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

[H-01] Missing state check for s_hasClaimedSnowman allows users to claim multiple airdrops via Signature Replay

velo

Root + Impact

Description

  • The airdrop protocol is intended to allow eligible participants to claim their reward exactly once. After a successful claim, the user's address is marked in the s_hasClaimedSnowman mapping to prevent further claims.

  • he claimSnowman function updates the s_hasClaimedSnowman mapping to true at the end of the execution but fails to verify this state at the beginning of the function. This omission allows a user to reuse a valid Merkle proof and signature multiple times to mint multiple NFTs.

function claimSnowman(address receiver, bytes32[] calldata merkleProof, uint8 v, bytes32 r, bytes32 s)
external
nonReentrant
{
@> // The contract fails to verify if s_hasClaimedSnowman[receiver] is already true
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();
}
​
bytes32 digest = getMessageHash(receiver);
if (!_isValidSignature(receiver, digest, v, r, s)) {
revert SA__InvalidSignature();
}
​
@> s_hasClaimedSnowman[receiver] = true;
i_snow.safeTransferFrom(receiver, address(this), amount);
i_snowman.mintSnowman(receiver, 1);
emit SnowmanClaimedSuccessfully(receiver, 1);
}

Risk

Likelihood:

  • The protocol lacks a validation check on the s_hasClaimedSnowman mapping at the function's entry point. // Occurs every time the function is called.

  • Cryptographic signatures and Merkle proofs remain valid for reuse because the underlying state (the user's token balance) can be restored to match the initial signed parameters.

Impact:

  • An attacker drains the NFT supply by repeatedly calling the claim function with the same signature.

  • The scarcity and economic value of the NFT collection are severely compromised through unauthorized inflation.

Proof of Concept

function testPoC_DoubleClaiming_Exploit() public {
// 1. Setup Alice's initial claim state
vm.prank(alice);
snow.approve(address(airdrop), 1 ether);
​
bytes32 digest = airdrop.getMessageHash(alice);
(uint8 v, bytes32 r, bytes32 s) = vm.sign(aliceKey, digest);
​
// 2. Alice performs her first legitimate claim
airdrop.claimSnowman(alice, AL_PROOF, v, r, s);
assertEq(nft.balanceOf(alice), 1);
​
// 3. Alice re-acquires tokens to match the signed amount
// simulating a transfer from another wallet
deal(address(snow), alice, 1);
vm.prank(alice);
snow.approve(address(airdrop), 1 ether);
​
// 4. Alice REPLAYS the same signature and proof successfully
airdrop.claimSnowman(alice, AL_PROOF, v, r, s);
​
// 5. Alice now has 2 NFTs using only 1 valid entry in the Merkle Tree
assertEq(nft.balanceOf(alice), 2);
}

Recommended Mitigation

function claimSnowman(address receiver, bytes32[] calldata merkleProof, uint8 v, bytes32 r, bytes32 s)
external
nonReentrant
{
+ if (s_hasClaimedSnowman[receiver]) {
+ revert SnowmanAirdrop__AlreadyClaimed();
+ }
uint256 amount = i_snow.balanceOf(receiver);
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 19 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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