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

claimSnowman sets s_hasClaimedSnowman but never checks it, allowing users to claim multiple times

brizybryan

Root + Impact

Description

  • The claimSnowman function is intended to allow each eligible address to claim their Snowman NFTs exactly once. The contract maintains a s_hasClaimedSnowman mapping and sets it to true after a claim, but never reads this mapping to gate entry into the function.

    A user who accumulates Snow tokens again after their first claim (via earnSnow() or buySnow()) can execute claimSnowman repeatedly, minting NFTs beyond their airdrop allocation each time their balance matches a valid Merkle leaf.

// Root cause in SnowmanAirdrop.sol, claimSnowman function (lines 69-99)
function claimSnowman(address receiver, bytes32[] calldata merkleProof, uint8 v, bytes32 r, bytes32 s)
external
nonReentrant
{
// @> There is no check: if (s_hasClaimedSnowman[receiver]) revert;
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 above
​
i_snowman.mintSnowman(receiver, amount);
}
​

Risk

Likelihood:

  • This occurs whenever a user who has already claimed acquires the same Snow balance again and calls claimSnowman a second time with the same valid Merkle proof.

  • Snow tokens can be freely re-earned weekly via earnSnow() or purchased via buySnow(), making re-accumulation trivial.

Impact:

  • Users can drain the protocol by repeatedly staking and claiming, minting unlimited NFTs.

  • The airdrop distribution is not enforced as one-per-address as intended.

Proof of Concept

This test shows that Alice can claim once, re-earn Snow tokens after a week, and claim a second time without any revert. The s_hasClaimedSnowman flag set during the first claim does not prevent the second claim because the function never reads it. After both claims, Alice holds 2 NFTs instead of the intended 1.

function testH02_DoubleClaimAllowed() public {
// Alice earns 1 Snow token and claims successfully
vm.prank(alice);
snow.earnSnow();
vm.prank(alice);
snow.approve(address(airdrop), type(uint256).max);
​
bytes32 alDigest = airdrop.getMessageHash(alice);
(uint8 v, bytes32 r, bytes32 s) = vm.sign(alKey, alDigest);
airdrop.claimSnowman(alice, AL_PROOF, v, r, s);
assert(nft.balanceOf(alice) == 1);
​
// Time passes, Alice earns Snow again
vm.warp(block.timestamp + 1 weeks);
vm.prank(alice);
snow.earnSnow();
​
// Alice claims AGAIN — no revert!
bytes32 alDigest2 = airdrop.getMessageHash(alice);
(uint8 v2, bytes32 r2, bytes32 s2) = vm.sign(alKey, alDigest2);
airdrop.claimSnowman(alice, AL_PROOF, v2, r2, s2);
​
// Alice now has 2 NFTs instead of the intended 1
assert(nft.balanceOf(alice) == 2);
}
​

Recommended Mitigation

Add a check at the top of claimSnowman that reads s_hasClaimedSnowman[receiver] and reverts if the address has already claimed. This ensures each address can only claim once, which is the purpose of the mapping that is already being written to.

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 4 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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