What Kinds of Security Problems Exist in Drupal?
This pie chart shows which are the most common kinds of problems in the project:
As you can see, XSS is the most common issue - almost covering 50%. Access Bypass, CSRF, SQL Injection, and Code Execution are the next most common making up a about a quarter of the weaknesses.
It's important to note that these are only vulnerabilities for which there has been a Security Announcement. Many more exist only on an individual site with improper configuration or a custom module or theme and can never be included in an analysis like this.