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

Missing check on s_hasClaimedSnowman in SnowmanAirdrop::claimSnowman allows free double-claiming via Snow::earnSnow

adri

Root + Impact

Description

  • bloc "Root cause"

function claimSnowman(address receiver, bytes32[] calldata merkleProof, uint8 v, bytes32 r, bytes32 s)
external
nonReentrant
{
...
// @> amount is recomputed live from current balance, never fixed at proof-generation time
uint256 amount = i_snow.balanceOf(receiver);
...
i_snow.safeTransferFrom(receiver, address(this), amount);
// @> s_hasClaimedSnowman is written here but never checked anywhere before this point
s_hasClaimedSnowman[receiver] = true;
...
i_snowman.mintSnowman(receiver, amount);
}

Risk

Likelihood:

  • Any address already eligible in the Merkle tree claims once, then calls Snow::earnSnow() (free, gated only by a global weekly timer) to rebuild the exact same Snow balance as their original leaf

  • The same Merkle proof and a freshly signed message (using the same balance-derived amount) are accepted again by claimSnowman, since s_hasClaimedSnowman is never read

  • This repeats indefinitely, once per cooldown window, with no upper bound

Impact:

  • Unlimited additional Snowman NFTs per eligible address over time, without ever purchasing Snow

  • Breaks the core "1 Snow held = 1 Snowman claimable once" invariant described for the protocol

Proof of Concept

This test performs a legitimate first claim for "alice" using the official test fixture,
then has her call the free Snow::earnSnow() after the weekly cooldown to rebuild the exact
Snow balance her original Merkle leaf encodes. She signs a new message for the same amount
and calls claimSnowman again with the same Merkle proof. The call succeeds and she receives
a second Snowman NFT.

function test_DoubleClaimViaFreeEarnSnow() public {
vm.prank(alice);
snow.approve(address(airdrop), 1);
bytes32 alDigest = airdrop.getMessageHash(alice);
(uint8 alV, bytes32 alR, bytes32 alS) = vm.sign(alKey, alDigest);
airdrop.claimSnowman(alice, AL_PROOF, alV, alR, alS);
assertEq(nft.balanceOf(alice), 1);
​
vm.warp(block.timestamp + 1 weeks + 1);
vm.prank(alice);
snow.earnSnow(); // free, resets alice's balance to match her original leaf
​
bytes32 alDigest2 = airdrop.getMessageHash(alice);
(uint8 alV2, bytes32 alR2, bytes32 alS2) = vm.sign(alKey, alDigest2);
vm.prank(alice);
snow.approve(address(airdrop), 1);
airdrop.claimSnowman(alice, AL_PROOF, alV2, alR2, alS2); // does not revert
​
assertEq(nft.balanceOf(alice), 2); // second NFT obtained with the same Merkle proof
}

Recommended Mitigation

Check s_hasClaimedSnowman before processing the claim, and keep the existing write as the
flag that prevents any subsequent attempt, regardless of the caller's current Snow balance.

+ 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();
}
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 22 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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