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

 * pod2g

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).