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

claimSnowman() Missing s_hasClaimedSnowman Check Allows Repeated Claims

tsaqifalk

Root + Impact

Description

The claimSnowman() function in SnowmanAirdrop.sol handles the distribution of Snowman NFTs to eligible users. Users submit a Merkle proof and ECDSA signature to claim their NFTs. The contract uses a s_hasClaimedSnowman mapping to track whether an address has already claimed, the mapping is set to true after a successful claim (line 94), and a public getter getClaimStatus() exists to query the status (line 138).

The function never checks s_hasClaimedSnowman[receiver] before processing a claim. The mapping is written but never read in the claim flow, allowing a user who has already claimed to earn additional Snow tokens (via earnSnow() or buySnow()) and claim again, receiving NFTs each time.

@> SnowmanAirdrop.sol:69-99

function claimSnowman(address receiver, bytes32[] calldata merkleProof, uint8 v, bytes32 r, bytes32 s)
external
nonReentrant
{
// ❌ Missing: require(!s_hasClaimedSnowman[receiver], "Already claimed");
​
if (receiver == address(0)) {
revert SA__ZeroAddress();
}
if (i_snow.balanceOf(receiver) == 0) {
revert SA__ZeroAmount();
}
// ... signature and merkle verification ...
​
i_snow.safeTransferFrom(receiver, address(this), amount);
​
s_hasClaimedSnowman[receiver] = true; // SET but never checked before
​
emit SnowmanClaimedSuccessfully(receiver, amount);
​
i_snowman.mintSnowman(receiver, amount);
}

Risk

Likelihood:

  • The mapping s_hasClaimedSnowman is declared, set to true after claiming, and exposed via a public getter, this demonstrates clear developer intent to enforce single-claim, making the missing check an implementation oversight rather than a design choice.

  • A user only needs 1 Snow token (free via earnSnow() after each weekly cooldown) and a valid signature to trigger a re-claim, no additional cost, special access, or complex setup required.

Impact:

  • Each re-claim mints additional Snowman NFTs to the user, inflating NFT supply beyond the intended allocation and destroying the scarcity mechanism that the staking/airdrop system was designed to enforce.

  • The user's Snow tokens transfer to the airdrop contract on each claim, allowing repeated draining of earned tokens into the contract rather than a single intended distribution per user.

Proof of Concept

// test/PoC_V2.t.sol
​
function test_PoC2_ClaimSnowman_MissingClaimedCheck() public {
// Get alice's proof from the merkle tree
bytes32[] memory alProof = m.getProof(leaves, 0);
​
// === FIRST CLAIM (legitimate) ===
vm.prank(alice);
snow.approve(address(airdrop), type(uint256).max);
​
bytes32 alDigest = airdrop.getMessageHash(alice);
(uint8 alV, bytes32 alR, bytes32 alS) = vm.sign(alKey, alDigest);
​
vm.prank(satoshi);
airdrop.claimSnowman(alice, alProof, alV, alR, alS);
​
assertEq(nft.balanceOf(alice), 1, "Alice got 1 NFT");
assertTrue(airdrop.getClaimStatus(alice), "ClaimStatus set to true");
// ClaimStatus is true but NEVER checked on re-entry
​
// === GET MORE SNOW ===
vm.prank(alice);
snow.earnSnow();
assertEq(snow.balanceOf(alice), 1, "Alice earned 1 Snow again");
​
// === SECOND CLAIM, should fail but DOESN'T ===
vm.prank(alice);
snow.approve(address(airdrop), type(uint256).max);
​
bytes32 alDigest2 = airdrop.getMessageHash(alice);
(uint8 alV2, bytes32 alR2, bytes32 alS2) = vm.sign(alKey, alDigest2);
​
vm.prank(satoshi);
airdrop.claimSnowman(alice, alProof, alV2, alR2, alS2);
​
// Re-claim succeeded, Alice now has 2 NFTs
assertEq(nft.balanceOf(alice), 2, "Alice got 2 NFTs, re-claim worked!");
}
​
// Test result: PASS, Alice claimed twice, got 2 NFTs

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();
}
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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