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

No Double-Claim Protection in claimSnowman()

muzammilmohammed678

Root + Impact

Description

  • Describe the normal behavior in one or more sentences
    The `claimSnowman()` function sets `s_hasClaimedSnowman[receiver] = true` after processing the claim, but doesn't check this flag before processing. While the balance check and Merkle proof provide some protection, a user could potentially claim multiple times if they manipulate their balance or if the Merkle tree allows multiple entries for the same address with different amounts.

  • Explain the specific issue or problem in one or more sentences
    The claim status is only set after the transfer and mint operations complete. There's no early check to prevent processing if the user has already claimed.

```solidity
// @> SnowmanAirdrop.sol:69-99
function claimSnowman(address receiver, bytes32[] calldata merkleProof, uint8 v, bytes32 r, bytes32 s)
external
nonReentrant
{
// ... validation ...
i_snow.safeTransferFrom(receiver, address(this), amount);
s_hasClaimedSnowman[receiver] = true; // @> Set after operations
// ... minting ...
}
```
​
If a user has multiple entries in the Merkle tree with different amounts, or if they can manipulate their balance to match different Merkle leaves, they could claim multiple times.
​

Risk

Likelihood:

  • * Requires Merkle tree to have duplicate entries for same address

    * Or requires balance manipulation to match different Merkle proofs

    * Lower likelihood due to Merkle proof validation

    * Could occur if Merkle tree generation is flawed

Impact:

  • * Users receiving more NFTs than intended

    * Double spending of Snow tokens

    * Unfair distribution of airdrop

    * Potential economic exploitation

Proof of Concept

```solidity
function testPotentialDoubleClaim() public {
// If Merkle tree has two entries for same address with different amounts
// User could claim twice by:
// 1. First claim with amount X
// 2. Transfer tokens to get balance Y
// 3. Second claim with amount Y (if Merkle proof exists)
// This depends on Merkle tree structure, but the contract
// doesn't explicitly prevent it
}
```
​

Recommended Mitigation

```diff
// SnowmanAirdrop.sol
​
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();
+ }
uint256 amount = i_snow.balanceOf(receiver);
if (amount == 0) {
revert SA__ZeroAmount();
}
// ... rest of function ...
i_snow.safeTransferFrom(receiver, address(this), amount);
s_hasClaimedSnowman[receiver] = true;
// ... minting ...
}
```
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge 17 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; } ```

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