Monero, a leading privacy-oriented cryptocurrency, supports a client/server operating mode that allows lightweight clients to avoid storing the entire blockchain, instead relying on a remote node to provide necessary information about the blockchain. However, a weakness of Monero's current blockchain data structure is that lightweight clients cannot authenticate the responses returned from a remote node. In this paper, we show that malicious responses from a remote node can lead to reduced privacy for the client. We discuss several lightweight mitigations that reduce the attack's effectiveness. To fully eliminate this class of attack, we also show how to augment Monero's blockchain data structure with an additional index that clients can use to authenticate responses from remote nodes. Our proposed solution could be implemented as a hard fork, or alternatively through a 'Refereed Delegation' approach without needing any fork. We developed a prototype implementation to demonstrate the feasibility of our proposal.