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

`SnowmanAirdrop::claimSnowman` never checks `s_hasClaimedSnowman`, allowing claims to be replayed and extra NFTs minted

colddecemberrrr

Root + Impact: the per-recipient claim flag is written but never read, so a recipient can re-use the same proof + signature to claim again and over-mint Snowman NFTs.

Description

  • Each eligible recipient should be able to claim their Snowman NFTs exactly once; s_hasClaimedSnowman exists to enforce this.

  • claimSnowman sets s_hasClaimedSnowman[receiver] = true but never reads it. The Merkle leaf and signature are bound to the fixed airdrop amount, so once a recipient's balance equals that amount again, the same proof + signature succeed again and mint a fresh batch of NFTs.

mapping(address => bool) private s_hasClaimedSnowman; // @> declared as replay guard...
​
function claimSnowman(address receiver, bytes32[] calldata merkleProof, uint8 v, bytes32 r, bytes32 s)
external
nonReentrant
{
@> // no check of s_hasClaimedSnowman here — nothing stops a repeat claim
...
i_snow.safeTransferFrom(receiver, address(this), amount);
@> s_hasClaimedSnowman[receiver] = true; // ...set, but NEVER read anywhere
emit SnowmanClaimedSuccessfully(receiver, amount);
i_snowman.mintSnowman(receiver, amount);
}

Risk

Likelihood:

  • Occurs whenever a recipient's Snow balance returns to the airdropped amount after their first claim — e.g. someone transfers Snow to them, or they re-earn it. The previously valid proof and signature then pass again.

Impact:

  • A recipient can mint more Snowman NFTs than their single entitlement and drain Snow into the contract repeatedly.

  • Breaks the "one claim per recipient / 1 NFT per Snow" invariant and inflates total NFT supply.

PROOF OF CONCEPT

function test_ClaimCanBeReplayed() public {
vm.prank(alice);
snow.approve(address(airdrop), type(uint256).max);
​
bytes32 digest = airdrop.getMessageHash(alice);
(uint8 v, bytes32 r, bytes32 s) = vm.sign(alKey, digest);
​
// first (legitimate) claim
vm.prank(satoshi);
airdrop.claimSnowman(alice, AL_PROOF, v, r, s);
assertEq(nft.balanceOf(alice), 1);
assertEq(snow.balanceOf(alice), 0);
​
// alice re-acquires exactly the airdrop amount (bob sends her 1 Snow)
vm.prank(bob);
snow.transfer(alice, 1);
assertEq(snow.balanceOf(alice), 1);
​
// the SAME proof + signature work again — s_hasClaimedSnowman is ignored
vm.prank(satoshi);
airdrop.claimSnowman(alice, AL_PROOF, v, r, s);
​
assertEq(nft.balanceOf(alice), 2); // double-minted
}

Run: forge test --mt test_ClaimCanBeReplayed → PASS (alice ends with 2 NFTs from one entitlement).

RECOMMENDED MITIGATION

error SA__InvalidProof();
error SA__InvalidSignature();
error SA__ZeroAddress();
error SA__ZeroAmount();
+ 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();
}

For defense in depth, also include a nonce in the signed EIP-712 message so a leaked signature cannot be reused.

Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 5 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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