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

SnowmanAirdrop::claimSnowman never checks s_hasClaimedSnowman, allowing repeated claims with the same Merkle proof

second_opinion

Root + Impact

Description

  • claimSnowman sets s_hasClaimedSnowman[receiver] = true after a successful claim, which shows the intent of one claim per allowlisted address.

  • The mapping is never read, so the only checks are the Merkle proof and the signature, both computed from the receiver live Snow balance. If the receiver holds the same balance again, the same proof and an identical signature digest are valid again and a second claim succeeds.

// src/SnowmanAirdrop.sol
function claimSnowman(address receiver, bytes32[] calldata merkleProof, uint8 v, bytes32 r, bytes32 s) external nonReentrant {
// @> no check of s_hasClaimedSnowman[receiver]
...
s_hasClaimedSnowman[receiver] = true; // @> written but never read
...
i_snowman.mintSnowman(receiver, amount);
}// Root cause in the codebase with @> marks to highlight the relevant section

Risk

Likelihood:

  • Requires the receiver to hold the same Snow balance again, which is easy for 1-unit allocations because Snow can be earned for free once a week through earnSnow.

  • Repeatable every week while farming is open, no special privileges are needed.

Impact:

  • Per-address allocation limits are bypassed and extra NFTs are minted beyond the airdrop list.

  • The recorded claim status is misleading, it is true while further claims still succeed.

Proof of Concept

Alice claims once with her proof, then earns 1 Snow again after a week and claims a second time with the same proof. The second claim succeeds and she owns 2 NFTs while getClaimStatus(alice) is already true. The test passes on the unmodified code.

function test_H2_claimCanBeRepeated() public {
_claimAsAlice(); // approve 1 Snow, sign getMessageHash(alice), call claimSnowman with Alice proof
assertEq(nft.balanceOf(alice), 1);
assertTrue(airdrop.getClaimStatus(alice)); // already "claimed"
​
vm.warp(block.timestamp + 1 weeks);
vm.prank(alice);
snow.earnSnow(); // free 1 Snow again
assertEq(snow.balanceOf(alice), 1);
​
_claimAsAlice(); // same proof succeeds again
assertEq(nft.balanceOf(alice), 2);
}
// Result: PASS

Recommended Mitigation

Check the s_hasClaimedSnowman mapping before processing a claim and revert if the receiver has already claimed.

+ 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();
}
...- remove this code
+ add this code
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 1 hour 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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