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

[H-1] Missing Claim Check Allows Multiple Claims

lmsand

Root + Impact

  • No Enforcement of hasClaimed Mapping

  • Impact: Allows unlimited claims, breaking core protocol logic and inflating NFT supply.

Description

  • The contract tracks and updates claim status using:

mapping(address => bool) private s_hasClaimedSnowman;
​
s_hasClaimedSnowman[receiver] = true;

  • However, the contract never checks this mapping before processing a claim in claimSnowman().

    As a result, there is no restriction preventing the same receiver from calling claimSnowman() multiple times, as long as the other conditions (valid signature, sufficient Snow balance, valid Merkle proof) are satisfied.

    This creates a logical flaw where the contract records claim status but does not enforce it, effectively rendering the mapping useless.

Risk

Likelihood: High

  • Reason: There is no check enforcing s_hasClaimedSnowman, so any user can repeatedly call the function with valid inputs.

Impact: High

  • A user can repeatedly call claimSnowman()

Each call:

  • Transfers Snow tokens from the user

  • Mints additional Snowman NFTs

  • Breaks the intended one-time claim design

  • Can lead to:

    • Unlimited NFT minting

    • Inflation of supply

    • Abuse of reward distribution

Proof of Concept:

Explanation:

The following test demonstrates that a single user can successfully call claimSnowman() multiple times due to the absence of a claim status check.

Step-by-step:

  1. Approval
    The user (alice) approves the SnowmanAirdrop contract to transfer her Snow tokens.

  2. First Claim

    • A valid EIP-712 signature is generated using Alice’s private key.

    • A valid Merkle proof (AL_PROOF) is provided.

    • The contract processes the claim:

      • Transfers Snow tokens from Alice

      • Mints a Snowman NFT

      • Sets s_hasClaimedSnowman[alice] = true

  3. State Reset for Test

    • Alice is re-funded with Snow tokens using deal() so she can pass the balanceOf check again.

    • A new valid signature is generated (since the signature depends on current balance).

  4. Second Claim (Exploit)

    • The same function is called again with valid inputs.

    • Because the contract does not check s_hasClaimedSnowman, the call succeeds.

    • Another NFT is minted for Alice.

  5. Result

    • Alice ends up with 2 NFTs instead of 1, proving that multiple claims are possible.

function test_DoubleClaim() public {
vm.prank(alice);
snow.approve(address(airdrop), type(uint256).max);
​
bytes32 digest = airdrop.getMessageHash(alice);
(uint8 v, bytes32 r, bytes32 s) = vm.sign(alKey, digest);
​
vm.prank(satoshi);
airdrop.claimSnowman(alice, AL_PROOF, v, r, s);
​
// Re-fund Alice so she can pass checks again
deal(address(snow), alice, 10 ether);
​
bytes32 newDigest = airdrop.getMessageHash(alice);
(uint8 v2, bytes32 r2, bytes32 s2) = vm.sign(alKey, newDigest);
​
vm.prank(satoshi);
airdrop.claimSnowman(alice, AL_PROOF, v2, r2, s2);
​
assert(nft.balanceOf(alice) == 2);
}
​

Recommended Mitigation

Add a claim validation check at the beginning of claimSnowman():

​
+ if (s_hasClaimedSnowman[receiver]) {
+ revert SA__AlreadyClaimed();
+}
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge 6 days 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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