Since this thread seems to have become quite popular, I thought I'd update it to give people all the newest information in one place.
Since I've made this post, there has been another OTA (build 12940) that improves bootloader security even further and prevents some potential root methods which were being developed for 12840. As of now, neither build 12840, build 12940, nor build 13300 has a published root method. New units have the patched bootloader preloaded from the factory and are not rootable. If you buy a unit at this point, there is a good chance that you will get one that is patched. (EDIT 2013-10-22: People are reporting that units they have purchased from Best Buy and Amazon are still running the vulnerable build. It is unclear if this is simply old stock or if there are still vulnerable units being produced.)
As for the methods described below, they cannot be performed through a shell (i.e. telnet) since the root filesystem is formatted as squashfs, which is read-only. Instead, the root images must be manually repacked for each OTA and flashed using a USB drive with an image such as FlashCast. @ddggttff3 maintains a FlashCast mod to update Chromecasts to the latest firmware without losing root, which can be found here.
For those of you who have managed to keep your vulnerable bootloaders, keep your eyes out. There should be some very cool releases in the near future.
As can be seen in this commit to Google's Chromecast source mirror, firmware version 1.1 adds a check for the result of image verification on line 755. This check will cause GTVHacker's USB image to fail to boot, and you will not be able to obtain root. Even if another root exploit is found, it seems very unlikely that it will be as clean or simple as the one which exists now, which simply uses version 0.7's unlocked bootloader to flash a new system image.
Unfortunately, I don't have a Chromecast to test on, so I cannot recommend a method of disabling OTAs. However, from looking at the system image, there are a few possibilities I see. THE FOLLOWING METHODS ARE UNTESTED AND ARE NOT GUARANTEED TO WORK OR LEAVE YOUR CHROMECAST IN A WORKING STATE. PERFORM THEM AT YOUR OWN RISK.
After telnetting into your rooted Chromecast or otherwise obtaining a root shell, you can try these two possible methods
- Rename otacerts.zip to otacerts.zip.bak in /system/etc/security/. This may remove the OTA signing keys and cause the Chromecast to reject any OTAs. However, I do not know whether this file is actually used or whether is simply a remnant from Chromecast's Android base.
- Replace /chrome/update_engine with an empty, executable, shell script (make sure to make a backup copy first). I am very unsure of this method, since it is simply going off the name of the update_engine binary. If update_engine happens to perform some task core to the system, doing this will leave your device in an unusable state. If this happens, simply re-rooting using GTVHacker's USB image should restore your system to how it was.
Again, I am not responsible for any bricked Chromecasts which may result from attempting this. If you do try either method, please report whether or not it appeared to work or have any ill effects.