Absinthe

Absinthe is the A5 userland jailbreak tool for iPhone 4S and iPad 2 on iOS 5.0 (iPhone 4S only), 5.0.1 (9A405) and iOS 5.0.1 build 9A406 on iPhone 4S. This tool was released on 20 January 2012, and is available in three incarnations:
 * A GUI tool with the same name, released by the Chronic Dev Team
 * A command-line version by the iPhone Dev Team, in case troubleshooting is required
 * An integration with redsn0w

There was also version 2.0 - 2.0.4 that supported all devices on iOS 5.1.1. This preinstalled Rocky Racoon from Cydia to untether.

The name Absinthe was chosen as an homage to greenpois0n, Chronic Dev Team's earlier jailbreak software. Absinthe is an alcoholic beverage that is often greenish in color and is poisonous in quantity.

Installation
The jailbreak packs Corona for the S5L8940 and the rest of the firmware patches in a regular iTunes backup and then restores this backup on to the device, which injects the required files. It is then completed by opening a web clip added to the home screen by Absinthe. If this doesn't work, for example, because the greenpois0n website is down due to the number of people attempting to jailbreak, an alternative is to enable a specially crafted VPN connection in the Settings app.

Exploit
This jailbreak uses the Racoon String Format Overflow Exploit and HFS Heap Overflow from Corona for untether. The exact exploit to jailbreak the two devices is not known yet, but it is assumed that the web clip loads a page that requires an IPsec VPN connection, which causes the racoon daemon to start and run the jailbreak payload.

However, some poking reveals some unique means of injection. At some point in the jailbreak process, BackupAgent is intentionally crashed to produce a crash log. The jailbreak payload reads this crash log in order to obtain the memory address of libcopyfile.dylib to continue with the exploit.

Opening the  file included with the GUI tool, which is the definition file for the web clip, an URL to the greenpois0n website is found, which shows a please wait message. The interesting part is the source code of this page has an image linked to, because   was designated as the trigger site for a VPN on-demand configuration and any attempt at accessing a resource on   will trigger the VPN. Note that there is no actual web server running on.