Snowman Merkle Airdrop
AI First Flight #10
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Submission Details
Impact: high
Likelihood: high
Invalid

Unbounded Mint Loop Causes Gas DoS for Large Claim Amounts

fabriziogiannitrapa

Unbounded Mint Loop Causes Gas DoS for Large Claim Amounts

Description

The claimSnowman flow converts 1 Snow token into 1 Snowman NFT. When a user claims, SnowmanAirdrop calls i_snowman.mintSnowman(receiver, amount) where amount equals the user's Snow balance. The mintSnowman function in Snowman.sol iterates in an unbounded loop, performing one ERC721 _safeMint per iteration. Each mint involves storage writes, event emission, and potentially an onERC721Received callback. With ~50k–100k+ gas per mint and a block gas limit of ~30M, users with roughly 300+ Snow tokens will exceed the block gas limit and their claim transaction will revert. They permanently lose the ability to claim their allocation.

// @> Root cause: unbounded loop over amount
function mintSnowman(address receiver, uint256 amount) external {
for (uint256 i = 0; i < amount; i++) { // @> amount can exceed gas limit
_safeMint(receiver, s_TokenCounter);
emit SnowmanMinted(receiver, s_TokenCounter);
s_TokenCounter++;
}
}

Risk

Likelihood (high):

  • Users acquire Snow via buySnow(amount) with arbitrary amounts, or via transfers from others.

  • Merkle tree and snapshot can include large allocations for whales or early participants.

  • No cap on Snow balance or claim amount; large holders are expected in normal operation.

Impact (high):

  • Users with large Snow balances cannot claim; transaction reverts with out-of-gas.

  • Snow is already transferred to the contract before mint; on revert the full tx rolls back, but the user remains permanently unable to claim.

  • Loss of airdrop entitlement for high-value participants.

Severity (high):

Proof of Concept

  1. Alice has 500 Snow (e.g. bought via buySnow or received via transfer).

  2. Alice calls claimSnowman with valid Merkle proof and signature.

  3. Snow is transferred from Alice to the airdrop contract.

  4. mintSnowman(alice, 500) is invoked. The loop runs 500 iterations.

  5. Each _safeMint costs ~50k–100k+ gas. Total gas exceeds block limit (~30M).

  6. Transaction reverts with out-of-gas. Alice cannot claim.

// Scenario: 500 Snow -> 500 mints -> OOG
// ~50k gas per _safeMint -> 500 * 50k = 25M gas (approaching block limit)
// 600+ mints would exceed 30M block gas limit
i_snowman.mintSnowman(alice, 500); // reverts OOG

Recommended Mitigation

Option 1: Use ERC1155 — Replace ERC721 Snowman with ERC1155. Use _mintBatch to mint multiple tokens in a single, gas-efficient operation.

- import {ERC721} from "@openzeppelin/contracts/token/ERC721/ERC721.sol";
+ import {ERC1155} from "@openzeppelin/contracts/token/ERC1155/ERC1155.sol";
​
- for (uint256 i = 0; i < amount; i++) {
- _safeMint(receiver, s_TokenCounter);
- emit SnowmanMinted(receiver, s_TokenCounter);
- s_TokenCounter++;
- }
+ uint256[] memory ids = new uint256[](amount);
+ uint256[] memory amounts = new uint256[](amount);
+ for (uint256 i = 0; i < amount; i++) {
+ ids[i] = s_TokenCounter++;
+ amounts[i] = 1;
+ }
+ _mintBatch(receiver, ids, amounts, "");

Option 2: Batched minting — Allow partial claims. Add a claimAmount parameter and cap per transaction. Users with large allocations claim in multiple transactions.

- function claimSnowman(address receiver, bytes32[] calldata merkleProof, uint8 v, bytes32 r, bytes32 s)
+ function claimSnowman(address receiver, uint256 claimAmount, bytes32[] calldata merkleProof, uint8 v, bytes32 r, bytes32 s)
external
nonReentrant
{
+ uint256 constant MAX_MINT_PER_TX = 100;
+ require(claimAmount <= MAX_MINT_PER_TX, "SA__ExceedsBatchLimit");
// ... verify user has at least claimAmount, update claimed tracking
- i_snowman.mintSnowman(receiver, amount);
+ i_snowman.mintSnowman(receiver, claimAmount);
}

Track claimedSoFar[receiver] and allow multiple claims until the full Merkle allocation is exhausted.

Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 12 hours ago
Submission Judgement Published
Invalidated
Reason: Incorrect statement

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