In this study, we consider fake message attacks in sparse mobile ad hoc networks, in which nodes are chronically isolated. In these networks, messages are delivered to their destination nodes using store-carry-forward routing, where they are relayed by some nodes. Therefore, when a node has messages in its buffer, it can falsify the messages easily. When malicious nodes exist in the network, they alter messages to create fake messages, and then they launch fake message attacks, that is, the fake messages are spread over the network. To analyze the negative effects of a fake message attack, we model the system dynamics without attack countermeasures using a Markov chain, and then formalize some performance metrics (i.e., the delivery probability, mean delivery delay, and mean number of forwarded messages). This analysis is useful for designing countermeasures. Moreover, we consider a hash-based countermeasure against fake message attacks using a hash of the message. Whenever a node that has a message and its hash encounters another node, it probabilistically forwards only one of them to the encountered node. By doing this, the message and the hash value can be delivered to the destination node via different relay nodes. Therefore, even if the destination node receives a fake message, it can verify the legitimacy of the received message. Through simulation experiments, we evaluate the effectiveness of the hash-based countermeasure.