Missing Double-Claim Check Allows Repeated Claims via s_hasClaimedSnowman
Description
claimSnowman() writes s_hasClaimedSnowman[receiver] = true after a successful claim, but never reads that mapping anywhere to block a repeat claim:
The only thing preventing an immediate re-claim is the earlier check i_snow.balanceOf(receiver) == 0 (which reverts SA__ZeroAmount()). That guard passes right after a claim, since the claim itself transfers the receiver's entire Snow balance away — leaving balance at 0. But amount and the digest/leaf used for verification are both recomputed live from current balance at call time, not pinned to a one-time allocation. If the receiver's Snow balance becomes non-zero again, they satisfy every check a second time.
Risk
Likelihood: Medium. This isn't a single-transaction exploit — it requires the receiver to first complete a legitimate claim, then bring their Snow balance back to a nonzero amount, which means either waiting out the 1-week earnSnow() cooldown or calling buySnow() again. It's not trivial, but it's well within reach of any user willing to wait a week or spend more Snow, and requires no special privileges or race conditions.
Impact: High. This breaks the airdrop's core invariant — each Merkle-committed allocation should be claimable exactly once. A user (or the same address controlled by an attacker) can repeatedly re-claim and mint additional Snowman NFTs beyond what the Merkle tree allocated them, directly inflating NFT supply and diluting the value of every legitimate holder's allocation.
Proof of Concept
Result: PASS. Alice ends up with 2 NFTs despite s_hasClaimedSnowman[alice] being true after her first claim — the flag exists in storage but enforces nothing.
Recommended Mitigation
Add this check at the top of claimSnowman(), before any transfer or mint logic executes.
Lead Judging Commences
[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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