[Bug 1822736] Re: Passwords longer than 255 characters break authentication

[Chris Guiver]

I followed Tom's REPRODUCTION test, and got exactly what Tom said I would.
I can confirm the issue with 256 character passwords.  
(I used `wc` to count characters in my buffer)

I could set the 256 character password (I used it with a backspace to
enter the old password, so it was only 1 character longer) but it
wouldn't then let me use it for the next `sudo whoami`.

x86 Lubuntu 19.04 (fully-updated) box

OS: Ubuntu Disco Dingo (development branch) i686 
Host: HP Compaq dx6120 MT(PL926AV) 
Kernel: 5.0.0-8-generic 
Uptime: 14 mins 
Packages: 1968 (dpkg) 
Shell: bash 5.0.2 
Theme: Arc-Darker [GTK3] 
Icons: Adwaita [GTK3] 
Terminal: qterminal 
CPU: Intel Pentium 4 2.80GHz (2) @ 2.790GHz 
GPU: NVIDIA GeForce 7600 GT 
Memory: 487MiB / 2953MiB 

test at dx6120-lubu:~$ apt-cache policy libpam0g
libpam0g:
  Installed: 1.3.1-5ubuntu1
  Candidate: 1.3.1-5ubuntu1
  Version table:
 *** 1.3.1-5ubuntu1 500
        500 http://ftp.iinet.net.au/pub/ubuntu disco/main i386 Packages
        100 /var/lib/dpkg/status

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https://bugs.launchpad.net/bugs/1822736

Title:
  Passwords longer than 255 characters break authentication

Status in pam package in Ubuntu:
  Confirmed

Bug description:
  DISCUSSION

  When a password longer than 255 characters is set for any user
  account, this user will become unable to authenticate when running
  'sudo' or 'passwd'.

  IMPACT

  This affects 18.04.2 systems, whether they were installed using
  Desktop (ubiquity) or Server (subiquity) installers. It may also
  affect other releases - this is yet untested.

  Tagged 'security' since these utilities then deny service to this
  user.

  REPRODUCTION

  # Add user 'test' with password 'testtest'
  sudo adduser --gecos '' test

  # Add user 'test' to the 'sudo' group
  sudo adduser test sudo

  # Become user 'test'
  sudo -iu test

  # Verify user 'test' can run commands via sudo
  sudo whoami

  # Change password of 'test' to this 255 character long password: 123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345
  passwd

  # Verify user 'test' can run commands via sudo with the new password set
  sudo -k
  sudo whoami    # should report "root"

  # Change password of 'test' to 'testtest':
  passwd

  # Verify user 'test' can run commands via sudo with the new password set
  sudo -k
  sudo whoami    # should report "root"

  # Change password of 'test' to this 256 character long password: 1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456
  passwd

  # Verify user 'test' can run commands via sudo with the new password set
  sudo -k
  sudo whoami    # should report "root"

  # This authentication fails, as sudo does not accept the 256 character password.
  # Attempting to change this password to a different value also fails:
  passwd

  # Effectively, user 'test' is now unable to use sudo, or to change
  their password.

  # The 'login' command, run by root, does, however, still enable user 'test' to login using the newly set 256 character password.
  # At the same time, a different restricted user who is a member of the 'sudo' group can still set a new password for 'test' (after authenticating to sudo with their own password) by supplying the current 256 character password using:
  sudo -u test passwd

  # Finally, to clean up
  sudo deluser --remove-home test

  ADDITIONAL OBSERVATIONS

  * A root-initiated 'login' command still allows this user to authenticate.
  * A different restricted user who is a member of the 'sudo' group can still set a new password for for this users' account (after authenticating to sudo with their own password) by supplying the >=256 character password

  CREDIT

  This was originally reported by 'Fieldy', I just reproduced it / filed
  this bug report.

  ProblemType: Bug
  DistroRelease: Ubuntu 18.04
  Package: libpam0g 1.1.8-3.6ubuntu2.18.04.1
  ProcVersionSignature: Ubuntu 4.18.0-16.17~18.04.1-generic 4.18.20
  Uname: Linux 4.18.0-16-generic x86_64
  ApportVersion: 2.20.9-0ubuntu7.6
  Architecture: amd64
  CurrentDesktop: ubuntu:GNOME
  Date: Tue Apr  2 09:39:39 2019
  SourcePackage: pam
  UpgradeStatus: No upgrade log present (probably fresh install)

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