“Hey, Pal! How are you? Remember that little project I had on the CubieTruck? I resumed hacking on it. But it’s a pity, I lost my root password… I have to start all over again. Any chance you remember what silly password we choose together?”
If only it was the user’s account password, it would be easy: log as root, change the password on behalf of the user, done. That time, who is going to save the root account? Is there any rootroot account, by any chance? No. But don’t worry, we’re going to hack into the root account nonetheless. But how? It’s Linux, it’s super secure, it’s the root account, I choose a 50-bits-of-entropy-random-super-secure password… Maybe. But almost any security will fail, once the hacker has access to the hardware. And today, we’re the hacker!
Let’s get access to the “hardware”: unplug the SD card! Then, plug it into your own system and mount the root partition of your Pi somewhere like
/tmp/Rasp. Further in this document, I will consider only rescuing the root account on a flash card, but the exact same technique would work on any machine where you can get access to the root partition outside of its original system. This means for example unplugging the main hard drive and plugging it into another computer, or booting a live distribution on the hardware from which you can mount the root partition.
Some bits of theory. For those who don’t care, jump to next paragraph. On a Linux system, the
/etc/passwd file stores the accounts information. Although that file also contained the passwords some years ago, the
/etc/shadow file now takes care of them. The reason is that the information in the
/etc/passwd file is necessary for a lot of common tools such as
ls, so that it isn’t possible to hide this file and it is actually readable from anyone on the system. Of course, passwords are not stored in plain text, but still it weakens the security as it allows anyone on the system to retrieve the hash and run an offline attack, like brute forcing. That’s why passwords are today stored in another file, the
/etc/shadow, which is only accessible to highly privileged processes, i.e. root. The old password field in the
/etc/passwd is now just an “
x“, telling the system to ask
Now that the root partition is mounted on a foreign system, the original permissions system that would prevent someone other than root to access and modify the
/etc/shadow doesn’t hold any more. Some strategies suggest simply emptying the password field, so that one could log into the account without passwords. But to my experience, this doesn’t work on systems with recent protection policies. Let’s not tickle the system too much, and try to find something more elegant.
First, there is a
--root option to the
passwd utility, which you can theoretically set to the mountpoint of the storage device
# passwd -R /tmp/Rasp root
but it miserably failed in my case. Nevermind, let’s do exactly the same, but manually. With a plus point: we will learn something in the process.
/etc/shadow, the passwords are hashed and salted. The method, the salt and the resulting hash are all stored in the form
$method_id$salt$hash. We decipher back the password, but now that we have access to the file, we can change this hash to be the one of a new password. Something that we will remember this time, if possible… After some research of a nice little tool to do the job, I found out that the best was a simple python script. Just start a python console and type
Python 3.5.0 (default, Sep 20 2015, 11:28:25)
[GCC 5.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import crypt
>>> crypt.crypt('cilyan.org', crypt.mksalt(crypt.METHOD_SHA512))
'cilyan.org' is our new password, and the resulting string is exactly what we have to place in the shadow file in the password field (just remember not to include the quotes). I used the SHA512 algorithm, which should also be standard to your system. You can check your standard method for your system as follows:
# grep ENCRYPT_METHOD /tmp/Rasp/etc/login.defs
Anyway, as the method used is stored in the file along the salt and hash, even if it isn’t the default one, you should be able to log in.
Replace the password in the shadow file, on the line for the root account, which should now look something like this
And if you are really, really lazy, you may just copy the above line, paste it in your shadow file, and log onto your Raspberry Pi using the password “
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