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

SnowmanAirdrop never checks s_hasClaimedSnowman - one signature can be replayed indefinitely for extra NFTs

zzzz

Root + Impact

Description

  • Normal behavior: s_hasClaimedSnowman exists to guarantee each address claims its Merkle-tree allocation exactly once.

  • claimSnowman writes the flag but never reads it, so the same signature and proof can be resubmitted forever, by anyone, whenever the receiver's balance returns to the committed amount.

// src/SnowmanAirdrop.sol
i_snow.safeTransferFrom(receiver, address(this), amount);
@> s_hasClaimedSnowman[receiver] = true; // written here, never read anywhere else in the file
emit SnowmanClaimedSuccessfully(receiver, amount);
i_snowman.mintSnowman(receiver, amount);

amount is re-derived from live balanceOf, not fixed at signing time, and v/r/s/the proof are plain calldata visible on-chain after the first claim — anyone can resubmit them.

Risk

Likelihood:

  • All 5 real fixture addresses have a snapshotted amount of exactly 1 — the same amount earnSnow() mints for free. Any claimant refills to that amount using only the ordinary free weekly mint, no second wallet or capital needed, then resubmits their own original signature.

  • No unusual precondition or timing coincidence required.

Impact:

  • The "claim once" guarantee is fully void — supply is bounded only by how many times Snow can be cycled through the function, not by the Merkle allocations.

  • A receiver who signs one delegated authorization loses control over how many times it's used, since anyone who saw the first transaction can replay it without further consent.

Proof of Concept

// test/PoC_ClaimReplay.t.sol
function test_ZeroCapitalReplayUsingOnlyTheOrdinaryFreeWeeklyEarn() public {
vm.prank(receiver);
freshSnow.earnSnow(); // becomes eligible the ordinary way
​
vm.prank(receiver);
freshSnow.approve(address(freshAirdrop), type(uint256).max);
bytes32 digest = freshAirdrop.getMessageHash(receiver);
(uint8 v, bytes32 r, bytes32 s) = vm.sign(receiverKey, digest);
vm.prank(relayer);
freshAirdrop.claimSnowman(receiver, EMPTY_PROOF, v, r, s); // 1st claim
assertEq(nft.balanceOf(receiver), 1);
​
vm.warp(block.timestamp + 1 weeks);
vm.prank(receiver);
freshSnow.earnSnow(); // refills to the committed amount, for free
​
vm.prank(relayer);
freshAirdrop.claimSnowman(receiver, EMPTY_PROOF, v, r, s); // SAME v,r,s, 2nd claim
assertEq(nft.balanceOf(receiver), 2);
}

forge test -vvvv shows the identical v=27, r=0x5773d1c5…, s=0x0a9a3330… accepted twice:

[PASS] test_ZeroCapitalReplayUsingOnlyTheOrdinaryFreeWeeklyEarn() (gas: 3428048)
Suite result: ok. 1 passed; 0 failed; 0 skipped

Recommended Mitigation

Check the flag the contract already maintains for exactly this purpose, first thing in the function:

+ 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();
...
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 3 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; } ```

Support

FAQs

Can't find an answer? Chat with us on Discord, Twitter or Linkedin.

Give us feedback!