Snowman Merkle Airdrop
First Flight #42
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Submission Details
Severity: low
Valid

Missing Claim Status Check Allows Multiple Claims in SnowmanAirdrop.sol::claimSnowman

whitehacker

Root + Impact

Root: The claimSnowman 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

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.

// 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;
}
Updates

Lead Judging Commences

yeahchibyke Lead Judge 5 months ago
Submission Judgement Published
Validated
Assigned finding tags:

Lack of claim check

The claim function of the Snowman Airdrop contract doesn't check that a recipient has already claimed a Snowman. This poses no significant risk as is as farming period must have been long concluded before snapshot, creation of merkle script, and finally claiming.

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