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

Missing Signature Replay Protection (Nonce Mechanism Absent)

hyer

Root + Impact

Description

  • The getMessageHash function currently constructs the EIP-712 message hash using only the receiver's address and the amount of tokens.

  • It does not include any form of replay protection parameters such as a nonce, contract address, or expiry time.

  • As a result, a valid signature signed by a user can be reused multiple times by anyone else to claim NFTs on behalf of the user, enabling replay attacks.

// Relevant part showing lack of nonce or other replay protection:
function getMessageHash(address receiver) 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})))
);
}
​

Risk

Likelihood:

  • Any attacker observing a valid signature from a user can reuse the signature multiple times to claim NFTs on the user's behalf.

  • This can happen anytime a user signs the message and shares the signature externally or via the transaction itself.

Impact:

  • Unauthorized repeated claims of Snowman NFTs for the same user, leading to token distribution abuse.

  • Users lose control over their claim signatures and cannot revoke or invalidate them.

Recommended Mitigation

  • Add a nonce parameter to the SnowmanClaim struct:

struct SnowmanClaim {
address receiver;
uint256 amount;
uint256 nonce;
}
​

  • Add a mapping(address => uint256) private s_nonces; to track the nonce for each user.

  • Include the nonce in the message hash calculation:

uint256 currentNonce = s_nonces[receiver];
bytes32 digest = _hashTypedDataV4(
keccak256(abi.encode(MESSAGE_TYPEHASH, receiver, amount, currentNonce))
);

  • After each successful claim, increment the nonce to prevent signature reuse:

s_nonces[receiver] = currentNonce + 1;
Updates

Lead Judging Commences

yeahchibyke Lead Judge 5 days 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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