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

SnowmanAirdrop.claimSnowman never reads s_hasClaimedSnowman — the same receiver can claim multiple times after re-acquiring Snow

fishyyh

SnowmanAirdrop.claimSnowman never reads s_hasClaimedSnowman

Impact

High Impact

Likelihood

Medium Likelihood

Root + Impact

Description

  • The airdrop contract tracks whether a receiver has already claimed via s_hasClaimedSnowman[receiver] and sets it to true at the end of claimSnowman.

  • The guard is written but never read: there is no if (s_hasClaimedSnowman[receiver]) revert at the top of the function. The implementation relies on the side-effect of safeTransferFrom(receiver, address(this), amount) zeroing the receiver's Snow balance. However, if the receiver re-acquires Snow (via buySnow or earnSnow) and their balance again matches the Merkle leaf, they can claim a second time.

// src/SnowmanAirdrop.sol
function claimSnowman(...) external nonReentrant {
if (receiver == address(0)) revert SA__ZeroAddress();
if (i_snow.balanceOf(receiver) == 0) revert SA__ZeroAmount();
@> // s_hasClaimedSnowman[receiver] is NEVER checked here
...
i_snow.safeTransferFrom(receiver, address(this), amount);
@> s_hasClaimedSnowman[receiver] = true; // set but never read
i_snowman.mintSnowman(receiver, amount);
}

Risk

Likelihood:

  • Any receiver who re-acquires Snow tokens to match their original Merkle leaf amount can claim again.

  • The protection is implicit and breaks under any future business logic change to the Snow transfer path.

Impact:

  • A receiver can obtain more Snowman NFTs than the Merkle tree entitles them to.

  • The s_hasClaimedSnowman mapping misleads auditors — it appears to guard claims but does not.

Proof of Concept

1) Alice is in the Merkle tree with 5 Snow
2) Alice holds 5 Snow -> calls claimSnowman -> succeeds -> s_hasClaimedSnowman[alice] = true
3) Alice buys 5 more Snow via buySnow
4) Alice calls claimSnowman again — no hasClaimed check -> passes -> claims again
5) Alice now holds 2x the intended Snowman NFTs

Recommended Mitigation

function claimSnowman(...) 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();
...
+ s_hasClaimedSnowman[receiver] = true; // CEI: state before external calls
i_snow.safeTransferFrom(receiver, address(this), amount);
- s_hasClaimedSnowman[receiver] = true;
i_snowman.mintSnowman(receiver, amount);
}
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 4 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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