The Cyrus IMAP Server ChangeLog states:
Fix possible single byte overflow in mailbox handling code. Fix possible single byte overflows in the imapd annotate extension. Fix stack buffer overflows in fetchnews (exploitable by peer news server), backend (exploitable by admin), and in imapd (exploitable by users though only on platforms where a filename may be larger than a mailbox name).
The 2.1.X series are reportedly only affected by the second issue. These issues may lead to execution of arbitrary code with the permissions of the user running the Cyrus IMAP Server.