Snowman Merkle Airdrop

Root + Impact

Description

• The contract imports SafeERC20 and uses safeTransferFrom in buySnow, but collectFee uses raw i_weth.transfer and discards the return value.

• The function also forwards the entire contract ETH balance, which can include ETH force-pushed via selfdestruct or trapped overpay from H-03 — bypassing any intended user refund.

Risk

Likelihood:

• Reason 1: When the fee token is swapped to a non-canonical WETH-like token that returns false on failure, funds silently stay in the Snow contract.

• Reason 2: With H-03 unfixed, every overpaid user's ETH is harvested by the collector rather than refunded.

Impact:

• Impact 1: Silent loss of WETH if a non-standard token is used.

• Impact 2: User overpay funds are absorbed by the protocol with no refund path.

Proof of Concept

The PoC demonstrates the force-push variant, which is the simplest to reproduce. A throwaway contract SelfDestructor is funded with 1 ETH and then self-destructs, sending its balance to the Snow contract — selfdestruct bypasses any receive() / fallback() checks, so this is unstoppable. When the legitimate collector later calls collectFee, the function forwards address(this).balance (which includes the force-pushed wei) to the collector. The assert that the collector received exactly 1 ETH proves the sweep semantics: the protocol cannot distinguish between fee revenue and arbitrarily injected ETH, so any future refund logic for the H-03 overpay case has nothing left to refund from.

Recommended Mitigation

The fix is twofold: switch to safeTransfer so any non-canonical WETH variant surfaces failures instead of swallowing them, and track ETH revenue in an explicit state variable so collectFee only forwards what the protocol actually accumulated through buySnow's exact-match path. The tracked variable is incremented only on the legitimate ETH-payment path, so force-pushed ETH and any H-03 overpay remain in the contract and can be refunded under a separate flow (or returned to users explicitly).