The 'dream team' of iOS hackers explains how the Corona jailbreak worked at HITBSecConf.

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GreenPois0n Absinthe was built upon @pod2g's Corona untether jailbreak to create the first public jailbreak for the iPhone 4S and iPad 2 on for the 5.0.1 firmware. In this paper, we present a chain of multiple exploits to accomplish sandbox breakout, kernel unsigned code injection and execution that result in a fully-featured and untethered jailbreak.

Corona is an acronym for "racoon", which is the primary victim for this attack. A format string vulnerability was located in racoon's error handling routines, allowing the researchers to write arbitrary data to racoon's stack, one byte at a time, if they can control racoon's configuration file. Using this technique researchers were able to build a ROP payload on racoon's stack to mount a rogue HFS volume that injects code at the kernel level and patch its code-signing routines.

The original Corona untether exploit made use of the LimeRa1n bootrom exploit as an injection vector, to allow developers to disable ASLR and sandboxing, and call racoon with a custom configuration script. This however left it unusable for newer A5 devices like the iPad2 and iPhone 4S, which weren't exploitable to LimeRa1n, so another injection vector was needed.
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You can get the full presentation notes from here.