Preventing Information Loss: Strengthening a Weak Link

[SP] So you think your network is secure? You've hardened your firewalls and briefed the employees on what is fair use of the computer systems. You patch security holes as quickly as they are found, and you've disabled every single service that is not mission critical. Heck, you've even switched over to an encrypted file system to prevent valuable data from making it outside the company if a machine is stolen. So why should you worry?