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.
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