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

claimSnowman() missing claim replay protection — receiver can claim unlimited Snowman NFTs

squashingbugsforlife

Root + Impact

Description

  • Normal behavior: claimSnowman() should allow each eligible receiver to claim their Snowman NFT exactly once. The contract sets s_hasClaimedSnowman[receiver] = true after a successful claim.

  • The issue: The s_hasClaimedSnowman mapping is set after each claim but is never checked before processing a new claim. There is no guard preventing a receiver from calling claimSnowman() repeatedly. As long as the receiver holds Snow tokens, they can drain the Snowman NFT supply by claiming over and over.

```solidity
// src/SnowmanAirdrop.sol#69-99
function claimSnowman(address receiver, bytes32[] calldata merkleProof, uint8 v, bytes32 r, bytes32 s)
external nonReentrant
{
// @> s_hasClaimedSnowman is NEVER CHECKED here
if (receiver == address(0)) revert SA__ZeroAddress();
if (i_snow.balanceOf(receiver) == 0) revert SA__ZeroAmount();
// ...
i_snow.safeTransferFrom(receiver, address(this), amount); // @> arbitrary from
s_hasClaimedSnowman[receiver] = true; // @> set but never read
i_snowman.mintSnowman(receiver, amount);
}
```

Risk

Likelihood:

  • Any eligible receiver can call claimSnowman() multiple times — no guard prevents it.

  • The receiver only needs to re-acquire Snow tokens between claims to repeat the exploit.

Impact:

  • Receiver claims far more Snowman NFTs than their Snow balance entitles them to.

  • An attacker supplying a victim's address as receiver can drain the victim's approved Snow tokens.

Proof of Concept

The following Foundry test demonstrates that a receiver can call claimSnowman() multiple times without restriction. The s_hasClaimedSnowman mapping is written on line 94 but is never read before processing a new claim, making the check completely ineffective. A receiver who reacquires Snow tokens can repeat the claim indefinitely, draining the entire NFT supply.

```solidity
function testDoubleClaim() public {
vm.startPrank(receiver);
​
// First claim — legitimate
snowmanAirdrop.claimSnowman(receiver, merkleProof, v, r, s);
uint256 nftsAfterFirst = snowman.balanceOf(receiver);
assertGt(nftsAfterFirst, 0);
​
// Receiver reacquires Snow tokens
snow.buySnow{value: buyFee * 10}(10);
​
// Second claim — succeeds, no revert
snowmanAirdrop.claimSnowman(receiver, merkleProof, v, r, s);
uint256 nftsAfterSecond = snowman.balanceOf(receiver);
​
// Receiver holds double the NFTs they should
assertGt(nftsAfterSecond, nftsAfterFirst);
vm.stopPrank();
}
```

Recommended Mitigation

The fix adds a claim guard at the very start of claimSnowman(), before any other validation runs. This ensures that once a receiver has successfully claimed, all future calls revert immediately. The check must come first — placing it after the balance check would still allow re-entry if the receiver holds tokens. A new custom error SA__AlreadyClaimed() is added to maintain the contract's error naming convention.

```diff
function claimSnowman(
address receiver,
bytes32[] calldata merkleProof,
uint8 v, bytes32 r, bytes32 s
) external nonReentrant {
+ // @> Check FIRST — before any other validation
+ if (s_hasClaimedSnowman[receiver]) revert SA__AlreadyClaimed();
if (receiver == address(0)) revert SA__ZeroAddress();
if (i_snow.balanceOf(receiver) == 0) revert SA__ZeroAmount();
// ... rest of function ...
s_hasClaimedSnowman[receiver] = true;
i_snowman.mintSnowman(receiver, amount);
}
​
+ error SA__AlreadyClaimed();
```
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 7 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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