Root + Impact

Description

• Normal behavior: Sellers might want to reduce the amount of their sell orders, triggering a partial refund. This should adjust internal state immediately so any callbacks from the refund see the correct, updated contract data.

• Issue: The contract refunds the token difference before updating order.amountToSell. This sequencing allows a re-entrancy exploit: if the refunded token is ERC777 or has a malicious fallback, the contract can be called again while it still reflects the outdated order state.

Risk

Likelihood:

• Triggered whenever a seller reduces their order and the refund uses a token with a callback or fallback capability.

• This is realistic in systems interacting with modern ERC standards or maliciously crafted tokens.

Impact:

• The seller could cancel the order or perform other manipulations during the callback, resulting in multiple refunds or inconsistent order data.

• Can lead to duplicated payouts or protocol accounting failures.

Proof of Concept

This PoC illustrates how a malicious token’s callback cancels the order while the contract still thinks the seller is entitled to a refund, creating multiple outflows from a single operation. This breaks the internal accounting assumptions of the contract.

Recommended Mitigation

By moving the state change before the external call, the contract ensures that any callback sees the updated state, closing off re-entrancy avenues. This preserves atomic operation integrity, a foundational Solidity design principle.