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

Missing Claim-Status Validation Allows Repeated Snowman NFT Airdrop Claims

daneknakamuradev

Root + Impact

The root cause is that claimSnowman() updates s_hasClaimedSnowman[receiver] after a successful claim but does not check this mapping before processing subsequent claims.

Description

The contract records whether a receiver has claimed, but fails to enforce this state during claim validation. If a user reacquires the required Snow token balance and satisfies the Merkle proof and signature checks, the same receiver may be able to claim again.

mapping(address => bool) private s_hasClaimedSnowman;
​
// Inside claimSnowman(), after a successful claim:
s_hasClaimedSnowman[receiver] = true;

Risk

Likelihood: High, assuming the user can reacquire the required token balance and obtain any required valid signature.

Impact: High, because repeated claims can undermine the intended one-time NFT distribution and increase NFT issuance beyond the intended allocation.

Proof of Concept

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;
​
// Demonstrates the missing claim-status validation.
contract ClaimReplayPoC {
mapping(address => bool) public hasClaimed;
​
// Vulnerable behavior: the function records the claim
// but never checks whether the receiver claimed before.
function claimSnowman(address receiver) external {
// Missing check:
// require(!hasClaimed[receiver], "Already claimed");
​
// Represents successful claim processing.
hasClaimed[receiver] = true;
}
}
Deploy ClaimReplayPoC.
​
Call claimSnowman(attackerAddress) from any account.
​
Call claimSnowman(attackerAddress) again.
​
Both transactions succeed because the function never checks hasClaimed[receiver].
​
Expected result: The second call should revert because the receiver has already claimed.
​
Note: This is a minimal demonstration of the missing check, not a complete exploit against the actual protocol. Confirm the real SnowmanAirdrop.sol claim logic, Merkle proof, and signature requirements before reporting this as an exploitable vulnerability.
​

Recommended Mitigation

//Define the custom error if it does not already exist:
error AlreadyClaimed();
function claimSnowman(
address receiver,
bytes32[] calldata proof,
bytes calldata signature
) external {
+ if (s_hasClaimedSnowman[receiver]) {
+ revert AlreadyClaimed();
+ }
​
// Existing Merkle proof and signature validation
// Existing token transfer and NFT minting logic
}
​
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 2 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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