If you pay attention to security issues in software, you've probably heard about man-in-the-middle attacks. These can occur more frequently than you expect, though inside of a company, it's more unlikely that you'll experience one if you have some fairly basic security controls on your network.

However, if you invite someone inside of your network, such as a consultant of some sort, you should be more vigilent. There's a great post showing how someone can execute a MITM attack against SQL Server. It's in depth, showing exactly how packet captures lead to the ability to hijack a session and create a new login.

If this seems like a lot of work, it is. However once the attack is built, this could easily be run by anyone inside of your network. I could see consultants running this type of attack and storing credentials they've created for use many months later. These could be sold later to someone that might use them in a website or other application to gather data from outside of your network.

Defending against these types of attacks is hard. Certainly not allowing free access for consultants is key, though a quick command line execution of a script might not be something that's easily noticed. At the very least, sysadmin logins should be monitored, and any changes to this list investigated immediately. However, I'd also say any built in server role changes should be checked and verified as being valid alterations. Even your SQL logins shouldn't change without administrators being aware.

Monitoring your systems is a big part of security. You might not prevent many of the attacks, but knowing they've taken place allows you to respond and potentially protect sensitive data.