For years Apple has been playing a game of cat and mouse with jailbreakers. Apple releases a new version of iOS, hackers get busy finding security vulnerabilities to exploit, a new jailbreak comes out utilizing those hard-earned exploits, and Apple fixes the security holes in a new iOS version. The cycle has been happening for years.
A team of hackers called the Evad3rs were responsible for the recent iOS 6 Evasi0n jailbreak, and Apple just released iOS 6.1.3 to patch the related security holes. Of the 6 exploits that were fixed, Apple has publicly credited 4 to the Evad3rs, all of which are chained together to give hackers root access to iOS.
iOS 6 has been jailbreakable for nearly 2 months, and over 18 million iOS devices have used Evasi0n to jailbreak, according to Forbes. That makes it the most popular jailbreak in history.
Unfortunately iOS 6.1.3 is the point of no return. You have to be on an older version of iOS 6 to jailbreak. It will probably be some time before a jailbreak like Evasi0n will be released again. The good news is that the Evad3rs have exploits already saved up for iOS 7.