An untethered jailbreak is a jailbreak wherein a user can reboot their device at will, and have their device start up with the jailbreak automatically applied without the assistance of a computer or a utility on the device.
These jailbreaks can be applied via multiple different methods, the most common of which being kernel exploits.
Most untethered jailbreaks rely on vulnerabilities in the kernel and early boot process, typically using a combination of codesigning bypasses and manipulating the system into executing a binary early in the boot process (or obtaining unsigned code execution via a vulnerability in an existing startup process). Once code execution has been obtained, a kernel exploit is used in order to patch the currently loaded kernel to allow for the rootfs to be remounted as read/write, and to allow for unsigned code execution.
Tools that use kernel exploits to achieve untethered jailbreaks:
- JailbreakMe 2.0 (star)/JailbreakMe 3.0 (saffron)
Older devices, such as the iPhone 3GS, iPod touch 2 (old bootrom) and earlier, have had vulnerabilities discovered in the BootROM that are able to be executed without the assistance of DFU mode (such as via a malformed image in the NOR) allowing for stages of the boot chain to be overwritten with custom code, such as a patched LLB/iBoot to allow for an unsigned kernel, and a custom boot logo. Examples of bootrom exploits that allow for untethered code execution are Pwnage, 24kpwn and alloc8.
Tools that use bootROM exploits to achieve untethered jailbreaks:
Some jailbreaks abuse vulnerabilities in the currently installed iBoot in order to patch out signature checks or load an alternative iBoot, therefore being able to load a patched and jailbroken kernel. Very few jailbreak utilities opt to use this method, as iBoot exploits are rare to come across and are able to be patched by Apple with software updates, thereby only being able to be used if blobs have been saved, or if the device was discontinued before Apple released a patch.