CRM Defender is a complete and robust module that introduces a safe barrier against brute-force and dictionary attacks. It produces a permanent ban of IP addresses from where a predefined number of failed attempts comes.
Other add-ons of interest
Glances - Integrate All Your Applications
Accomplish More Every Day. Spend your time on getting things done, not on finding information. Glances securely connects all of your apps and provides a simplified view of the data you need right when you need it, in real time.
SecuritySuite - Enhanced SuiteCRM Teams
Control what your users can access and save time, money, and frustrations. Lock down sensitive data in SuiteCRM to specific groups or teams. Supports unlimited assigned users, unlimited group assignments to records, custom layouts for each group, login/sudo capabilities and much more.