From the article:

Is open source software more secure? To most Linux enthusiasts, the answer is obvious: open source means more people can look for bugs and a faster dissemination of bug fixes. Obviously, yes. But noted security expert Gene Spafford says that this may not necessarily be true. According to the Purdue professor of computer science and co-author of Practical Unix & Internet Security, good security begins with good design and neither Windows nor Linux have much to brag about in that category.