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

SnowmanAirdrop::claimSnowman() never checks s_hasClaimedSnowman, allowing repeat claims from a single Merkle leaf

foxcryptie

Description

  • claimSnowman() is meant to grant a Snowman NFT exactly once per whitelisted address, in exchange for staking the Snow amount recorded in that address's Merkle leaf.

  • s_hasClaimedSnowman[receiver] is written to true after a successful claim, but no code path in the contract ever reads it — the once-only guard is dead. The real block on an immediate re-claim is accidental: claimSnowman recomputes amount = i_snow.balanceOf(receiver) and the leaf as (receiver, amount), so once the receiver re-acquires exactly the Snow amount from their original leaf (all leaves in this airdrop are amount = 1), the same proof validates again with a freshly self-signed EIP-712 digest.

function claimSnowman(address receiver, bytes32[] calldata merkleProof, uint8 v, bytes32 r, bytes32 s)
external
nonReentrant
{
...
i_snow.safeTransferFrom(receiver, address(this), amount);
​
@> s_hasClaimedSnowman[receiver] = true; // written here, never read anywhere in the contract
​
emit SnowmanClaimedSuccessfully(receiver, amount);
i_snowman.mintSnowman(receiver, amount);
}
​

Risk

Likelihood:

  • Any whitelisted address that re-acquires exactly the Snow amount from its original leaf — via the free weekly earnSnow() or by paying through buySnow() — can immediately reuse the same Merkle proof to claim again.

  • The original claim's signature (v, r, s) itself remains valid indefinitely and can be replayed verbatim for a repeat claim, since neither the signature nor the leaf is ever marked as consumed.

Impact:

  • A whitelisted address can extract one additional Snowman NFT per Snow token it re-acquires, so the airdrop's "one NFT per eligible address" guarantee does not hold — demonstrated reaching a 3× claim from a single tree entry in testing.

  • The Merkle tree stops functioning as a once-only entitlement and degrades into a reusable "balance == leaf amount" filter.

Proof of Concept

function test_H4_double_claim_same_leaf() public {
(uint8 v, bytes32 r, bytes32 s) = _aliceApproveAndSign();
vm.prank(bob);
airdrop.claimSnowman(alice, AL_PROOF, v, r, s);
assertEq(nft.balanceOf(alice), 1, "first claim ok");
assertTrue(airdrop.getClaimStatus(alice), "flag set... but never checked");
​
vm.warp(block.timestamp + 1 weeks);
vm.prank(alice);
snow.earnSnow(); // free weekly re-acquisition of the same leaf amount
​
(v, r, s) = _aliceApproveAndSign(); // fresh self-signature, trivially available
vm.prank(bob);
airdrop.claimSnowman(alice, AL_PROOF, v, r, s);
assertEq(nft.balanceOf(alice), 2, "claimed twice from one tree entry");
}
​
function test_H4b_original_signature_replays() public {
// the FIRST claim's own (v, r, s) is replayed verbatim for a second claim —
// no new signature needed at all
}
Run: forge test --match-test "test_H4_double_claim_same_leaf|test_H4b_original_signature_replays" -vv → both PASS

Recommended Mitigation

if (!MerkleProof.verify(merkleProof, i_merkleRoot, leaf)) {
revert SA__InvalidProof();
}
​
+ if (s_hasClaimedSnowman[receiver]) {
+ revert SA__AlreadyClaimed();
+ }
+
i_snow.safeTransferFrom(receiver, address(this), amount);
s_hasClaimedSnowman[receiver] = true;
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 1 hour 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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