eWeek Labs: Open Source Quicker at Fixing Flaws
The real question for it organizations isn't whether open-source software is more secure than proprietary software but which type of software is fixed fastest.

Humans code both open-source and proprietary software, which means that mistakes will be made, and the resulting holes need to be reported and closed. Right now, open source has it all over proprietary software when it comes to owning up to and resolving problems.
http://www.eweek.com/article2/0,3959,562226,00.asp