su produces a "su: incorrect password' error with valid password

Issue

• Local users can no longer use 'su' command to get root rights.
• Users cannot escalate to root user using su using valid password.
• It is possible to login as root user directly but not possible to escalate to root using su.

• Users cannot use su - command to switch user getting "su: Authentication failure".

  Aug 11 17:01:49 testserver unix_chkpwd[1234]: check pass; user unknown
  Aug 11 17:01:49 testserver unix_chkpwd[1234]: password check failed for user (testuser)
  Aug 11 17:01:49 testserver su: pam_unix(su-l:auth): authentication failure; logname=redhat uid=40110 euid=40110 tty=pts/1 ruser=redhat rhost=  user=testuser
  

• Only this generic message is generated in logs which is the same as supplying the wrong password:

  Dec 14 16:49:57 testserver su(pam_unix)[23992]: authentication failure; logname=test uid=500 euid=500 tty=pts/2 ruser=test rhost=  user=root
  

• Only this generic message is generated in logs which is the same as supplying the wrong password

• Being root you can use su to switch to another user, but regular users are not able to run su

  • RHEL7/8/9:

  [user@localhost ~]$ su
  Password:
  su: Authentication failure
  
  [user@localhost ~]$ su user2
  Password:
  su: Authentication failure
  

  • RHEL4/5/6:

  [user@localhost ~]$ su
  Password:
  su:  incorrect password
  
  [user@localhost ~]$ su user2
  Password:
  su:  incorrect password