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

Missing `s_hasClaimedSnowman` check allows users to claim airdrop NFTs infinitely by re-acquiring Snow tokens

ashfiexe


Description:

The Snowman contract's mintSnowman() function is intended to be called only by the SnowmanAirdrop contract after a user has passed Merkle proof verification, signature validation, and staked Snow tokens. However, the function has no access control whatsoever — no onlyOwner modifier, no role check, no caller whitelist. Any external address can call mintSnowman() directly to mint unlimited NFTs for free, rendering the entire airdrop mechanism, Merkle tree verification, ECDSA signature checks, and Snow token staking completely useless.

Root cause:

// Snowman.sol
contract Snowman is ERC721, Ownable {
// ...
​
function mintSnowman(address receiver, uint256 amount) external { @> // No access control modifier
for (uint256 i = 0; i < amount; i++) {
_safeMint(receiver, s_TokenCounter); @> // Anyone can mint to any address
emit SnowmanMinted(receiver, s_TokenCounter);
s_TokenCounter++;
}
}
}

Risk**:**

Likelihood:

  • Any user can call the function at any time with no prerequisites

  • No special conditions or edge cases are required — the function is simply unprotected

Impact:

  • Unlimited NFT minting at zero cost, completely devaluing all Snowman NFTs

  • The entire airdrop system (Merkle proofs, signatures, Snow staking) is bypassed

  • Users who paid ETH/WETH for Snow tokens lose all value

Proof of Concept:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.24;
​
import {Test} from "forge-std/Test.sol";
import {Snow} from "../src/Snow.sol";
import {Snowman} from "../src/Snowman.sol";
import {SnowmanAirdrop} from "../src/SnowmanAirdrop.sol";
import {MockWETH} from "../src/mock/MockWETH.sol";
import {Helper} from "../script/Helper.s.sol";
​
contract PoC1_UnauthorizedMint is Test {
Snow snow;
Snowman nft;
SnowmanAirdrop airdrop;
MockWETH weth;
Helper deployer;
​
address attacker = makeAddr("attacker");
​
function setUp() public {
deployer = new Helper();
(airdrop, snow, nft, weth) = deployer.run();
}
​
function testAnyoneCanMintUnlimitedSnowmanNFTs() public {
// Verify attacker starts with nothing
assertEq(nft.balanceOf(attacker), 0);
​
// Attacker calls mintSnowman directly — no Snow, no proof, no signature
vm.prank(attacker);
nft.mintSnowman(attacker, 100);
​
// Attacker now has 100 NFTs, completely bypassing airdrop
assertEq(nft.balanceOf(attacker), 100);
​
// Attacker can mint even more
vm.prank(attacker);
nft.mintSnowman(attacker, 1000);
​
assertEq(nft.balanceOf(attacker), 1100);
}
}

Recommended Mitigation:

function mintSnowman(address receiver, uint256 amount) external onlyOwner {
- function mintSnowman(address receiver, uint256 amount) external {
+ function mintSnowman(address receiver, uint256 amount) external onlyOwner {
for (uint256 i = 0; i < amount; i++) {
_safeMint(receiver, s_TokenCounter);
emit SnowmanMinted(receiver, s_TokenCounter);
s_TokenCounter++;
}
}

Additionally, in the deploy script, transfer ownership of Snowman to the SnowmanAirdrop contract so only it can call mintSnowman:

// After deploying airdrop:
snowman.transferOwnership(address(airdrop));
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 2 hours 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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