Vandalism and Hacking
Vandals often use hacking techniques to deface a website or destroy data and files, but there are also those who just want to steal resources (make use of other peoples' servers without their knowledge or permission) or to cover their tracks by stealthily making use of hardware owned by legitimate businesses to carry out processing for illegal operations or to relay spam and viruses to others.
Vulnerable extensions list
Even most of security-conscientious Joomla webmasters aren't knowing the existence of this list, maintained at Joomla docs site. There are listed all components with known security problems, and very important to know, the items once appeared on the list aren't removed when the problem is fixed, because large majority of Joomla webmasters aren't upgrading their site as new versions are coming out for the add-ons used. So worth checking, even if the components you use are listed on GREEN - aka fixed -, you may run in trouble, because hackers are knowing the list - and are pro-actively seeking Joomla sites using the insecure add-ons. So you will become easily a target even if you have the secure version...
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