I find the title irritating, as it seems to impugn open source software as a category. Why is the licensing model even relevant? A better phrasing would be something like "...flaw in a popular web framework" or "...flaw in Apache Struts."
Licensing model directly affects the availability of source code, and this technically facilitates exploit discovery.
Also, do we know that e.g. this and Heartbleed were discovered by reading the source? If they weren't then the availability of the source code is inconsequential IMO.