SHA-1 Image Segment Overflow

SHA-1 Image Segment Overflow or SHAtter was an exploit that allowed unsigned code execution from a flaw in the bootrom. It was never used in a public jailbreak due to the exploit used in limera1n being released first. SHAtter was patched in the A5 devices and therefore, never released.

Compatibility

SHAtter only works with A4 devices:

• iPad 1G
• iPhone 4 (both models)
• iPod touch 4G
• Apple TV 2G

Credit

• vulnerability: posixninja (7 May 2010), also discovered independently by geohot
• research: posixninja, pod2g, also MuscleNerd
• exploit: pod2g

Background Info

In April 2010 pod2g wrote a USB fuzzer and tested every single USB control message possible on his iPod touch 2G. The fuzzer found 2 vulnerabilities:

• a heap overflow caused by usb_control_msg(0xA1, 1)
• a way to dump the bootrom using USB descriptors request

The team tested these two vulnerabilities on newer devices (iPhone 3GS, iPod touch 3G, iPad) and both were already fixed by Apple.

posixninja continued the fuzzing on these devices and found that with a particular sequence of USB messages it was possible to dump the BSS+Heap+Stack (on new gens only). Having a memory dump is really helpful to make exploits and it was also the first time we had this kind of dump. (Previous bootrom exploits like the 0x24000 Segment Overflow were done blind!)

Also, his first attempts to dump the memory resulted in rebooting the device. Interesting! We'll see after that this reboot is the base of the SHAtter exploit.

Research began to figure out why the device would reboot. posixninja found the reason and proposed different ideas to exploit this. He also reversed tons of assembly code of the bootrom in this period, giving a support discussion to the team. We're not talking about days, but months of work. So, major props to posixninja: SHAtter would not have been possible without the clever vulnerability he found and the research he did on the bootrom.

In the meanwhile, pod2g helped on the USB reversing side and found a way to have more control over the size of the USB packets sent. The finer-grained control of the packet sizes is the key of SHAtter.

posixninja and pod2g worked on exploiting the vulnerability for days. Every attempt was a failure because the idea to attack the stack and bypass the IMG3 control routines was just impossible. It took them weeks to understand why they failed and why they couldn't exploit it this way.

They both gave up in July and focused on other subjects.

Vulnerability

Explanation by p0sixninja at MyGreatFest:

It tricked the bootrom to think the size of the image uploading was larger then what it actually was. Then when it would try to load the image, it would see that it was wrong. Then it would try to wipe out the entire image with all zeros and go past it and start wiping out bootrom.

Exploitation was done by overwriting SHA-1 registers to zeros so then when it went to check images it would copy part of image into memory address zero (where the bootrom is). It would take the image uploaded and copy it over top of the bootrom (which turns out to be writable over the data portion).