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

Merkle Leaf Calculation Uses Current Balance Instead of Snapshot

seenu1947

Root + Impact

Description

The claimSnowman() function calculates the Merkle leaf using the user's current Snow token balance at the time of claiming, rather than using a fixed snapshot amount. This means if a user's balance changes after the Merkle tree is generated, their proof becomes invalid and they cannot claim, even though they were legitimately eligible.

The Merkle tree is generated off-chain with specific balance amounts, but the on-chain verification uses balanceOf(receiver) which can change at any time.

// src/SnowmanAirdrop.sol:84-89
function claimSnowman(address receiver, bytes32[] calldata merkleProof, uint8 v, bytes32 r, bytes32 s)
external
nonReentrant
{
// ...
uint256 amount = i_snow.balanceOf(receiver); // @> Uses CURRENT balance
bytes32 leaf = keccak256(bytes.concat(keccak256(abi.encode(receiver, amount))));
if (!MerkleProof.verify(merkleProof, i_merkleRoot, leaf)) {
revert SA__InvalidProof(); // @> Fails if balance changed
}
// ...
}

Risk

Likelihood:

  • Users naturally buy, sell, or transfer Snow tokens

  • Balance changes are common in normal protocol usage

  • No warning to users about balance requirements

  • Time gap between Merkle tree generation and claiming

Impact:

  • Legitimate users cannot claim if their balance changed

  • Users must maintain exact balance from snapshot

  • Poor user experience and confusion

  • May require Merkle tree regeneration

  • Users might miss airdrop window

Proof of Concept

function testClaimFailsAfterBalanceChange() public {
// T0: Merkle tree generated with Alice having 10 Snow tokens
// Merkle leaf = keccak256(keccak256(abi.encode(alice, 10)))
// T1: Alice buys 5 more Snow tokens
vm.prank(alice);
snow.buySnow{value: 5 ether}(5);
assertEq(snow.balanceOf(alice), 15); // Balance changed from 10 to 15
// T2: Alice tries to claim with her original proof
vm.prank(alice);
snow.approve(address(airdrop), 15);
// Claim fails because:
// - Proof is for (alice, 10)
// - Current balance is 15
// - Leaf calculated: keccak256(keccak256(abi.encode(alice, 15)))
// - Proof expects: keccak256(keccak256(abi.encode(alice, 10)))
// - MISMATCH → InvalidProof
vm.expectRevert(SnowmanAirdrop.SA__InvalidProof.selector);
airdrop.claimSnowman(alice, proof, v, r, s);
// Alice cannot claim even though she was eligible!
}

Recommended Mitigation

+ error SA__InsufficientBalance();
​
function claimSnowman(
address receiver,
+ uint256 claimAmount,
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();
}
- if (i_snow.balanceOf(receiver) == 0) {
- revert SA__ZeroAmount();
- }
+ if (i_snow.balanceOf(receiver) < claimAmount) {
+ revert SA__InsufficientBalance();
+ }
- if (!_isValidSignature(receiver, getMessageHash(receiver), v, r, s)) {
+ if (!_isValidSignature(receiver, getMessageHash(receiver, claimAmount), v, r, s)) {
revert SA__InvalidSignature();
}
- uint256 amount = i_snow.balanceOf(receiver);
- bytes32 leaf = keccak256(bytes.concat(keccak256(abi.encode(receiver, amount))));
+ bytes32 leaf = keccak256(bytes.concat(keccak256(abi.encode(receiver, claimAmount))));
if (!MerkleProof.verify(merkleProof, i_merkleRoot, leaf)) {
revert SA__InvalidProof();
}
- i_snow.safeTransferFrom(receiver, address(this), amount);
+ i_snow.safeTransferFrom(receiver, address(this), claimAmount);
s_hasClaimedSnowman[receiver] = true;
- emit SnowmanClaimedSuccessfully(receiver, amount);
+ emit SnowmanClaimedSuccessfully(receiver, claimAmount);
- i_snowman.mintSnowman(receiver, amount);
+ i_snowman.mintSnowman(receiver, claimAmount);
}
- function getMessageHash(address receiver) public view returns (bytes32) {
+ function getMessageHash(address receiver, uint256 amount) public view returns (bytes32) {
- if (i_snow.balanceOf(receiver) == 0) {
- revert SA__ZeroAmount();
- }
-
- uint256 amount = i_snow.balanceOf(receiver);
-
return _hashTypedDataV4(
keccak256(abi.encode(MESSAGE_TYPEHASH, SnowmanClaim({receiver: receiver, amount: amount})))
);
}
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 4 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.&#x20; ```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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