This is a statistic that's quoted a lot -- often 80% of breaches occur internally -- but there's varying data on it. The CSI/FBI Computer Crime and Security Survey has interesting data on this, but I would also recommend checking out other surveys from security magazines and research firms. A Google search using the words "information security survey" turns up some good info.
The bottom line is no one really knows for sure, but it's a considerable amount. Based on what I see, the odds are that more external attacks occur, but are often blocked by basic perimeter security controls. On the other hand, even though internal security breaches probably occur less often, when they do, it's bad news. I wrote an article intellectual property leakage (which is what most internal breaches are about) for the Feb. 2004 issue of Information Security magazine that covers this subject and provides a survey of vendor solutions.
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This was first published in March 2004