming multiple times. The function should check this mapping at the beginning to reject duplicate claims.

The function never checks the s_hasClaimedSnowman mapping before processing a claim, meaning users who acquire additional Snow tokens after their first claim could potentially claim again.

// SnowmanAirdrop.sol
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: check if s_hasClaimedSnowman[receiver] is true
​
// ... verification logic ...
​
s_hasClaimedSnowman[receiver] = true; // Set but never checked above
// ... minting logic ...
}

Risk

Likelihood:HIGH

  • Reason 1 :Any user who has already claimed and acquires more tokens matching a valid Merkle proof can attempt to claim again

  • Reason 2: Combined with other vulnerabilities, this could be exploited systematically

Impact:HIGH

  • Impact 1:Users can receive more NFTs than their allocation

  • Impact 2:
    Economic value dilution for legitimate single-claim users

Proof of Concept

This test shows that after Alice successfully claims her airdrop, the s_hasClaimedSnowman flag is set to true but never checked on subsequent calls. If Alice acquires more tokens and has a valid proof, nothing prevents her from claiming again since the check is missing.

function testDoubleClaim() public {
address alice = makeAddr("alice");
// First claim succeeds
vm.prank(alice);
airdrop.claimSnowman(alice, merkleProof, v, r, s);
assertTrue(airdrop.getClaimStatus(alice));
// Alice acquires more tokens
// Second claim is not blocked by s_hasClaimedSnowman check
vm.prank(alice);
airdrop.claimSnowman(alice, merkleProof2, v2, r2, s2);
}

Recommended Mitigation

Add a check at the beginning of claimSnowman to verify the receiver hasn't already claimed. This ensures each address can only claim once, maintaining fair distribution of the airdrop.

+ 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();
}
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge 7 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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