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Last night, 60 Minutes broadcast an excellent, in-depth analysis of the Stuxnet Worm and how it was used to infiltrate and damage the Iranian nuclear program. Let's put politics aside for a moment (as I always try to do on this blog). Anyone who wants (or needs) to understand how malicious code may be used to wreak havoc upon a thought-to-be-secure system should watch this video.
Particularly, pay close attention to how the worm was introduced into the facility's computers. I guarantee, it'll be the best 15 minutes you can invest before you sit down and formulate your security plan.