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

M-03 Missing Double-Claim Check in claimSnowman()

mnusurov

Root + Impact

Description

  • Normal behavior: The claimSnowman() function should check s_hasClaimedSnowman[receiver] before processing a claim, preventing users from claiming their airdrop more than once.

  • Issue: The s_hasClaimedSnowman mapping is written at line 94 but never checked before the claim is processed. While the current implementation burns the user's Snow tokens (preventing re-claim with the same proof), the mapping exists specifically to prevent double-claiming and its read-check is missing.

// src/SnowmanAirdrop.sol:69-99
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();
​
if (!_isValidSignature(receiver, getMessageHash(receiver), v, r, s)) {
revert SA__InvalidSignature();
}
​
uint256 amount = i_snow.balanceOf(receiver);
bytes32 leaf = keccak256(bytes.concat(keccak256(abi.encode(receiver, amount))));
if (!MerkleProof.verify(merkleProof, i_merkleRoot, leaf)) {
revert SA__InvalidProof();
}
​
i_snow.safeTransferFrom(receiver, address(this), amount);
​
@> s_hasClaimedSnowman[receiver] = true; // Written but never checked!
// Missing: if (s_hasClaimedSnowman[receiver]) revert AlreadyClaimed();
}

Risk

Likelihood:

  • The mapping s_hasClaimedSnowman is explicitly maintained but never read defensively

  • If the token burn logic changes or users receive more tokens, double-claiming becomes possible

Impact:

  • The contract maintains state (s_hasClaimedSnowman) that suggests double-claim protection, but it is not enforced

  • This is a defense-in-depth issue — the economic constraint (token burn) currently prevents abuse, but the explicit check is missing

Proof of Concept

This POC demonstrates that s_hasClaimedSnowman is written but never checked, representing a missing defensive check.

// From test/audit/M-03-missing-claim-check.t.sol
function testExploitVulnerability() public {
// Alice claims her airdrop
vm.startPrank(alice);
snow.approve(address(airdrop), 1);
vm.stopPrank();
​
bytes32 digest = airdrop.getMessageHash(alice);
(uint8 v, bytes32 r, bytes32 s) = vm.sign(alKey, digest);
​
vm.prank(alice);
airdrop.claimSnowman(alice, AL_PROOF, v, r, s);
assertTrue(airdrop.getClaimStatus(alice)); // mapping is set
​
// The mapping s_hasClaimedSnowman is set to true
// But there is NO check preventing re-claim if balance allows
console2.log("s_hasClaimedSnowman is written but never checked");
}

Recommended Mitigation

Add a check on s_hasClaimedSnowman[receiver] at the start of claimSnowman() to enforce double-claim protection.

// src/SnowmanAirdrop.sol:69-99
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();
// ... rest of function
}
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge 8 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; } ```

Support

FAQs

Can't find an answer? Chat with us on Discord, Twitter or Linkedin.

Give us feedback!