Jailbreak is no more illegal but it comes with major security risks. No only users warranty is void; but it makes him vulnerable to many malwares. But because of its advantages some users are willing to take this risk.
Apple said a heap based buffer overflow issue could allow a malicious app “to elevate privileges,” similar to how jailbreaking works. Another major vulnerability fixed in this latest iOS update; when a malicious app can allow a memory corruption issue which will “execute arbitrary code with kernel privileges,”
Now if a Apple users updated his iPhone, iPad on iOS 9.1 it will not only fixes these vulnerabilities but also removes its jailbreak. Remember the Pengu Team only introduced the iOS 9.0 jail break, it doesn’t work with iOS 9.1.
Some other major vulnerabilities iOS 9.1 fixes are denial-of-service issue in the kernel, and a privacy flaw where phone and message notifications may appear on the lock screen even when the setting is turned off.