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

[H-3] claimSnowman() has no checks to ensure that an address has already claimed and a person can claim twice

soarinskysagar

[H-3] SnowmanAirdrop::claimSnowman() has no checks to ensure that an address has already claimed and a person can claim twice

Description

  • Airdrop contract uses markle root to check for eligibility

  • However, address remains eligible even if person has claimed once, since there are no other checks

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:

  • very high

Impact:

  • Users claiming more than once breaks invariants

Proof of Concept

Add the following test case to the test suite of SnowmanAirdrop. The contract has a merkle hash where alice's wallet with 1 Snow token is eligible for minting the airdrop. Therefore, we are minting snowman once. Then, Alice's wallet address becomes 0. However, we are transferring Alice another 1 Snow token and thus she again becomes eligible for the airdrop and can claim an NFT, as proved by the test

function test_claimAgain() public {
assert(nft.balanceOf(alice) == 0);
vm.prank(alice);
snow.approve(address(airdrop), 2);
​
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(nft.ownerOf(0) == alice);
​
vm.prank(bob);
snow.transfer(alice, 1);
​
vm.prank(satoshi);
airdrop.claimSnowman(alice, AL_PROOF, alV, alR, alS);
​
assert(nft.balanceOf(alice) == 2);
assert(nft.ownerOf(1) == alice);
}

Recommended Mitigation

Add a mapping from address to bool which keeps track of the people who have claimed the token, and add a check using the mapping to prevent people from claiming again

+ mapping(address=>bool) public hasClaimed;
.
.
.
function claimSnowman(address receiver, bytes32[] calldata merkleProof, uint8 v, bytes32 r, bytes32 s)
external
nonReentrant
{
+ assert(hasClaimed[msg.sender] != true)
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();
}
+ hasClaimed[msg.sender] = 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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