[Bug 1927078] Re: Don't allow useradd to use fully numeric names

[Steve Beattie]

The Ubuntu Security team is +1 on disallowing purely numeric usernames,
as they are too easily confused with UIDs.

I think our preference would be to disallow leading numeric digits
entirely so that for example, 0x0 and 0o0 would be blocked as well, to
try to prevent both user and programmatic confusion.

Probably adduser should also be made consistent with whatever change is
made to useradd. The package provided adduser.conf files do have a
NAME_REGEX option (in addition to the --force-badname option) but AFAICT
is commented out by default (the commented out regex is
"^[a-z][-a-z0-9_]*\$" but I'm not sure that's appropriate in a UTF-8
world.)

It would be good to have testcase and documentation for this captured
somewhere.

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Title:
  Don't allow useradd to use fully numeric names

Status in shadow package in Ubuntu:
  New
Status in shadow source package in Focal:
  New
Status in shadow source package in Groovy:
  New
Status in shadow source package in Hirsute:
  New
Status in shadow source package in Impish:
  New

Bug description:
  [Description]

  Fully numeric names support in Ubuntu is inconsistent in Focal onwards
  because systemd does not like them[1] but are still allowed by default
  by useradd, leaving the session behavior in hands of the running
  applications. Two examples:

  1. After creating a user named "0", the user can log in via ssh or
  console but loginctl won't create a session for it:

  root at focal:/home/ubuntu# useradd -m 0
  root at focal:/home/ubuntu# id 0
  uid=1005(0) gid=1005(0) groups=1005(0)

  ..

  0 at 192.168.122.6's password:
  Welcome to Ubuntu 20.04.2 LTS (GNU/Linux 5.8.0-48-generic x86_64)

  Last login: Thu Apr  8 16:17:06 2021 from 192.168.122.1
  $ loginctl
  No sessions.
  $ w
   16:20:09 up 4 min,  1 user,  load average: 0.03, 0.14, 0.08
  USER     TTY      FROM             LOGIN@   IDLE   JCPU   PCPU WHAT
  0        pts/0    192.168.122.1    16:17    0.00s  0.00s  0.00s w  

  And pam-systemd shows the following message:

  Apr 08 16:17:06 focal sshd[1584]: pam_unix(sshd:session): session opened for user 0 by (uid=0)
  Apr 08 16:17:06 focal sshd[1584]: pam_systemd(sshd:session): pam-systemd initializing
  Apr 08 16:17:06 focal sshd[1584]: pam_systemd(sshd:session): Failed to get user record: Invalid argument

  
  2. With that same username, every successful authentication in gdm will loop back to gdm again instead of starting gnome, making the user unable to login.

  
  Making useradd fail (unless --badnames is set) when a fully numeric name is used will make the default OS behavior consistent.

  
  [Other info]

  - Upstream does not support fully numeric usernames
  - useradd has a --badnames parameter that would still allow the use of these type of names

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