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

User Can claim Unlimited amount of NFTs

lemrabetmoctar1

Missing check if teh user had claimed before, User can claim NFTs more than once

Description

  • Users that have snow man tokens should be able to claim one NFTs and no more

  • User can claim unlimited amount of NFTs because in the calim function the didn't check if the user had claim before so if the user has the same amount of snow tokens that he had before at the first time he call teh function he can call it again and claim NFT

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(); //@audit-issue: signature can be used multiple times if the amount still the same
} //audit-issue: there is no check if the user had calaimed before
​
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; //@audit-issue: they didn't check if the user aready claimed so he can calim again
​
emit SnowmanClaimedSuccessfully(receiver, amount);
​
i_snowman.mintSnowman(receiver, amount);
}

Risk

Likelihood:

  • anytime user has the same amount of tokens that he had in teh first claim he can call the function and claim teh NFT

  • everyweek/ any time he bough snow man tokens

Impact:

  • Loosing NFT

  • attacker can collect unlimited amount of NFTs and that broke the invarient of `claiming once `


Proof of Concept

Here is the full step by step attack senario with the test function using foundry

1- attacker call the claim function with balance = 1 token
2- the balance of attacker reset to 0 token
3- attacker wait one weeks or bough a new token now the balance = 1 tokens
4- call the function again with the same signuture
​
code :
function test_claimingTwiceWithSameAmount() public {
// ===== FIRST CLAIM =====
assert(nft.balanceOf(bob) == 0);
​
vm.prank(bob);
snow.approve(address(airdrop), type(uint256).max); // better
​
bytes32 digest = airdrop.getMessageHash(bob);
(uint8 v, bytes32 r, bytes32 s) = vm.sign(bobKey, digest);
​
vm.prank(satoshi);
airdrop.claimSnowman(bob, BOB_PROOF, v, r, s);
​
assertEq(nft.balanceOf(bob), 1);
assertEq(nft.ownerOf(0), bob);
​
// ===== RESTORE SAME AMOUNT =====
vm.warp(block.timestamp + 1 weeks);
​
vm.prank(bob);
snow.earnSnow();
​
uint256 newBalance = snow.balanceOf(bob);
console2.log("Bob balance after earn:", newBalance);
​
// ===== SECOND CLAIM =====
vm.prank(satoshi);
airdrop.claimSnowman(bob, BOB_PROOF, v, r, s);
​
// 🚨 PROOF OF BUG
assertEq(nft.balanceOf(bob), 2);
}

Recommended Mitigation

To protect agains this bug you haev two opetions one to check if the user has claimed before and the seconde to use nonce to avoide using teh same signuture twice

+ mapping(address => uint256) public nonces;
+ if (s_hasClaimedSnowman[receiver] = true);
+ revert;
​
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge 2 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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