Security by obscurity doesn't work. Microsoft software has always been closed source and it has never prevented hackers making exploits.

Open source software allows hundreds of thousands of people to comb over the code and find/fix vulnerabilities much easier.

It's also true that because of the way Linux is developed, security flaws in Linux are patched much faster than in other projects, with Linux patching issues in an average of 25 days compared to Microsoft's 83 days. And the gap is widening, recently Linux has got that down to 15 days.

There's a reason companies go with Linux for servers that handle sensitive information or are business-critical. And there's a reason why the best encryption algorithms are all open source.