How to get root on Ubuntu 20.04 by pretending nobody’s /home

Last modified on November 11, 2020

I'm eager on Ubuntu, so I might fancy to attend invent it as actual as possible. I even acknowledge now not too way back spent reasonably a type of time purchasing for safety vulnerabilities in Ubuntu’s system corporations, and it has principally been an train in frustration. I even acknowledge discovered (and reported) a pair of factors, however the bulk had been low severity. Ubuntu is originate supply, which plan that many people acknowledge checked out the supply code ahead of me, and it seems to be like fancy your complete simple bugs acknowledge already been discovered. In different phrases, I don’t want this weblog put up to provide you with the have an effect on that Ubuntu is crammed with trivial safety bugs; that’s now not been my have an effect on to this degree.

This weblog put up is prepared an astonishingly simple plan to escalate privileges on Ubuntu. With a pair of simple instructions inside the terminal, and some mouse clicks, a similar previous individual can originate an administrator account for themselves. I even acknowledge made a brief demo video, to display how simple it's a methods.

Or now not it's peculiar for a vulnerability on a latest working system to be this easy to milk. I even acknowledge, on some events, written 1000's of strains of code to milk a vulnerability. Latest exploits contain advanced trickery, fancy the utilization of a reminiscence corruption vulnerability to forge faux objects inside the heap, or changing a file with a symlink with microsecond accuracy to milk a TOCTOU vulnerability. So on this point in time it’s fairly uncommon to salvage a vulnerability that doesn’t require coding experience to milk. I furthermore believe the vulnerability is simple to notice, even want to you originate now not acknowledge any prior knowledge of how Ubuntu works or any safety analysis expertise.

Disclaimer: For anyone to milk this vulnerability, they want make a alternative up entry to to the graphical desktop session of the system, so this enterprise impacts desktop customers most though-provoking.

Right here's a excessive degree thought of the exploitation steps, as proven inside the demo video.

First, originate a terminal and originate a symlink on your personal dwelling listing:

ln -s /dev/zero .pam_environment

(If that doesn’t work as a result of a file named .pam_environment already exists, then legitimate briefly rename the passe file in order that you'd presumably perchance possibly restore it later.)

Subsequent, originate “Region & Language” inside the system settings and postpone a thought at to substitute the language. The dialog field will freeze, so legitimate ignore it and return to the terminal. At this degree, a program named accounts-daemon is though-provoking 100% of a CPU core, so your pc may perchance possibly truthful turn into sluggish and start to decide on up scorching.

In the terminal, delete the symlink. Otherwise you may perchance possibly lock your self out of your acknowledge account!

rm .pam_environment

The following step is to ship a SIGSTOP sign to accounts-daemon to discontinue it from thrashing that CPU core. But to elevate out that, you first want to know accounts-daemon’s course of identifier (PID). In the video, I elevate out that by working excessive, which is a utility for monitoring the working processes. Because accounts-daemon is caught in an limitless loop, it speedy goes to the tip of the checklist. Another plan to salvage the PID is with the pidof utility:

$ pidof accounts-daemon

Armed with accounts-daemon’s PID, you may perchance possibly use extinguish to ship the SIGSTOP sign:

extinguish -SIGSTOP 597

Your pc can postpone a breather now.

Right right here is the indispensable step. You’re going to log off of your account, however first you have to to negate a timer to reset accounts-daemon after which you'll be able to acknowledge logged out. Otherwise you’ll legitimate be locked out and the exploit will fail. (Don’t alarm if this happens: each factor shall be wait on to similar previous after a reboot.) Right this is how to negate the timer:

nohup bash -c "sleep 30s; extinguish -SIGSEGV 597; extinguish -SIGCONT 597"

The nohup utility is a simple plan to go away a script working after which you'll be able to acknowledge logged out. This uncover tells it to chase a bash script that does three issues:

  1. Sleep for 30 seconds. (You legitimate want to give your autonomous time to log off. I negate it to 10 seconds for the video.)
  2. Ship accounts-daemon a SIGSEGV sign, that may perchance possibly truthful invent it atomize.
  3. Ship accounts-daemon a SIGCONT sign to deactivate the SIGSTOP, which you despatched earlier. The SIGSEGV obtained’t postpone attain until the SIGCONT is obtained.

As quickly as completed, log off and wait a pair of seconds for the SIGSEGV to detonate. If the exploit is favorable, then you definately may perchance possibly be offered with a collection of dialog containers which help you originate a model contemporary individual account. The contemporary individual account is an administrator account. (In the video, I chase identification to display that the contemporary individual is a member of the sudo neighborhood, which plan that it has root privileges.)


Terminate with me! Even want to you originate now not acknowledge any prior knowledge of how Ubuntu (or additional particularly, GNOME) works, I reckon I'll signal this vulnerability to you. There are in fact two bugs involved. The foremost is in accountsservice, which is a service that manages individual accounts on the pc. The 2nd is in GNOME Demonstrate Manager (gdm3), which, amongst different issues, handles the login present conceal conceal. I’ll signal each of these bugs individually beneath.

accountsservice denial of service (GHSL-2020-187, GHSL-2020-188 / CVE-2020-16126, CVE-2020-16127)

The accountsservice daemon (accounts-daemon) is a instrument service that manages individual accounts on the machine. It might most definitely elevate out issues fancy originate a model contemporary individual account or substitute an individual’s password, nonetheless it might presumably perchance effectively furthermore elevate out a lot much less security-pretty issues fancy substitute an individual’s icon or their most well-liked language. Daemons are packages that chase inside the background and lift out now not acknowledge their acknowledge individual interface. On the alternative hand, the packages settings dialog field can focus on with accounts-daemon via a message system often known as D-Bus.

System Settings: Users

System Settings: Region & Language

In the exploit, I make the most of the packages settings dialog field to substitute the language. A similar previous individual is allowed to substitute that environment on their acknowledge account - administrator privileges are now not required. Below the hood, the packages corporations dialog field sends the org.freedesktop.Accounts.User.SetLanguage uncover to accounts-daemon, via D-Bus.

It seems to be like that Ubuntu makes use of a modified mannequin of accountsservice that contains some further code that doesn’t exist inside the upstream mannequin maintained by freedesktop. Ubuntu’s patch provides a unbiased named is_in_pam_environment, which appears for a file named .pam_environment inside the individual’s dwelling listing and reads it. The denial of service vulnerability works by making .pam_environment a symlink to /dev/zero. /dev/zero is a particular file that doesn’t in fact exist on disk. It is equipped by the working system and behaves fancy an infinitely lengthy file whereby each byte is zero. When is_in_pam_environment tries to learn .pam_environment, it can get redirected to /dev/zero by the symlink, after which can get caught in an limitless loop as a result of /dev/zero is infinitely lengthy.

There’s a 2nd section to this worm. The exploit contains crashing accounts-daemon by sending it a SIGSEGV. Completely a similar previous individual shouldn’t be allowed to atomize a instrument service fancy that? They shouldn’t, however accounts-daemon inadvertently permits it by shedding privileges legitimate ahead of it begins discovering out the individual’s .pam_environment. Dropping privileges plan that the daemon briefly forfeits its root privileges, adopting as an alternative the lower privileges of the individual. Sarcastically, that’s supposed to be a safety precaution, the unbiased of which is to give safety to the daemon from a malicious one who does one factor fancy symlinking their .pam_environment to /and many others/shadow, which is a extraordinarily fairly file that similar previous customers aren’t allowed to learn. Unfortunately, when accomplished incorrectly, it furthermore grants the individual permission to ship the daemon indicators, which is why we’re in a position to ship accounts-daemon a SIGSEGV.

gdm3 privilege escalation ensuing from unresponsive accounts-daemon (GHSL-2020-202 / CVE-2020-16125)

GNOME Demonstrate Manager (gdm3) is a elementary ingredient of Ubuntu’s individual interface. It handles issues fancy beginning and stopping individual classes after they log interior and out. It furthermore manages the login present conceal conceal.

gdm3 login screen

Another ingredient dealt with by gdm3 is the preliminary setup of a model contemporary pc. Can acknowledge to you put in Ubuntu on a model contemporary pc, one of many essential foremost issues that you have to to elevate out is originate an individual account. The preliminary individual account wants to be an administrator in order that you'd presumably perchance possibly proceed establishing the machine, doing issues fancy configuring the wifi and inserting in functions. Right this is a screenshot of the preliminary setup present conceal conceal (taken from the exploit video):


The dialog field that you just witness inside the screenshot is a separate utility, known as gnome-preliminary-setup. It is triggered by gdm3 when there are zero individual accounts on the system, which is the anticipated enterprise at some degree of the preliminary setup of a model contemporary pc. How does gdm3 verify what variety of customers there are on the system? You in all chance already guessed it: by asking accounts-daemon! So what happens if accounts-daemon is unresponsive? The linked code is right here.

It makes use of D-Bus to ask accounts-daemon what variety of customers there are, however since accounts-daemon is unresponsive, the D-Bus plan name fails ensuing from a timeout. (In my trying out, the timeout took spherical 20 seconds.) On account of the timeout error, the code would now not negate the signal of priv->have_existing_user_accounts. Unfortunately, the default signal of priv->have_existing_user_accounts is counterfeit, now not gorgeous, so now gdm3 thinks that there are zero individual accounts and it launches gnome-preliminary-setup.

I in fact acknowledge a confession to invent: I discovered this worm solely by accident. Right this is the message that I despatched to my colleagues at roughly 10pm BST on October 14:

I legitimate purchased LPE by accident, however I'm now not fairly particular how to reproduce it. 🤦

Right right here’s what happened: I had discovered a pair of denial-of-service vulnerabilities in accountsservice. I assumed to be them low severity, however used to be writing them up for a vulnerability file to ship to Ubuntu. Around 6pm, I finished work and closed my pc lid. Later inside the night, I opened the pc lid and located that I used to be locked out of my account. I had been experimenting with the .pam_environment symlink and had forgotten to delete it ahead of closing the lid. No mountainous deal: I veteran Ctrl-Alt-F4 to originate a console, logged in (the console login used to be now not tormented by the accountsservice DOS), and killed accounts-daemon with a SIGSEGV. I didn’t want to use sudo ensuing from the privilege shedding vulnerability. The following ingredient I knew, I used to be taking a thought on the gnome-preliminary-setup dialog containers, and used to be amazed to salvage that I utilized in an effort to originate a model contemporary individual with administrator privileges.

Unfortunately, after I tried to reproduce the similar sequence of steps, I couldn’t make a alternative up it to w

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