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

Missing state check in claimSnowman allows infinite Merkle proof and signature replays, draining NFTs

gopi_vaka

Root + Impact

Description

  • The SnowmanAirdrop contract intends to allow eligible users to claim a Snowman NFT exactly once. To enforce this, the contract defines a mapping s_hasClaimedSnowman which is updated to true after a successful claim.

  • While the claimSnowman function updates the s_hasClaimedSnowman mapping at the end of its execution, it never actually checks this mapping at the beginning. Because the Merkle leaf is constructed using only the receiver and amount (lacking a nonce), an attacker can simply transfer more Snow tokens into their wallet and reuse the exact same signature and Merkle proof to repeatedly call claimSnowman(), infinitely minting NFTs.

function claimSnowman(address receiver, bytes32[] calldata merkleProof, uint8 v, bytes32 r, bytes32 s)
external
nonReentrant
{
// @> BUG: Missing check for `if(s_hasClaimedSnowman[receiver]) revert();`
if (receiver == address(0)) { revert SA__ZeroAddress(); }
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; // @> State is updated, but never checked!
​
emit SnowmanClaimedSuccessfully(receiver, amount);
i_snowman.mintSnowman(receiver, amount);
}

Risk

Likelihood:

  • High. Exploiting this requires no special privileges, flash loans, or complex contract deployments. An attacker only needs to be a valid recipient in the Merkle tree and have access to secondary Snow tokens.

Impact:

  • High. Complete breakdown of the NFT rarity and distribution mechanics. An attacker can infinitely inflate the supply of Snowman NFTs, draining value from legitimate participants and breaking the 1:1 integration between the ERC20 staking and ERC721 claiming.

Proof of Concept

Narrative Setup: The following Foundry test proves the missing state check allows for infinite claiming. We simulate a valid user (Alice) who successfully claims her Snowman NFT. To execute the replay attack, Alice simply acquires more Snow tokens (e.g., from a secondary wallet) matching her original balance, and calls claimSnowman again using her original signature and proof. The transaction succeeds, minting her a second NFT.

Execution Steps:

  1. Alice legitimately claims her airdrop using her valid signature and Merkle proof.

  2. Alice transfers fresh Snow tokens into her wallet so her balance matches her original Merkle leaf amount.

  3. Alice calls claimSnowman a second time with the exact same parameters.

  4. The test asserts that Alice's NFT balance is now 2, proving the replay was successful.

How to run this test: Place the following code inside a Foundry test file connected to the SnowmanAirdrop contract and run it. (Note: aliceProof, v, r, and s represent valid cryptographic inputs generated during setup).

function test_POC_InfiniteSignatureAndProofReplay() public {
// 1. Initial State: Alice has 100 Snow tokens and a valid proof/signature.
// Assume Alice's valid parameters are already generated:
// bytes32[] memory aliceProof;
// uint8 v; bytes32 r; bytes32 s;
uint256 claimAmount = snow.balanceOf(alice);
​
// 2. Alice claims her first NFT legitimately.
vm.prank(alice);
airdrop.claimSnowman(alice, aliceProof, v, r, s);
assertEq(snowmanNft.balanceOf(alice), 1);
assertTrue(airdrop.getClaimStatus(alice)); // The state is now TRUE
​
// 3. THE ATTACK: Alice acquires more Snow tokens from a secondary source
// so her balance matches the original leaf requirement.
vm.prank(secondaryWallet);
snow.transfer(alice, claimAmount);
​
// 4. Alice replays the exact same transaction.
// Because the contract never checks `getClaimStatus()`, it succeeds.
vm.prank(alice);
airdrop.claimSnowman(alice, aliceProof, v, r, s);
​
// 5. THE PROOF: Alice now has 2 NFTs from a 1-time airdrop.
assertEq(snowmanNft.balanceOf(alice), 2);
console.log("BUG: Alice successfully bypassed the claim limit using replay!");
}

Recommended Mitigation

A standard ECDSA nonce mapping is insufficient here, as the static Merkle proof could still be replayed. The protocol must enforce a strict state-lock using the existing mapping to immediately revert duplicate claims, preserving the Checks-Effects-Interactions pattern

+ 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(); }
// ... execution continues ...
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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