Open source is not less secure by definition, as you want it to look like, SoN9ne, many proprietary solutions have security issues as well, and it's not always the companies that patches their software better and/or faster than the open source teams does. Proprietary software has it's cons in that you need to pay, probably both a one-time fee and a monthly or yearly fee for service and/or updates, and if you don't play, you won't get your security updates. Of course, you can never be sure that the open source team do release a security upgrade, but if they upgrade once in a while, it is not a problem. And you can't stand there without updates as you don't need to remember to pay the bill, as there are none.
Another con with proprietary solutions is that you, at most of the time, aren't allowed to alter the product if you need to, and if you need new functionality, you need to buy it, instead of download it for free from the open source website. With for example Drupal 7, you don't even have to download your new addons (modules, themes or other updates) yourself, the system itself can help you with this. Of course, it might be a security hole, but _everything_ on the net might have one or more security holes, open source or proprietary or any other classification.
most of the proprietary systems has as much limitations as the open source ones, or wait, proprietary... just the word mentions another bunch of limitations open source doesn't have.
Regarding vulnerabilities, proprietary doesn't give a guarantee that there are no vulnerabilities, but if you're lucky, and the Terms Of Service gives you an opening to have some money back. If anyone of those companies would leave a opening for it. likely? well, probably not.
So, it's not only open source that has it's pros and cons, proprietary solutions has theirs as well.
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I study Information Systems at
Karlstad University when I'm not on CodeCall