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

s_hasClaimedSnowman is written but never checked — unlimited replay of the same claim.

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Root + Impact

Description

  • A given (receiver, amount) allocation should be claimable exactly once — that's what s_hasClaimedSnowman exists for.

  • The mapping is set to true at the end of claimSnowman(), but it is never read or checked anywhere in the function. Nothing actually blocks a second, third, or Nth claim.


Because amount is re-derived from i_snow.balanceOf(receiver) on every call (not a stored snapshot), the only thing stopping an immediate second claim is that the first claim drained the receiver's Snow balance to 0. That's easily undone.

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();
// @> no `if (s_hasClaimedSnowman[receiver]) revert ...` guard anywhere
...
i_snow.safeTransferFrom(receiver, address(this), amount);
​
s_hasClaimedSnowman[receiver] = true; // @> written, but never read/enforced
​
​
emit SnowmanClaimedSuccessfully(receiver, amount);
i_snowman.mintSnowman(receiver, amount);
}

Risk

Likelihood:

  • Occurs any time a receiver (or a relayer holding their valid signature) re-acquires exactly the same Snow balance used in their original claim — e.g. buying it back via buySnow(), or accumulating it again via earnSnow() over time — and resubmits the same, still-valid signature and Merkle proof.

  • The signature and Merkle leaf are static (no nonce, no post-claim state binding), so the exact same calldata that worked once works again indefinitely.

Impact:

  • Unlimited extra Snowman NFTs minted per address by repeatedly re-buying/re-claiming.

  • Fully breaks the "one allocation, one claim" invariant the mapping was designed to enforce.

Proof of Concept

+ // Alice's committed leaf/allocation: (alice, 5 Snow)
+ vm.prank(relayer);
+ airdrop.claimSnowman(alice, proof, v, r, s); // succeeds: pulls 5 Snow, mints 5 Snowman
+ // s_hasClaimedSnowman[alice] = true — but nothing enforces it
​
+ vm.prank(alice);
+ snow.buySnow(5); // Alice simply re-buys the exact same amount she started with
​
+ vm.prank(relayer);
+ airdrop.claimSnowman(alice, proof, v, r, s); // SAME signature, SAME proof — succeeds AGAIN
+ // Alice now has 10 Snowman NFTs from a single allocation meant to grant 5

Recommended Mitigation

error SA__InvalidProof();
error SA__InvalidSignature();
error SA__ZeroAddress();
error SA__ZeroAmount();
+ 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 about 3 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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