Linux – pam_cracklib: preventing users from using passwords that contain common words

The simple solution may be to simply remove execute permissions on system binaries. If you want to prevent users from compiling things or running things from directories they have write access you can create a separate partition and mount those file-systems with the noexec option.

man mount (noexec option)

Do not allow direct execution of any binaries on the mounted file system. (Until recently it was possible to run binaries anyway using a command like /lib/ld*.so /mnt/binary. This trick fails since Linux 2.4.25 / 2.6.0.)

I believe another way to accomplish this you would need to use something like AppArmor or SELinux.

The handiest answer I've come across is to use the passwd command in conjunction with dscl. Here is the output from an interactive session (with the passwords replaced by asterices):

$ dscl -u diradmin -p ces 
Password: 
 > cd /LDAPv3/127.0.0.1/
/LDAPv3/127.0.0.1 > auth diradmin *****
/LDAPv3/127.0.0.1 > passwd Users/Atwo807 *****
/LDAPv3/127.0.0.1 > passwd Users/Atwo249 *****
/LDAPv3/127.0.0.1 > passwd Users/doesnotexist foobar
passwd: Invalid Path
<dscl_cmd> DS Error: -14009 (eDSUnknownNodeName)
/LDAPv3/127.0.0.1 > exit
Goodbye

Here is a python script to make the changes. You will need the pexpect module (sudo easy_install pexpect should get it for you; I don't think you need the dev tools installed).

#!/usr/bin/env python

import pexpect

def ChangePasswords(host, path, diradmin, diradmin_password, user_passwords, record_type='Users'):
    """Changes passwords in a Open Directory or similar directory service.

    host = the dns name or IP of the computer hosting the directory
    path = the pathname to the directory (ex. '/LDAPv3/127.0.0.1')
    diradmin = the directory administrator's shortname (ex. 'diradmin')
    diradmin_password = the directory administrator's password
    user_passwords = a dictionary mapping record names (typically, user's short
                     names) onto their new password
    record_type = the sort of records you are updating.  Typically 'Users'

    Returns a tuple.  The first entry is a list of all records (users) who
        failed to update.  The second entry is a list of all records (users)
        who successfully updated.
    """

    failed_list = []
    succeeded_list = []
    prompt = " > "

    child = pexpect.spawn("dscl -u %s -p %s" % (diradmin, host))

    if not (ReplyOnGoodResult(child, "Password:", diradmin_password) and
       ReplyOnGoodResult(child, prompt, "cd %s" % path) and
       ReplyOnGoodResult(child, prompt,
                        "auth %s %s"  % (diradmin, diradmin_password)) and
       ReplyOnGoodResult(child, prompt, None)):
        print "Failed to log in and authenticate"
        failed_list = user_passwords.keys()
        return (failed_list, succeeded_list)

    # We are now logged in, and have a prompt waiting for us
    expected_list = [ pexpect.EOF, pexpect.TIMEOUT,
                     '(?i)error', 'Invalid Path', prompt ]
    desired_index = len(expected_list) - 1
    for record_name in user_passwords:        
        #print "Updating password for %s" % record_name,

        child.sendline("passwd %s/%s %s" % (record_type, record_name,
                                            user_passwords[record_name]))
        if child.expect(expected_list) == desired_index:
            #print ": Succeeded"
            succeeded_list.append(record_name)
        else:
            #print ": Failed"
            failed_list.append(record_name)
            child.expect(prompt)

    child.sendline("exit")
    child.expect(pexpect.EOF)

    return (failed_list, succeeded_list)


def ReplyOnGoodResult(child, desired, reply):
    """Helps analyze the results as we try to set passwords.

    child = a pexpect child process
    desired = The value we hope to see 
    reply = text to send if we get the desired result (or None for no reply)
    If we do get the desired result, we send the reply and return true.
    If not, we return false."""

    expectations = [ pexpect.EOF, pexpect.TIMEOUT, '(?i)error', desired ]
    desired_index = len(expectations) - 1

    index = child.expect(expectations)
    if index == desired_index:
        if reply:
            child.sendline(reply)
        return True
    else:
        return False

You can use it as follows:

# This example assumes that you have named the script given above 'pwchange.py'
# and that it is in the current working directory
import pwchange 

(failed, succeeded) = pwchange.ChangePasswords("ces", "/LDAPv3/127.0.0.1", 
     "diradmin", "******", 
     { 'Atwo807' : '*****', 'Atwo249' : '*****', 
       'Nonexist' : 'foobar', 'Bad' : 'bad' })

print failed, succeeded
['Bad', 'Nonexist'] ['Atwo249', 'Atwo807']