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How to add a user that accept any password as a valid password?

PS:

  • look at /etc/pam.d/passwd. – Ipor Sircer Dec 13 '16 at 18:40
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This configuration was tested on Ubuntu 16.04.1 LTS Server. Modify /etc/pam.d/common-auth.

# [...]
auth [success=2 default=ignore] pam_unix.so nullok_secure
auth [success=1 default=ignore] pam_succeed_if.so user = the_username
# [...]
auth requisite pam_deny.so
# [...]
auth required pam_permit.so
# [...]
  • The success=x portion tells PAM to skip x rules on success.
  • Substitute the real username for the_username, above.

So, all users first try to authenticate with pam_unix.so, which requires a correct username and password in order to succeed. If authentication succeeds with pam_unix.so, then proceed to pam_permit.so. That's the default behavior.

If authentication failed with pam_unix.so, proceed to pam_succeed_if.so, which succeeds whenever a user enters the username of the_username, using whatever password was entered. If both pam_unix.so and pam_succeed_if.so fail, then proceed to pam_deny.so; otherwise, proceed to pam_permit.so.

A word of caution: be very careful on a live system because it's easy to make a mistake and lock yourself out, probably requiring a fix via rescue media.

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