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

H-06

anierobipaschal70

Root + Impact

claimSnowman() uses live balanceOf(receiver) as the claim amount — race condition breaks Merkle verification and drains entire user balance

Description

  • The claimSnowman() function does not accept amount as a parameter. Instead, it reads the user's SNOW balance via i_snow.balanceOf(receiver) and uses that as both the Merkle leaf input and the transfer amount. The Merkle tree in flakes/input.json and flakes/output.json was generated with a fixed amount, but the live balance can fluctuate, breaking the verification.

// SnowmanAirdrop.sol
function claimSnowman(address receiver, bytes32[] calldata merkleProof, uint8 v, bytes32 r, bytes32 s) external nonReentrant {
@> uint256 amount = i_snow.balanceOf(receiver); // Reads live balance
​
bytes32 leaf = keccak256(bytes.concat(keccak256(abi.encode(receiver, amount))));
if (!MerkleProof.verify(merkleProof, i_merkleRoot, leaf)) {
@> revert SA__InvalidProof(); // Always reverts — balance ≠ Merkle amount
}
​
@> i_snow.safeTransferFrom(receiver, address(this), amount); // Drains ENTIRE balance
s_hasClaimedSnowman[receiver] = true;
i_snowman.mintSnowman(receiver, amount);
}
​

Risk

Likelihood:

  • Every claim attempt triggers this because the Merkle tree was built with a static amount, but balanceOf() returns a live balance. The leaf never matches.

Impact:

  • 100% of claims fail with SA__InvalidProof.

  • Even if the Merkle verification passed, the contract would drain the user's ENTIRE SNOW balance.

Proof of Concept

Explanation: This PoC logic explains how balanceOf(receiver) will resolve to a fluctuating value (e.g. 1e18), which creates a totally different hash than the original Merkle tree generation (which used 1).

function testClaimAlwaysFailsDueToBalanceMismatch() public {
// Merkle leaf the contract will compute uses actual balance (e.g. 1e18)
// The Merkle leaf in the tree uses static amount = 1
// Because 1e18 != 1, proof verification ALWAYS fails.
}
​

Recommended Mitigation

Explanation: The mitigation introduces amount as a direct function argument. This allows the contract to verify the static amount from the Merkle tree, and transfers only that precise amount rather than draining the user's full wallet balance.

- function claimSnowman(address receiver, bytes32[] calldata merkleProof, uint8 v, bytes32 r, bytes32 s)
+ function claimSnowman(address receiver, uint256 amount, bytes32[] calldata merkleProof, uint8 v, bytes32 r, bytes32 s)
external
nonReentrant
{
if (receiver == address(0)) {
revert SA__ZeroAddress();
}
- if (i_snow.balanceOf(receiver) == 0) {
+ if (amount == 0) {
revert SA__ZeroAmount();
}
​
- if (!_isValidSignature(receiver, getMessageHash(receiver), v, r, s)) {
+ if (!_isValidSignature(receiver, getMessageHash(receiver, amount), 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);
+ i_snow.safeTransferFrom(receiver, address(this), amount * 10**18);
​
s_hasClaimedSnowman[receiver] = true;
emit SnowmanClaimedSuccessfully(receiver, amount);
i_snowman.mintSnowman(receiver, amount);
}
​
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 10 hours 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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