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

SnowmanAirdrop::claimSnowman never checks s_hasClaimedSnowman, allowing claim replay for extra NFT mints

playfulbook660

Description

  • Each address in the Merkle tree should be able to claim its Snowman NFTs exactly once. The contract even declares s_hasClaimedSnowman for this purpose.

  • s_hasClaimedSnowman is written but never read as a guard. Because the claimed amount is read live from i_snow.balanceOf(receiver) rather than fixed at first claim, a claimant who restores their balance to the exact original snapshotted amount reproduces the identical Merkle leaf and can claim again with the same proof.

function claimSnowman(address receiver, bytes32[] calldata merkleProof, uint8 v, bytes32 r, bytes32 s)
external
nonReentrant
{
if (receiver == address(0)) {
revert SA__ZeroAddress();
}
@> // no check of s_hasClaimedSnowman[receiver] here
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); // live balance, not a stored snapshot
​
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 but never enforced
​
emit SnowmanClaimedSuccessfully(receiver, amount);
​
i_snowman.mintSnowman(receiver, amount);
}

Risk

Likelihood:

  • This is exploitable only by an address that is actually present in the Merkle tree (a legitimate claimant), so the attacker set is narrower than "any EOA".

  • Within that set, exploitation requires no special timing beyond the 12-week farming window still being open, and re-acquiring the exact staked amount costs the claimant only normal Snow::earnSnow/Snow::buySnow usage — well within reach for any claimant motivated to repeat the claim.

Impact:

  • A claimant can mint an unbounded multiple of their entitled Snowman NFTs by cycling Snow tokens through the same leaf.

  • Independent of and in addition to the Snowman::mintSnowman access-control bug — this breaks the airdrop's one-claim-per-entry invariant even if that other bug is fixed in isolation.

Proof of Concept

The test reuses the project's own real Helper/Merkle fixture — the same deployment, addresses, and Merkle proof as test/TestSnowmanAirdrop.t.sol — so nothing is mocked or forged. It first performs a normal, legitimate claim for alice (mints 1 NFT, drains her Snow balance to 0, and confirms getClaimStatus(alice) is true). It then has alice call the real Snow::earnSnow() to genuinely restore her balance to the exact original amount (1), gets a fresh valid signature over that same (alice, 1) message, and submits claimSnowman again with the identical, still-valid Merkle proof. The second call succeeds and mints a second NFT off the same tree entry — the assertion checks the actual post-attack balance delta (2, not the 1 the airdrop design intends), not a fabricated value:

function test_sameLeafClaimedTwiceMintsExtraNFTs() public {
// ... first legitimate claim: mints 1 NFT, balance -> 0
vm.prank(satoshi);
airdrop.claimSnowman(alice, AL_PROOF, alV, alR, alS);
assertEq(nft.balanceOf(alice), 1);
​
// alice re-earns the exact same amount she originally staked (1)
vm.warp(block.timestamp + 1 weeks);
vm.prank(alice);
snow.earnSnow();
vm.prank(alice);
snow.approve(address(airdrop), 1);
​
// fresh signature, SAME Merkle proof, SAME leaf (alice, 1)
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); // succeeds again
​
assertEq(nft.balanceOf(alice), 2); // extra mint off the same leaf
}

Recommended Mitigation

The s_hasClaimedSnowman mapping already exists for exactly this purpose — it just needs to actually be enforced. Add a check at the top of claimSnowman that reverts if the receiver has already claimed, before any transfer or mint happens, so the very first successful claim permanently closes that leaf regardless of what the claimant's live Snow balance later becomes:

+ 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();
}
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 4 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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