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

`SnowmanAirdrop::claimSnowman` never reads `s_hasClaimedSnowman` — the one-time-claim guard is dead code (repeat claims)

hopeh

Description

The airdrop is meant to be claimable only once per recipient. claimSnowman records that a receiver has claimed by writing s_hasClaimedSnowman[receiver] = true, and the contract even exposes getClaimStatus to read that flag — so a one-time guard is clearly the intent.

However, claimSnowman never reads s_hasClaimedSnowman before processing a claim. There is no if (s_hasClaimedSnowman[receiver]) revert ... anywhere, so the flag is pure dead code and the one-time restriction is never enforced. The Merkle leaf and the signature are both keyed only on (receiver, amount), so whenever the receiver's balance equals the committed amount again, the exact same proof and signature re-validate and another mint occurs.

// SnowmanAirdrop.sol
function claimSnowman(address receiver, bytes32[] calldata merkleProof, uint8 v, bytes32 r, bytes32 s)
external
nonReentrant
{
// @> no `if (s_hasClaimedSnowman[receiver]) revert;` check here
// ... signature + merkle checks on (receiver, amount) ...
i_snow.safeTransferFrom(receiver, address(this), amount);
s_hasClaimedSnowman[receiver] = true; // @> written, but never read
i_snowman.mintSnowman(receiver, amount);
}

Risk

Likelihood: Medium

  • The guard is unconditionally bypassed; the only precondition for a repeat is that balanceOf(receiver) == amount again (the value committed in the Merkle tree).

  • Because claims can be submitted on a receiver's behalf with their stored (v, r, s), anyone can top the receiver's balance back up to amount (a plain Snow transfer) and re-submit the same proof + signature — no cooperation from the receiver is required.

Impact: High

  • The one-time airdrop becomes repeatable: the sanctioned distribution can be exceeded and Snowman NFTs over-minted arbitrarily many times for the same allocation, breaking the airdrop's accounting.

Proof of Concept

function test_claimCanBeReplayed() public {
// 1) First (legitimate) claim succeeds; receiver's Snow is pulled into the airdrop.
vm.prank(anyone);
airdrop.claimSnowman(receiver, proof, v, r, s);
assertEq(snowman.balanceOf(receiver), amount);
​
// 2) Restore receiver's balance to the committed `amount` (e.g. a transfer from a whale).
vm.prank(whale);
snow.transfer(receiver, amount);
​
// 3) The SAME proof + signature validate again -> a second mint, no revert.
vm.prank(anyone);
airdrop.claimSnowman(receiver, proof, v, r, s); // @> succeeds again
assertGt(snowman.balanceOf(receiver), amount);
}

The second claimSnowman does not revert, proving the one-time guard is not enforced and the claim can be replayed.

Recommended Mitigation

Read the flag at the start of the function and revert on a repeat (and add the missing 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(); // @> enforce once
// ... existing checks ...
s_hasClaimedSnowman[receiver] = true;
i_snowman.mintSnowman(receiver, amount);
}
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge 7 days 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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