Monero is known as a cryptocurrency for its ability to provide greater anonymity. At its core is the RingCT protocol that can hide the sender and the amount of money in a transaction. However, the Snowden revelation alerts us that the implementation of cryptographic algorithms in practice might be substituted covertly which would result in a complete breach of the security of the cryptosystem. In this work, we turn to evaluate the potential hazards of algorithm substitution attacks (ASAs) against the RingCT protocol and explore feasible countermeasures. In specific, we first present the ASA model for RingCT where the goals of adversary include diminishing sender anonymity and recovering the spending key, then propose concrete ASAs against RingCT protocols that are undetectable in terms of the output of algorithms. Finally, we show how to thwart ASAs on RingCT protocols with reverse firewalls.