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

Missing Claimed-State Enforcement Allows Multiple Claims

sexretxt

Root + Impact

Description

Normal Behavior

Each eligible address should be allowed to claim Snowman NFTs only once.
After a successful claim, the contract should permanently prevent any further claims from the same address.

Issue

Although the contract records claim status using the s_hasClaimedSnowman mapping, this state is never checked before processing a claim.

mapping(address => bool) private s_hasClaimedSnowman;
​
function claimSnowman(...) external nonReentrant {
...
@> s_hasClaimedSnowman[receiver] = true;
...
}

Because no validation exists at the start of claimSnowman, an address can repeatedly invoke the function and mint Snowman NFTs multiple times.

This breaks the core airdrop invariant that each address may claim only once.

Risk

Likelihood:

  • Reason 1: The claimed-state mapping is written but never read.

  • Reason 2: The function allows repeated calls from the same address.

  • Reason 3: No nonce or replay protection exists.

Impact:

  • Impact 1: Snowman NFTs can be minted multiple times by the same address.

  • Impact 2: Airdrop fairness and supply assumptions are invalidated.

  • Impact 3: Protocol guarantees become unenforceable.

Proof of Concept

This issue occurs due to the absence of claimed-state validation.

1. An eligible address calls claimSnowman() successfully.
2. The contract sets s_hasClaimedSnowman[address] = true.
3. The same address calls claimSnowman() again.
4. No claimed-state check exists.
5. The second call succeeds and mints additional Snowman NFTs.

Recommended Mitigation

Enforce the claimed-state at the beginning of the claimSnowman function.

function claimSnowman(...) external nonReentrant {
+ if (s_hasClaimedSnowman[receiver]) {
+ revert SA__AlreadyClaimed();
+ }
...
}

This ensures that each address can only claim once and preserves the intended airdrop invariant.

⚠️ Note on Overlap

This issue is logically distinct from replayable signatures:

  • H-1 focuses on signature replay

  • H-3 focuses on missing state enforcement

Both independently allow multiple claims and should be reported separately.

Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge 1 day 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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