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

Claimed users can repeatedly claim Snowman NFTs because claim status is never checked

phat1911

Description

Root + Impact

Description

SnowmanAirdrop.claimSnowman() is intended to let each eligible receiver claim their Snowman NFTs once. The contract even stores claim status in s_hasClaimedSnowman.

However, the function only sets the claim status after a successful claim:

s_hasClaimedSnowman[receiver] = true;

It never checks this mapping before processing a new claim.

As a result, a receiver who has already claimed can claim again after regaining the same Snow balance used in the Merkle leaf. The Merkle leaf is computed from the receiver and their current Snow balance:

uint256 amount = i_snow.balanceOf(receiver);
​
bytes32 leaf = keccak256(bytes.concat(keccak256(abi.encode(receiver, amount))));

This breaks the intended one-time claim behavior and allows eligible users to mint more NFTs than their allocation.

Risk

Likelihood:

  • The contract already exposes getClaimStatus(), showing that one-time claim tracking is intended.

  • s_hasClaimedSnowman[receiver] is set after claiming but is never checked.

  • Eligible users can regain Snow by using earnSnow() or buySnow() during the farming period.

Impact:

  • Eligible users can claim more NFTs than allocated.

  • The airdrop distribution can be inflated beyond the Merkle allocation.

  • Snowman NFT ownership no longer accurately represents a single valid Snow-backed claim.

Proof of Concept

Add this test to the test suite:

function testClaimCanBeRepeatedAfterRegainingSameSnowBalance() public {
Helper deployer = new Helper();
(SnowmanAirdrop airdrop, Snow snow, Snowman nft,) = deployer.run();
(address alice, uint256 alKey) = makeAddrAndKey("alice");
address gasPayer = makeAddr("gasPayer");
​
vm.startPrank(alice);
snow.approve(address(airdrop), 1);
vm.stopPrank();
​
bytes32 digest = airdrop.getMessageHash(alice);
(uint8 v, bytes32 r, bytes32 s) = vm.sign(alKey, digest);
​
vm.prank(gasPayer);
airdrop.claimSnowman(alice, AL_PROOF, v, r, s);
​
assertEq(nft.balanceOf(alice), 1);
assertTrue(airdrop.getClaimStatus(alice));
​
vm.warp(block.timestamp + 1 weeks);
vm.prank(alice);
snow.earnSnow();
​
vm.prank(alice);
snow.approve(address(airdrop), 1);
​
bytes32 secondDigest = airdrop.getMessageHash(alice);
(uint8 v2, bytes32 r2, bytes32 s2) = vm.sign(alKey, secondDigest);
​
vm.prank(gasPayer);
airdrop.claimSnowman(alice, AL_PROOF, v2, r2, s2);
​
assertEq(nft.balanceOf(alice), 2);
}

The test passes, showing that Alice can claim once, regain 1 Snow, and claim again with the same Merkle proof.

Recommended Mitigation

Check whether the receiver has already claimed before verifying and processing the claim.

+ 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();
}
​
if (!_isValidSignature(receiver, getMessageHash(receiver), v, r, s)) {
revert SA__InvalidSignature();
}
​
uint256 amount = i_snow.balanceOf(receiver);
​
bytes32 leaf = keccak256(bytes.concat(keccak256(abi.encode(receiver, amount))));
​
if (!MerkleProof.verify(merkleProof, i_merkleRoot, leaf)) {
revert SA__InvalidProof();
}
​
i_snow.safeTransferFrom(receiver, address(this), amount);
​
s_hasClaimedSnowman[receiver] = true;
​
emit SnowmanClaimedSuccessfully(receiver, amount);
​
i_snowman.mintSnowman(receiver, amount);
}
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 10 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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