Yes, it is possible that your Mac could get hijacked. On August 17, 2015, PC World posted an article about how your Mac could get hijacked. PC World reports, "An Italian teenager has found two zero-day vulnerabilities in Apple’s OS X operating system that could be used to gain remote access to a computer."
The good news for Apple is that it doesn't appear to be in OS X El Capitan.
Is there a patch? Sort of, but it doesn't come from Apple. And because it takes two bugs together to cause the kind of memory corruption that would allow a hacker to gain root access, it's kind of unlikely that any Mac user would be affected. In other words, if you're not a high profile target, this should be the least of your worries.