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

claimSnowman Does Not Check s_hasClaimedSnowman, Enabling Repeated Claims

sp0ck0x

Root + Impact

Description

  • The SnowmanAirdrop contract is designed so that each eligible address can claim Snowman NFTs exactly once by
    submitting a valid Merkle proof and EIP-712 signature. After claiming, the receiver's Snow tokens are transferred
    to the contract and they receive NFTs equal to their Snow balance.

  • Although s_hasClaimedSnowman[receiver] is set to true after a successful claim, it is never read at the start
    of claimSnowman(). A receiver who re-acquires the same number of Snow tokens can call claimSnowman() again with
    an identical Merkle proof and signature — receiving additional NFTs indefinitely.

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 is never read here — already-claimed receivers proceed freely
​
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);
​
// @> Written to true, but this write is never used as a guard above
s_hasClaimedSnowman[receiver] = true;
​
emit SnowmanClaimedSuccessfully(receiver, amount);
i_snowman.mintSnowman(receiver, amount);
}

Risk

Likelihood: High

  • Any Merkle-eligible address can re-acquire Snow tokens through earnSnow() (free, once per week) or buySnow()
    (paid) during the 12-week farming window, making the precondition for a second claim trivially achievable.

  • The re-acquired amount matches the original Merkle leaf exactly (tree stores amount = 1 for each address in the
    reference setup), so the same proof and signature are reusable without generating any new credentials.

Impact:

  • Every eligible address can claim an unbounded number of Snowman NFTs over the course of the farming period,
    completely undermining the intended 1-per-address distribution.

  • Because the Merkle leaf amount is read from the live balanceOf at claim time, each additional claim also
    inflates the number of Snow tokens permanently locked in the airdrop contract.

Proof of Concept

function test_exploit_doubleClaim() public {
// Alice starts with 1 Snow token — legitimate claim
assertEq(snow.balanceOf(alice), 1);
​
vm.prank(alice);
snow.approve(address(airdrop), 1);
bytes32 digest = airdrop.getMessageHash(alice);
(uint8 v, bytes32 r, bytes32 s) = vm.sign(aliceKey, digest);
​
vm.prank(satoshi);
airdrop.claimSnowman(alice, AL_PROOF, v, r, s);
​
assertEq(nft.balanceOf(alice), 1);
assertTrue(airdrop.getClaimStatus(alice)); // marked true, but never checked
​
// Alice re-acquires 1 Snow token via earnSnow
vm.warp(block.timestamp + 1 weeks + 1);
vm.prank(alice);
snow.earnSnow();
​
// Exact same proof and signature — digest is identical since amount == 1 again
bytes32 digest2 = airdrop.getMessageHash(alice);
assertEq(digest, digest2);
​
vm.prank(alice);
snow.approve(address(airdrop), 1);
​
vm.prank(satoshi);
airdrop.claimSnowman(alice, AL_PROOF, v, r, s); // reuses old signature
​
// Alice now holds 2 NFTs — double claim confirmed
assertEq(nft.balanceOf(alice), 2);
}

Recommended Mitigation

Add an already-claimed guard as the first check in claimSnowman():

+ error SA__AlreadyClaimed();
​
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();
​
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);
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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