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

SnowmanAirdrop::claimSnowman never reads s_hasClaimedSnowman, letting eligible users claim repeatedly

duranovskyy

Root + Impact

SnowmanAirdrop::claimSnowman writes s_hasClaimedSnowman[receiver] = true but never reads it, so there is no guard against a repeat claim. An address in the Merkle set can claim more than once and mint Snowman NFTs beyond its allocation.

Description

  • Each eligible address is meant to claim its Snowman NFTs exactly once; the contract keeps an s_hasClaimedSnowman flag for this.

  • The flag is set during a claim but is never checked in claimSnowman. The only thing that blocks an immediate repeat is that the receiver's Snow is pulled to zero — so once the receiver restores their Snow to the exact amount in their Merkle leaf, the claim succeeds again.

function claimSnowman(address receiver, bytes32[] calldata merkleProof, uint8 v, bytes32 r, bytes32 s)
external
nonReentrant
{
@> // missing: no check of s_hasClaimedSnowman[receiver] — nothing blocks a repeat claim
...
i_snow.safeTransferFrom(receiver, address(this), amount);
@> s_hasClaimedSnowman[receiver] = true; // written, but never read anywhere
...
i_snowman.mintSnowman(receiver, amount);
}

Risk

Likelihood:

  • A user restores their Snow balance to the exact amount in their Merkle leaf (Snow is freely obtainable) and calls claimSnowman again; the missing flag check lets the call through.

Impact:

  • An address in the Merkle set can claim repeatedly, minting Snowman NFTs beyond its one-time allocation and inflating supply.

  • Honest one-time claimers are diluted.

Proof of Concept

test/snow/AirdropDoubleClaim.t.sol — a claimer in a single-leaf tree claims, refills Snow to the exact leaf amount, and claims again. Snowman balance goes 10 → 20, [PASS].

function test_claimerCanClaimTwice() public {
snow.mint(claimer, AMOUNT);
vm.prank(claimer);
snow.approve(address(airdrop), type(uint256).max);
_claim(); // Snowman: 10
​
snow.mint(claimer, AMOUNT); // restore exactly AMOUNT
_claim(); // second claim NOT blocked
assertEq(snowman.balanceOf(claimer), 2 * AMOUNT); // 20
}

Recommended Mitigation

Check the flag at the start of claimSnowman (and declare 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();
+ }
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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