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

M-3 — Claim replay: claim-once flag is never read

rubabbel

Root + Impact

claimSnowman sets s_hasClaimedSnowman[receiver] = true but never checks it. The signature commits only (receiver, amount) with no nonce or deadline. If the receiver's balance is restored to the signed amount, the same signature and Merkle proof can be replayed to mint additional NFTs.

Description

// src/SnowmanAirdrop.sol
47: mapping(address => bool) private s_hasClaimedSnowman; // written but never read
...
69: function claimSnowman(address receiver, bytes32[] calldata merkleProof, uint8 v, bytes32 r, bytes32 s) external nonReentrant {
@> // no check: require(!s_hasClaimedSnowman[receiver])
...
92: i_snow.safeTransferFrom(receiver, address(this), amount);
@>94: s_hasClaimedSnowman[receiver] = true; // latch set, never enforced

Risk

Likelihood: Medium — requires the receiver to regain exactly the signed balance (any transfer can supply it).

Impact: Medium — a receiver can claim more than once, over-minting NFTs; with a max approval a third party can replay on the receiver's behalf.

Proof of Concept

function test_PoC4_ClaimReplay() public {
_deploy();
(address alice, uint256 aliceKey) = makeAddrAndKey("alice");
address bob = makeAddr("bob");
​
bytes32 leaf = keccak256(bytes.concat(keccak256(abi.encode(alice, uint256(1)))));
SnowmanAirdrop airdrop = new SnowmanAirdrop(leaf, address(snow), address(nft));
​
vm.prank(alice); snow.earnSnow();
vm.warp(block.timestamp + 1 weeks);
vm.prank(bob); snow.earnSnow();
​
vm.prank(alice);
snow.approve(address(airdrop), type(uint256).max);
​
bytes32 digest = airdrop.getMessageHash(alice);
(uint8 v, bytes32 r, bytes32 s) = vm.sign(aliceKey, digest);
bytes32[] memory proof = new bytes32[](0);
​
airdrop.claimSnowman(alice, proof, v, r, s);
assertEq(nft.balanceOf(alice), 1);
​
vm.prank(bob); snow.transfer(alice, 1); // restore balance to signed amount
​
airdrop.claimSnowman(alice, proof, v, r, s); // exact replay
assertEq(nft.balanceOf(alice), 2);
}

Recommended Mitigation

Enforce the claim flag and add a nonce/deadline to the signed struct.

+ error SA__AlreadyClaimed();
...
function claimSnowman(...) external nonReentrant {
+ if (s_hasClaimedSnowman[receiver]) revert SA__AlreadyClaimed();
...
s_hasClaimedSnowman[receiver] = true;

+ Add nonce and deadline to SnowmanClaim and include them in MESSAGE_TYPEHASH and getMessageHash.

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   **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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