How secure is Magento?

I used magento and as my experience says the security is very very good, not perfect of course it still an open-source platform. Magento has great community that fix security threads every day. 

 

I think the90% of security threats came from the developer not the platform

 

You can download books about security 

 

Actually there is no exact answer to your question as the security of any framework or system is not measured by itself but by the website that are using it and how it's using it.but to comfort you, yes Magento is one of the well known and well build ecommerce systems on the internet but that doesn't mean it's unhackable or100% secure.
if you didn't pay attention to the following you will lose you website security:- use a good hosting service (do search for the cheapest) use a good backup plans don't forget the periodical maintainance pay attection to the extensions and plugins you use in your website keep all the passwords to the system strong and complex I know that's maybe disappointing to you but I will leave you will a good story to explain my point.My family rely on me to maintain the family laptops clean and secure cause they use it alot in online shopping and keeping up with the bank records among other things.each time I check these laptops I make a clean windows installations, get a nice and strong antivirus, install a powerful firewall and after6 month when I return back to the same laptop what I find is a horribly infected laptop with virus and malware, what happen what goes wrong, the antivirus is begging for an update, the firewall need a serious security update, alot of installed programs are actually a malware and spyware.so i reached a solution of controling these laptops remotely every two weeks to keep them clean and secure.
my advice, build the website good and keep a good eye on your backup and your servers and pay attention to the firewall warnings and everything will be alright.

Magento is opensource as an ecomerce solution as compare to many others ecommerce solutions available in the market. But each opensource has its own advantages and disadvantages. There is an opensource, a security risk is always there.

Thank you Amgad Nasr for the answare, i'm not disappointed from the story is good to share expirience.
This question is to hear more about the security level of Magento, and of course the web maintenance & support is on the top of the list.
But what about the core after all there is the place where the magic is happen.