Snowman Merkle Airdrop
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Submission Details
Severity: low
Valid

SnowmanAirdrop::claimSnowman() does not check s_hasClaimedSnowman before minting, allowing repeated claims against the same Merkle allocation

themoonkami

Root + Impact

Description

  • Each eligible address should be able to claim Snowman NFTs only once, based on their snow token balance at snapshot. After a successful claim, the address should no longer be able to reuse the same allocation or Merkle proof to claim again.

  • claimSnowmansets s_hasClaimedSnowman[receiver]to truebut doesn't check this value before processing a claim. An attacker can therefore restore their Snow balance to their snapshot allocation via buySnow, sign a fresh EIP-712 for that restored balance and repeatedly claim another set of NFTs using the same original valid Merkle Prood.

function claimSnowman(address receiver, bytes32[] calldata merkleProof, uint8 v, bytes32 r, bytes32 s)
external
nonReentrant
{
@> // there is no check here for s_hasClaimedSnowman[receiver] before proceeding
if (receiver == address(0)) {
revert SA__ZeroAddress();
}
if (i_snow.balanceOf(receiver) == 0) {
revert SA__ZeroAmount();
}
...
i_snow.safeTransferFrom(receiver, address(this), amount);
@> s_hasClaimedSnowman[receiver] = true; // written here, but never read before this point
emit SnowmanClaimedSuccessfully(receiver, amount);
i_snowman.mintSnowman(receiver, amount);
}

Risk

Likelihood: High

  • An attacker calls buySnow() to restore their Snow balance to exactly their original snapshot allocation, then resubmits claimSnowman() with a freshly signed message and their original Merkle proof. Since the leaf is computed purely from (receiver, amount), restoring the exact original amount reproduces the exact same valid leaf every time.

  • This requires no special timing, no race condition, and no interaction with any other user; a single address can repeat the cycle indefinitely, bounded only by how many times the attacker is willing to pay the buySnow() fee to restore their balance.

Impact: High

  • An attacker can mint an unbounded number of additional Snowman NFTs beyond their single legitimate allocation, extracting value directly from the protocol at the cost of only the buySnow() fee per repetition. This is a profitable, repeatable exploit for Snowman NFTs that carry secondary market or utility value.

  • The core protocol invariant which is that each eligible address may claim its allocation exactly once, is completely broken, undermining the integrity of the entire Merkle airdrop distribution.

Proof of Concept

function testDoubleClaimExploit() public {
// The first valid claim
vm.prank(alice);
snow.approve(address(airdrop), type(uint256).max);
​
bytes32 alDigest1 = airdrop.getMessageHash(alice);
(uint8 alV1, bytes32 alR1, bytes32 alS1) = vm.sign(alKey, alDigest1);
​
vm.prank(satoshi);
airdrop.claimSnowman(alice, AL_PROOF, alV1, alR1, alS1);
​
assert(nft.balanceOf(alice) == 1);
assert(snow.balanceOf(alice) == 0);
​
// Alice buys the exact amount of snow tokens she had at snapshot time
uint256 fee = snow.s_buyFee();
deal(alice, fee);
​
vm.prank(alice);
snow.buySnow{value: fee}(1);
​
assert(snow.balanceOf(alice) == 1);
​
// Approve again for the second claim
vm.prank(alice);
snow.approve(address(airdrop), type(uint256).max);
​
// Since the amount and key are the same, the signature will be valid still
bytes32 alDigest2 = airdrop.getMessageHash(alice);
(uint8 alV2, bytes32 alR2, bytes32 alS2) = vm.sign(alKey, alDigest2);
​
vm.prank(satoshi);
airdrop.claimSnowman(alice, AL_PROOF, alV2, alR2, alS2);
​
// Alice has now claimed TWICE against the same allocation
assert(nft.balanceOf(alice) == 2);
}

This test demonstrates that alice, a legitimately allocated claimant, can claim her single allocation, restore her Snow balance to the exact snapshot amount via buySnow(), and successfully claim a second time using her original, unchanged Merkle proof. Running forge test --mt testDoubleClaimExploit confirms this passes, with alice ending the test holding 2 Snowman NFTs against a snapshot allocation of only 1.

This is further shown by an invariant which is checked by a companion handler contract that repeatedly interleaves calls to claimSnowman() and buySnow() across five known, legitimately allocated claimants, in random order and sequence.

function invariant_noClaimantExceedsOneNFT() public view {
for (uint256 i = 0; i < 5; i++) {
address claimant = handler.getClaimant(i);
assertLe(
nft.balanceOf(claimant),
1,
"Claimant holds more Snowman NFTs than their single legitimate allocation"
);
}
}

Recommended Mitigation

This mitigation adds a single check at the top of claimSnowman(), using the s_hasClaimedSnowman mapping that already exists in the contract but was never read before this point. The fix requires no new state, only wiring in a check against state that was already being written on every successful claim. A new custom error, SA__AlreadyClaimed(), is also introduced to give this specific revert case a clear, distinct name.

+ 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();
}
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 8 hours ago
Submission Judgement Published
Validated
Assigned finding tags:

[L-01] Missing Claim Status Check Allows Multiple Claims in SnowmanAirdrop.sol::claimSnowman

# Root + Impact &#x20; **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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