unix_chkpwd man page
unix_chkpwd — Helper binary that verifies the password of the current user
unix_chkpwd is a helper program for the pam_unix module that verifies the password of the current user. It also checks password and account expiration dates in shadow. It is not intended to be run directly from the command line and logs a security violation if done so.
It is typically installed setuid root or setgid shadow.
The interface of the helper - command line options, and input/output data format are internal to the pam_unix module and it should not be called directly from applications.
Written by Andrew Morgan and other various people.