Snowman Merkle Airdrop
Findings
Critical
[C-1] Snowman::mintSnowman has no access control
Description: mintSnowman is external with no caller restriction, so it isn't limited to the SnowmanAirdrop contract.
Risk: Likelihood: High — trivially callable by anyone. Impact: High — unlimited free NFT minting, bypassing all airdrop logic.
Proof of Concept:
solidity
Recommended Mitigation:
solidity
[C-2] Snow::s_earnTimer is global, not per-user
Description: s_earnTimer is a single uint256 shared by all users instead of tracked per address.
Risk: Likelihood: High — triggered by normal use. Impact: High — protocol-wide denial of service on Snow farming; only one address can earn per week.
Proof of Concept:
solidity
Recommended Mitigation:
solidity
[C-3] claimSnowman never checks s_hasClaimedSnowman
Description: The mapping is written but never read/required to be false before minting proceeds.
Risk: Likelihood: High — repeatable by any claimant. Impact: High — unbounded re-claims inflate NFT supply beyond intended allocation.
Proof of Concept:
solidity
Recommended Mitigation:
solidity
High
[H-1] Incorrect EIP-712 type string breaks signature verification
Description: MESSAGE_TYPEHASH uses "addres" instead of "address".
Risk: Likelihood: High — affects every standard wallet signature. Impact: High — legitimate claims fail verification.
Proof of Concept: Signing via ethers/viem with the correct address type produces a typehash that will never match the on-chain constant, so _isValidSignature always fails for standard-signed messages.
Recommended Mitigation:
solidity
[H-2] Signature/proof validated against a live, mutable balanceOf
Description: amount is derived from i_snow.balanceOf(receiver) at call time rather than a fixed, signed value.
Risk: Likelihood: Medium — requires an active griefer or balance drift since signing/tree snapshot. Impact: High — permanently blocks a targeted user's claim; breaks proofs generally.
Proof of Concept:
solidity
Recommended Mitigation: Pass amount as an explicit parameter, sign/prove against it, and check balanceOf(receiver) >= amount.
Medium
[M-1] buySnow keeps mismatched ETH with no refund
Description: Any msg.value not exactly equal to the fee falls into the WETH branch, which still pulls WETH and keeps the ETH sent.
Risk: Likelihood: Medium — any imprecise payment. Impact: Medium — user funds stuck, recoverable only by the collector.
Proof of Concept: Call buySnow{value: 1}(1) when the WETH fee path is taken — the 1 wei ETH is retained with no refund or revert.
Recommended Mitigation:
solidity
[M-2] Unbounded mint loop risks gas-limit DoS
Description: amount = balanceOf(receiver) feeds an uncapped loop of _safeMint calls.
Risk: Likelihood: Low-Medium — requires a large accumulated balance. Impact: Medium — claim permanently reverts for that user.
Proof of Concept: A receiver with thousands of Snow calls claimSnowman, exceeding block gas limit.
Recommended Mitigation: Cap mintable amount per claim or batch more efficiently.
[M-3] collectFee bypasses SafeERC20
Description: Uses raw i_weth.transfer instead of safeTransfer, ignoring the return value.
Risk: Likelihood: Low — depends on WETH implementation. Impact: Medium — silent transfer failure possible with non-standard tokens.
Proof of Concept: N/A — implementation-dependent; flagged as a best-practice violation given SafeERC20 is already imported and used elsewhere.
Recommended Mitigation:
solidity
[M-4] Claimed Snow tokens are permanently locked
Description: Tokens transferred to SnowmanAirdrop on claim are never burned or made recoverable.
Risk: Likelihood: High — occurs on every claim. Impact: Low-Medium — funds locked, not lost to an attacker, but unrecoverable as designed.
Proof of Concept: N/A — direct consequence of the current transfer-only implementation.
Recommended Mitigation: Add a real _burn call, or an owner-controlled rescue function.
Low
[L-1]
Snowman::tokenURI's zero-address check is unreachable dead code under OZ v5 (ownerOfreverts first).[L-2]
SM__NotAllowedis declared but unused — wire into the [C-1] fix.[L-3]
buySnow(0)succeeds silently; add a zero-amount guard.[L-4]
s_buyFeeis never updated post-construction — markimmutable.[L-5] No zero-check on
_merkleRootin the constructor.[L-6]
buySnowperforms an external call before updatings_earnTimer— tighten CEI ordering.
Lead Judging Commences
[H-01] Unrestricted NFT Minting in Snowman.sol
# Root + Impact ## Description * The Snowman NFT contract is designed to mint NFTs through a controlled airdrop mechanism where only authorized entities should be able to create new tokens for eligible recipients. * The `mintSnowman()` function lacks any access control mechanisms, allowing any external address to call the function and mint unlimited NFTs to any recipient without authorization, completely bypassing the intended airdrop distribution model. ```Solidity // Root cause in the codebase function mintSnowman(address receiver, uint256 amount) external { @> // NO ACCESS CONTROL - Any address can call this function for (uint256 i = 0; i < amount; i++) { _safeMint(receiver, s_TokenCounter); emit SnowmanMinted(receiver, s_TokenCounter); s_TokenCounter++; } @> // NO VALIDATION - No checks on amount or caller authorization } ``` ## Risk **Likelihood**: * The vulnerability will be exploited as soon as any malicious actor discovers the contract address, since the function is publicly accessible with no restrictions * Automated scanning tools and MEV bots continuously monitor new contract deployments for exploitable functions, making discovery inevitable **Impact**: * Complete destruction of tokenomics through unlimited supply inflation, rendering all legitimate NFTs worthless * Total compromise of the airdrop mechanism, allowing attackers to mint millions of tokens and undermine the project's credibility and economic model ## Proof of Concept ```Solidity // SPDX-License-Identifier: MIT pragma solidity ^0.8.24; import {Test, console2} from "forge-std/Test.sol"; import {Snowman} from "../src/Snowman.sol"; contract SnowmanExploitPoC is Test { Snowman public snowman; address public attacker = makeAddr("attacker"); string constant SVG_URI = "data:image/svg+xml;base64,PHN2Zy4uLi4+"; function setUp() public { snowman = new Snowman(SVG_URI); } function testExploit_UnrestrictedMinting() public { console2.log("=== UNRESTRICTED MINTING EXPLOIT ==="); console2.log("Initial token counter:", snowman.getTokenCounter()); console2.log("Attacker balance before:", snowman.balanceOf(attacker)); // EXPLOIT: Anyone can mint unlimited NFTs vm.prank(attacker); snowman.mintSnowman(attacker, 1000); // Mint 1K NFTs console2.log("Final token counter:", snowman.getTokenCounter()); console2.log("Attacker balance after:", snowman.balanceOf(attacker)); // Verify exploit success assertEq(snowman.balanceOf(attacker), 1000); assertEq(snowman.getTokenCounter(), 1000); console2.log(" EXPLOIT SUCCESSFUL - Minted 1K NFTs without authorization"); } } ``` <br /> PoC Results: ```Solidity forge test --match-test testExploit_UnrestrictedMinting -vv [⠑] Compiling... [⠢] Compiling 1 files with Solc 0.8.29 [⠰] Solc 0.8.29 finished in 1.45s Compiler run successful! Ran 1 test for test/SnowmanExploitPoC.t.sol:SnowmanExploitPoC [PASS] testExploit_UnrestrictedMinting() (gas: 26868041) Logs: === UNRESTRICTED MINTING EXPLOIT === Initial token counter: 0 Attacker balance before: 0 Final token counter: 1000 Attacker balance after: 1000 EXPLOIT SUCCESSFUL - Minted 1K NFTs without authorization Suite result: ok. 1 passed; 0 failed; 0 skipped; finished in 4.28ms (3.58ms CPU time) Ran 1 test suite in 10.15ms (4.28ms CPU time): 1 tests passed, 0 failed, 0 skipped (1 total tests) ``` ## Recommended Mitigation Adding the `onlyOwner` modifier restricts the `mintSnowman()` function to only be callable by the contract owner, preventing unauthorized addresses from minting NFTs. ```diff - 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++; } } ```
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