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

[Tom Reynolds]

It is yet unclear what the root cause of this issue is - libpam, crypt,
passwd and sudo seem like primary suspects. The 256 character password
is hashed to a value which still allows a TTY login to succeed. Also,
passwd run by a different user in the context of the affected user
(using sudo) still works. So apparently it only breaks for the logged-in
user. This may help identify the root cause.

pam_unix(8) (on bionic) states:
  The maximum length of a password supported by the pam_unix module via the
  helper binary is PAM_MAX_RESP_SIZE - currently 512 bytes. The rest of the
  password provided by the conversation function to the module will be 
  ignored.

Regarding passwd, I was told this on IRC:
  passwd uses getpass() to read the password from the user, and testing just 
  getpass() alone shows it will return up to 4095 bytes long (i.e. truncates 
  at that length).

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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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