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

Title: Merkle Leaf Uses Live Token Balance Instead of Snapshot — Claims Can Be Permanently Blocked

harmonygemini8

Description

The airdrop is built around a Merkle tree whose leaves are computed off-chain from a snapshot: for each receiver, leaf = keccak256(keccak256(abi.encode(receiver, snapshotAmount))). However, both claimSnowman() and getMessageHash() read i_snow.balanceOf(receiver) at the moment the transaction executes, instead of using the snapshot amount encoded in the tree.

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);
...
i_snowman.mintSnowman(receiver, amount);

The signature digest is built the same way in getMessageHash():

uint256 amount = i_snow.balanceOf(receiver);
return _hashTypedDataV4(keccak256(abi.encode(MESSAGE_TYPEHASH, SnowmanClaim({receiver: receiver, amount: amount}))));

Because the Merkle tree was generated with the snapshot amount (1 per user in the provided setup), any change to the receiver's balance between snapshot and claim makes the computed leaf fail verification, or makes the signature digest differ from what was signed.

A malicious actor can exploit this to permanently grief an eligible user: sending 1 wei of Snow to the victim changes the victim's balance, so the victim can never claim. Even a fresh signature over the new balance fails the Merkle proof, because the new amount is not present in the tree.

Risk

Impact: Medium — eligible users permanently lose their airdrop entitlement (both the NFT and the Snow tokens that would have been transferred). The airdrop becomes unusable for anyone whose balance changes after the snapshot.

Likelihood: High — Snow is an actively-trading ERC20 (earned weekly, bought with ETH/WETH, transferable), so balance changes are common, and the grief is trivial to trigger with 1 wei.

Proof of Concept

The test below shows Alice signing over her snapshot balance (1), then Bob sending her 1 wei. The claim now reverts with SA__InvalidSignature because getMessageHash() returns a digest over the changed balance:

function testPoc3_BalanceChangeBreaksClaim() public {
uint256 amount = snow.balanceOf(alice);
vm.prank(alice);
snow.approve(address(airdrop), amount);
​
bytes32 digest = airdrop.getMessageHash(alice);
(uint8 v, bytes32 r, bytes32 s) = vm.sign(alKey, digest);
​
vm.prank(bob);
snow.transfer(alice, 1);
​
vm.expectRevert(abi.encodeWithSelector(SnowmanAirdrop.SA__InvalidSignature.selector));
vm.prank(bob);
airdrop.claimSnowman(alice, AL_PROOF, v, r, s);
}

Test result: [PASS] testPoc3_BalanceChangeBreaksClaim — the claim reverts after a 1-wei balance change.

Recommended Mitigation

Use the fixed snapshot amount in both the Merkle leaf and the message hash. Pass amount as an explicit parameter (validated against the leaf) instead of reading balanceOf():

function claimSnowman(address receiver, uint256 amount, bytes32[] calldata merkleProof, uint8 v, bytes32 r, bytes32 s) external nonReentrant {
if (s_hasClaimedSnowman[receiver]) revert AlreadyClaimed();
bytes32 leaf = keccak256(bytes.concat(keccak256(abi.encode(receiver, amount))));
if (!MerkleProof.verify(merkleProof, i_merkleRoot, leaf)) revert SA__InvalidProof();
if (!_isValidSignature(receiver, getMessageHash(receiver, amount), v, r, s)) revert SA__InvalidSignature();
i_snow.safeTransferFrom(receiver, address(this), amount);
s_hasClaimedSnowman[receiver] = true;
...
}
​
function getMessageHash(address receiver, uint256 amount) public view returns (bytes32) {
return _hashTypedDataV4(keccak256(abi.encode(MESSAGE_TYPEHASH, SnowmanClaim({receiver: receiver, amount: amount}))));
}
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge 1 day ago
Submission Judgement Published
Validated
Assigned finding tags:

[M-01] DoS to a user trying to claim a Snowman

# Root + Impact ## Description * Users will approve a specific amount of Snow to the SnowmanAirdrop and also sign a message with their address and that same amount, in order to be able to claim the NFT * Because the current amount of Snow owned by the user is used in the verification, an attacker could forcefully send Snow to the receiver in a front-running attack, to prevent the receiver from claiming the NFT.  ```Solidity function getMessageHash(address receiver) public view returns (bytes32) { ... // @audit HIGH An attacker could send 1 wei of Snow token to the receiver and invalidate the signature, causing the receiver to never be able to claim their Snowman uint256 amount = i_snow.balanceOf(receiver); return _hashTypedDataV4( keccak256(abi.encode(MESSAGE_TYPEHASH, SnowmanClaim({receiver: receiver, amount: amount}))) ); ``` ## Risk **Likelihood**: * The attacker must purchase Snow and forcefully send it to the receiver in a front-running attack, so the likelihood is Medium **Impact**: * The impact is High as it could lock out the receiver from claiming forever ## Proof of Concept The attack consists on Bob sending an extra Snow token to Alice before Satoshi claims the NFT on behalf of Alice. To showcase the risk, the extra Snow is earned for free by Bob. ```Solidity function testDoSClaimSnowman() public { assert(snow.balanceOf(alice) == 1); // Get alice's digest while the amount is still 1 bytes32 alDigest = airdrop.getMessageHash(alice); // alice signs a message (uint8 alV, bytes32 alR, bytes32 alS) = vm.sign(alKey, alDigest); vm.startPrank(bob); vm.warp(block.timestamp + 1 weeks); snow.earnSnow(); assert(snow.balanceOf(bob) == 2); snow.transfer(alice, 1); // Alice claim test assert(snow.balanceOf(alice) == 2); vm.startPrank(alice); snow.approve(address(airdrop), 1); // satoshi calls claims on behalf of alice using her signed message vm.startPrank(satoshi); vm.expectRevert(); airdrop.claimSnowman(alice, AL_PROOF, alV, alR, alS); } ``` ## Recommended Mitigation Include the amount to be claimed in both `getMessageHash` and `claimSnowman` instead of reading it from the Snow contract. Showing only the new code in the section below ```Python function claimSnowman(address receiver, uint256 amount, bytes32[] calldata merkleProof, uint8 v, bytes32 r, bytes32 s) external nonReentrant { ... bytes32 leaf = keccak256(bytes.concat(keccak256(abi.encode(receiver, amount)))); if (!MerkleProof.verify(merkleProof, i_merkleRoot, leaf)) { revert SA__InvalidProof(); } // @audit LOW Seems like using the ERC20 permit here would allow for both the delegation of the claim and the transfer of the Snow tokens in one transaction i_snow.safeTransferFrom(receiver, address(this), amount); // send ... } ```

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