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

[Marc Deslauriers]

Thanks for taking the time to report this bug and helping to make Ubuntu
better. We appreciate the difficulties you are facing, but this appears
to be a "regular" (non-security) bug.  I have unmarked it as a security
issue since this bug does not show evidence of allowing attackers to
cross privilege boundaries nor directly cause loss of data/privacy.
Please feel free to report any other bugs you may find.

** Information type changed from Public Security to Public

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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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