Huge numbers of people are constantly testing and reworking open source security.
Once closed source has a bad encryption found or accepts certain strings for SQL Queries, it becomes a feeding frenzy, and the people who set it up never put any counter measures in place because a small team never had any chance of approaching the vast well of human knowledge on security.
Just look at the news: US Government Facility Hacked, Credit Company Hacked, Industrial Plant Hacked, Proprietary Vehicle Hacked, etc.