It is interesting that although Microsoft operating systems still have a considerable number of vulnerabilities, they are no longer in the top 3. Apple
with OS X and iOS is at the top, followed by Linux kernel.
2014 was a tough year for Linux users from a security point of view, coupled with the fact that some of the most important security issues of the year were reported for applications that usually run on Linux systems. Heartbleed, for example, is a critical security vulnerability detected in OpenSSL while Shellshock is a vulnerability that affects GNU Bash.