A permanently unlocked boot loader (BL) on a Nexus device is a big security risk.
It's only recommended for a pure developer phone.
An insecure BL enables all sorts of fastboot commands that can be used for e.g. doing the following:
- Make a copy of the device
E.g. by booting a custom image (adb boot boot.img), then copying partition dumps
- Erase data using fastboot erase
- Flash arbitrary Android firmware, recovery images or radio firmware
fastboot flash radio|recovery|boot|...
- Install a root kit (boot custom recovery, then modify system files)
- Steal Google/Facebook/whatever accounts stored on the phone
In case someone has their device encrypted, full access is not immediately possible. However there exists cracking software to brute force the used encryption PIN/passphrase. Altering the firmware is always possible (i.e. an evil maid attack, aka. installing a rootkit/password logger/etc.)
This is the normal process of modding a Nexus device:
- unlock the bootloader (fastboot oem unlock)
- boot/install insecure/open custom recovery
- install custom firmware
To secure your modded device you should do this afterwards:
- install a safe recovery image again (custom recoveries like TWRP/CWM are unsafe)
use the extracted recovery.img from Google's factory firmware file, available here
- relock the bootloader (fastboot oem lock)
This breaks the usual update mechanisms like RomManager. In case you need to upgrade, you can run flash_image recovery recovery.img (as root on the regular Android OS, it bypasses the BL lock) to install a temporary custom recovery again and use this once to upgrade the firmware and then remove the custom recovery again by doing a flash_image recovery stock.img. I know, this is not very user friendly right now, but let's hope the situation improves at the custom recovery front.
Some background on what's possible: http://hak5.org/episodes/hak5-1205 and http://www.youtube.com/watch?v=uM2_CpuzQ0o