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

s_hasClaimedSnowman is written but never checked, removing one-time-claim protection

aclod
# [L-2] `s_hasClaimedSnowman` is written but never checked, so the intended one-time-claim protection does not exist
​
## Summary
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`claimSnowman` sets `s_hasClaimedSnowman[receiver] = true` after a successful claim, clearly intending to prevent a recipient from claiming more than once. However, this mapping is never read anywhere in the claim path, so the protection it represents is not enforced. The only thing preventing an immediate re-claim is the incidental `balanceOf(receiver) == 0` check, and the claim signature carries no nonce — so a recipient who re-acquires the same `Snow` amount can replay the same signature and Merkle proof to claim again.
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## Vulnerability Details
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In `claimSnowman` the flag is written but never used as a guard:
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```solidity
// ... signature + merkle checks ...
i_snow.safeTransferFrom(receiver, address(this), amount);
​
s_hasClaimedSnowman[receiver] = true; // written...
​
emit SnowmanClaimedSuccessfully(receiver, amount);
i_snowman.mintSnowman(receiver, amount);
```
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There is no `if (s_hasClaimedSnowman[receiver]) revert ...;` anywhere. The function's only de-facto single-claim guard is:
​
```solidity
if (i_snow.balanceOf(receiver) == 0) {
revert SA__ZeroAmount();
}
```
​
This is incidental, not intentional, and is bypassable over time:
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1. Recipient claims; their `Snow` is transferred to the airdrop contract, so `balanceOf == 0`.
2. Recipient re-acquires `Snow` (via `earnSnow` / `buySnow`) up to the exact snapshot `amount` encoded in their Merkle leaf.
3. The same Merkle proof still verifies (root is unchanged) and the same signature still validates (no nonce, no `used` flag is enforced), so `claimSnowman` succeeds again and mints another batch of NFTs.
​
## Impact
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- The intended "claim only once" guarantee is not actually enforced; the `s_hasClaimedSnowman` mapping is effectively dead code.
- A recipient can mint `Snowman` NFTs more than once by rebuilding their `Snow` balance to the snapshot amount, inflating the NFT supply beyond entitlement.
​
Real-world impact is limited by the need to re-acquire the exact snapshot amount, so this is **Low** severity, but the missing guard is a clear defect.
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## Recommended Mitigation
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Enforce the flag that the contract already maintains:
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```diff
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();
}
```
​
(Add the corresponding `SA__AlreadyClaimed` error.) For full robustness, also include a nonce in the signed `SnowmanClaim` message.
​
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 1 hour 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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