As I have one of these and spent several hours trying different rooting methods for the phone, including rolling back the firmware and many other techniques, I can honestly say that I have found the easiest, safest and fastest way to root this particular model of phone.
First of all, please be aware that doing any of the below can void your phones warranty, I would also like to thank the original developers of all apps/programs used to root the phone as well as everyone at Sony for making such a great little device to start with.
1. Own a Sony Xperia Miro ST23i running Firmware 11.0.A.5.5 (this is the current latest firmware for the phone) … If you are not sure what Firmware you are on, go to Settings, About Phone and look at the Build Number for reference.
2. If you are on the correct firmware then continue to 3, if you are on an earlier firmware then use Sony PC Companion to update your phone to the correct version.
3. Download Unlock Root software from http://unlockroot.com/ on your PC or laptop (Note … the software is free to use to root the phone as well as many others … BUT … there is a lot of adware that you need to deselect or avoid during the install process). I will not be held responsible for your PC becoming infected or just blowing up during this process, same goes for the phone. Thanks to the developers behind this software.
4. If you wish to unlock your bootloader (will allow you to flash kernels and firmwares via flashtool) go to http://unlockbootloader.sonymobile.com/ and follow the process they give (there aren’t many custom kernels currently for the Miro and some ROMs don’t need an unlocked bootloader, you can also still update or repair via SE PC Companion if your bootloader is locked – I believe that you can also lose DRM if you unlock the bootloader so you may want to skip this step and just come back to it later if you need to)
5. Connect the phone to your PC via the supplied USB lead, let the drivers install if they aren’t already then in your phones settings go to Developer options and turn USB debugging on.
6. Run the Unlock Root program and click on Root, there may be a small delay and then the program should identify the device as the ST23, the program will then root the phone, during the process it will ask you if you wish to install a battery saving feature on the phone, I have always selected no so I’m not sure how effective the feature is (install that at your own risk), it will then ask if it can reboot the phone to complete the process. When you click yes the phone should automatically reboot.
7. Wait for the phone to restart fully, there should now be an app called SuperSU on the phone, open it and it will ask to update the binaries, select yes and it should update (you now have root access) you can stop at this point and there are some good benefits from basic root such as removing pre-installed apps (bloatware) like Google+, etc as well as being able to move more apps to the internal storage, etc. Note that you cannot install a new ROM at this point, for that you need a recovery installed.
If you wish to install a custom ROM, continue to add a recovery, for this we will use Clockwork Mod Recovery (CWM)
8. Download and unzip cwm_miro.zip from http://forum.xda-developers.com/showthread.php?p=34527749, please click the thanks button for ‘srl3gx’ for his post as you download.
9. From the unzipped folder you want to run install.bat, this will talk to the phone (still USB connected in developer mode) and add CWM.
10. Once complete, you are now able to install custom ROMs onto your unlocked Miro, there are a couple of special note I feel I should mention that you may wish to try.
Gimlo ROM http://forum.xda-developers.com/showthread.php?t=2123835 (This is a version of the standard IceCreamSandwich offered by Sony but you get to choose what is installed as default with a couple of nice options thrown in such as the Xperia T Keyboard and the abilty to leave a lot of possibly unused apps out – the end result is a familiar yet lightweight and customized version of that beautiful Sony interface)
CyanogenMod10 http://forum.xda-developers.com/showthread.php?t=1997251 (CyanogenMod is probably one of the best known of all ROMs, this will upgrade your phone to JellyBean offering some great features at the same time as removing most of the bloatware. The main downfall of this ROM is losing the Sony interface but you gain a smoother system and things like Google Now, system toggles, etc – Note that your Bootloader will need to be unlocked for this ROM – see step 4)
I would also recommend The Adrenaline Engine http://forum.xda-developers.com/showthread.php?t=2157068 which offers quite a few small tweaks to make the whole system that bit smoother and can be used in conjunction with either of the above or on the stock Sony ROM.
I wish you the best with your phone and hope that the above proved helpful.
For any further information have a look at http://forum.xda-developers.com/forumdisplay.php?f=1980 for lots of possible ROMs to suit your needs or just general FAQ’s, just don’t forget to thank the creators for their efforts.