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

Unlimited Snowman NFT mint via replayable signature

sexretxt

Root + Impact

Description

Normal Behavior

Each eligible address should be able to claim Snowman NFTs only once, using a valid EIP-712 signature to authorize the claim. After a successful claim, subsequent attempts should be rejected.

Issue

The claimSnowman function never checks whether an address has already claimed, even though the contract tracks this state in s\_hasClaimedSnowman.

As a result, the same signed message can be reused indefinitely to mint Snowman NFTs multiple times.

// Root cause in the codebase
​
function claimSnowman(...) external nonReentrant {
...
if (!_isValidSignature(receiver, getMessageHash(receiver), v, r, s)) {
revert SA__InvalidSignature();
}
​
// ❌ Missing check:
// require(!s_hasClaimedSnowman[receiver])
​
i_snow.safeTransferFrom(receiver, address(this), amount);
​
@> s_hasClaimedSnowman[receiver] = true;
​
emit SnowmanClaimedSuccessfully(receiver, amount);
​
i_snowman.mintSnowman(receiver, amount);
}

The s_hasClaimedSnowman mapping is written but never enforced, making it ineffective as a replay protection mechanism.

Risk

Likelihood:

  • Reason 1: The same EIP-712 signature remains valid forever.

  • Reason 2: No nonce, deadline, or claimed-state validation exists.

  • Reason 3: The attack works every time claimSnowman is called.

Impact:

  • Impact 1: Unlimited Snowman NFTs can be minted by a single address.

  • Impact 2: NFT supply and fairness assumptions are completely broken.

  • Impact 3: Trust in the airdrop and staking mechanism is undermined.

Proof of Concept

The vulnerability can be exploited because the contract does not verify whether a receiver has already claimed a Snowman NFT before processing a new claim. As a result, the same valid EIP-712 signature can be reused multiple times.

1. Receiver signs a valid SnowmanClaim message.
2. Receiver calls claimSnowman() with the signature → succeeds.
3. s_hasClaimedSnowman[receiver] is set to true.
4. Receiver calls claimSnowman() again with the SAME signature.
5. No claimed-state check exists → call succeeds again.
6. Snowman NFTs are minted repeatedly with no limit.

This process can be repeated indefinitely, allowing a single address to mint an unbounded number of Snowman NFTs using one signature.

Recommended Mitigation

Add a claimed-state check at the beginning of claimSnowman to prevent replayed claims.

function claimSnowman(...) external nonReentrant {
+ if (s_hasClaimedSnowman[receiver]) {
+ revert SA__AlreadyClaimed();
+ }
​
if (!_isValidSignature(receiver, getMessageHash(receiver), v, r, s)) {
revert SA__InvalidSignature();
}
​
...
}

Optionally, strengthen replay protection further by:

  • Including a nonce in the signed message, or

  • Invalidating signatures after a successful claim.

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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