limera1n is a bootrom exploit found by geohot that works on all A4 and below devices. The last devices being the iPhone 4 GSM (1,1)(1,3) and the iPhone 4 CDMA (3,3). It is the last public bootrom exploit to be released to this day. A bootrom exploit is an exploit that can be run on a device with any iOS version. Basically meaning the device can be jailbroken for life. The reason I am talking about limera1n is because it will be mentioned often in my posts. I like to mess around with this exploit a lot and find out all of the cool stuff I can do with it. It has been made into a tethered jailbreak (Mac, Windows) by geohot which you can untether using the 0x24000 Segment Overflow exploit or the Packet Filter Kernel Exploit.
limera1n For Newer Devices
As of now there are no public bootrom exploits for any newer devices. iH8sn0w has found a very powerful iBoot exploit for A5 and A5X devices, but has not shared it publicly. He has not done this because he says that with just a bit of tweaking he can make it work on newer devices (A5+). If he were to release it anytime soon, Apple is in a position to release a new software update without any problem, effectivly patching it on all of the devices the iBoot exploit would work on.