Where are queued messages stored and how secure is the storage?

File-scoped queued messages are stored in a database on the same filesystem as the file to which they are scoped. The database is readable to the super-user only, and the ToolTalk database server (running as root) only gives the messages to processes owned by a user with read access on the file.

Session-scoped queued messages are stored in the address space of the ttsession that manages the session. ttsession only gives the messages to a process that has satisfied the authentication mode in which the ttsession is running.