useradd grants root privileges to admin user

[Bugra Aydogar]

Hi,

As part of the Ubuntu Core 20, if a developer creates a user named either admin or Admin, it will have root privileges by default. I think that this is a security issue but it requires input from the security engineers/specialists.

Basically, if you look at the sudoers file under `/etc/sudoers`, you will realize the following line;

```
# Members of the admin group may gain root privileges
%admin ALL=(ALL) ALL
```

And if someone creates a user by using admin as name, the system provide root privilege.

```
sudo useradd -s /bin/bash -u 8003 -d /home/Admin --extrausers Admin
iotuc@ubuntu:~$ su Admin
Password:
Admin@ubuntu:/home/iotuc$ cat /etc/sudoers
cat: /etc/sudoers: Permission denied
Admin@ubuntu:/home/iotuc$ sudo cat /etc/sudoers
[sudo] password for Admin:
#
# This file MUST be edited with the 'visudo' command as root.
#
# Please consider adding local content in /etc/sudoers.d/ instead of
# directly modifying this file.
#
# See the man page for details on how to write a sudoers file.
#
Defaults env_reset
Defaults mail_badpass
Defaults secure_path="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/snap/bin"
# Host alias specification
# User alias specification
# Cmnd alias specification
# User privilege specification
root ALL=(ALL:ALL) ALL
# Members of the admin group may gain root privileges
%admin ALL=(ALL) ALL
# Allow members of group sudo to execute any command
%sudo ALL=(ALL:ALL) ALL
# See sudoers(5) for more information on "#include" directives:
#includedir /etc/sudoers.d
Admin@ubuntu:/home/iotuc$
```

Similarly, if I create a username called aydogar, it does not have root privilege and this is expected.

```
sudo useradd -s /bin/bash -u 8004 -d /home/aydogar --extrausers aydogar
iotuc@ubuntu:~$ su aydogar
Password:
aydogar@ubuntu:/home/iotuc$ cat /etc/sudoers
cat: /etc/sudoers: Permission denied
aydogar@ubuntu:/home/iotuc$ sudo cat /etc/sudoers
[sudo] password for aydogar:
aydogar is not in the sudoers file. This incident will be reported.
aydogar@ubuntu:/home/iotuc$
```

I think, this is a bug but would love here other ideas and inputs.

[Lars Düsing (lars.duesing)]

This is no bug. useradd adds automatically the user to a same named group. So admin user would get added to admin group. This is intended behavior. admin group has sudo rights. This is also intended.

[Oliver Grawert (ogra)]

On UbuntuCore the default user added by console-conf is automatically granted passwordless sudo.

the /etc/group file is on a readonly filesystem and contains the sudo group, this usually prevents you from accidentially adding extra sudo users (UbuntuCore exclusively uses /var/lib/extrausers for addtional users and snapd (being the management authority) normally creates snippets in /etc/sudoers.d on a per user basis)

in earlier UbuntuCore releases the admin group still existed in the readonly /etc/group so adding such a user would have been prevented by the readonly state of the file.

now you can unconditionally create such a user and automatically get sudo privs without having to go through a system-user assertion managed by snapd (which is the only process that can create snippets in /etc/sudoers.d on these systems)

if not considered a security hole (due to the fact that you can circumvent the documented locked down user creation process via system-user-assertions for this type of systems) this is at least a heavy inconsistency ... we should either add an admin group to the core snaps readonly /etc/group or wipe the admin line from sudoers so we match the documented behavior.