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

claimSnowman never checks s_hasClaimedSnowman allowing repeated NFT claims

softbutsavage

Root + Impact

Description

claimSnowman() sets s_hasClaimedSnowman[receiver] = true after a successful
claim but never reads it back. There is no guard preventing a second claim.
Since Snow tokens are transferred away during claiming, the balance check
(i_snow.balanceOf(receiver) == 0) temporarily prevents re-entry — but a
user can re-acquire Snow tokens through earnSnow() for free once per week.
Because all Merkle entries in this deployment use amount=1, every recipient
can earn 1 Snow per week and re-submit the same valid proof indefinitely,
claiming an additional Snowman NFT each time until the supply is exhausted.

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(); }
​
// @> s_hasClaimedSnowman[receiver] is never checked here
​
if (!_isValidSignature(receiver, getMessageHash(receiver), v, r, s)) {
revert SA__InvalidSignature();
}
​
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);
​
// @> flag is set but has no effect — nothing reads it
s_hasClaimedSnowman[receiver] = true;
​
emit SnowmanClaimedSuccessfully(receiver, amount);
i_snowman.mintSnowman(receiver, amount);
}

Risk

Likelihood:

Every eligible recipient can exploit this no special access or funds required. earnSnow() provides 1 free Snow token per week, matching the Merkle entry
amount for all recipients in this deployment

Impact:

Attacker drains the entire Snowman NFT supply, receiving unlimited NFTs
at zero cost beyond waiting one week between claims. Legitimate users who have not yet claimed may find no NFTs remaining

Proof of Concept

Run: forge test --match-path test/audit/RepeatedClaim.t.sol -vv
​
Alice makes her first legitimate claim (1 Snowman NFT). The contract sets
s_hasClaimedSnowman[alice] = true. After one week, Alice calls earnSnow()
to receive 1 free Snow token — matching her original Merkle entry amount.
She then calls claimSnowman() again with the same valid proof and receives
a second NFT. This repeats every week. The test confirms Alice holds 3 NFTs
after 3 claims while getClaimStatus(alice) returns true throughout, proving
the flag is set but never enforced.
​
softbutsavage@penguin:~/2025-06-snowman-merkle-airdrop$ forge test --match-path test/audit/RepeatedClaim.t.sol -vv
[⠒] Compiling...
[⠆] Compiling 1 files with Solc 0.8.34
[⠔] Solc 0.8.34 finished in 5.23s
Compiler run successful!
​
Ran 1 test for test/audit/RepeatedClaim.t.sol:RepeatedClaimTest
[PASS] testRepeatedClaimDrainsNFTSupply() (gas: 461615)
Suite result: ok. 1 passed; 0 failed; 0 skipped; finished in 44.39ms (13.51ms CPU time)
​
Ran 1 test suite in 181.00ms (44.39ms CPU time): 1 tests passed, 0 failed, 0 skipped (1 total tests)
softbutsavage@penguin:~/2025-06-snowman-merkle-airdrop$
​

Recommended Mitigation

Add a check at the top of claimSnowman() that reverts if the receiver has
already claimed. The mapping is already being written correctly — it just
needs to be read before proceeding.
​
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(); }
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 2 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; } ```

Support

FAQs

Can't find an answer? Chat with us on Discord, Twitter or Linkedin.

Give us feedback!