Been Cracked? Just Put PAM On It!

PAM and Passwords

We begin by taking a look at how PAM may be used to control password choices and password aging. Example 5-3 shows the /etc/pam.d/passwd configuration file. Notice that there are two entries with the password module type. This is an example of stacked entries. Let's go through these entries in detail. It gets a little complicated, but once we get through it, the rest of this chapter should be easier to understand.

Example 5-3 The /etc/pam.d/passwd Configuration File

Auth    required  /lib/security/pam_pwdb.so
Account  required  /lib/security/pam_pwdb.so
Password  required  /lib/security/pam_cracklib.so retry=3
Password  required  /lib/security/pam_pwdb.so use_authtok

The Password Database Library The /lib/security/pam_pwdb module interacts with and requires the password database library (pwdb library, libpwdb, found in /lib). The purpose of the pwdb library is to provide a centralized database for lookups of information associated with users and groups. Specifically, it provides the source of passwords for pam_pwdb.

The pwdb library requires an /etc/pwdb.conf configuration file. Example 5-4 shows a sample file. The file contains two distinct sections—the first, preceded by the user: keyword, pertains to information associated with users.

Example 5-4 The /etc/pwdb.conf File

# This is the configuration file for the pwdb library #
user:
  unix+shadow
  nis+unix+shadow 
group:
  unix+shadow
  nis+unix+shadow 

The second, preceded by the group: keyword, pertains to information associated with groups. After the section header, you see keywords concatenated with + symbols, called lists. Each list represents the collection of databases that are merged to form the records for each user or group. For example, the unix+shadow list under the user: section is a list consisting of the contents of the /etc/passwd and /etc/shadow files. The nis+unix+shadow entry specifies the list containing NIS ³ (formerly yp) records as well as the contents of the /etc/passwd and /etc/shadow files. The entries for groups are entirely similar.

When the pam_pwdb module is invoked, it in turn invokes the pwdb library.

The pwdb library will find the first entry that matches the user or group passed to it by pam_pwdb, based on the entries in /etc/pwdb.conf. Thus order is important in that file. The lists that appear first are searched first and pwdb stops at the first match.

The pam_pwdb Module The pam_pwdb module is capable of operating in support of all four module types.

Module type auth. When the auth type is specified, it functions to authenticate the user by prompting the user for a password and querying pwdb with the username/password pair. It can take the following arguments: debug, use_first_pass, try_first_pass, nullok, and nodelay. All other arguments supported by pam_pwdb but not by the auth module type are silently ignored. Any other arguments will be logged as errors through syslog, but will not affect the function of the module. The first three arguments are described in Table 5.3 on page 85.

The nullok argument allows accounts with no passwords. Of course, you would never specify this argument, right? The default behavior, therefore, is that this module treats accounts with no passwords as if they were locked accounts. This is good!

The nodelay argument causes this module to return immediately on failure. Normally this module will delay prior to reporting an authentication failure, making it slower for an attacker to try to guess passwords.

So what is the purpose of the line:

auth required /lib/security/pam_pwdb.so 

Table 5.4 Arguments for pam_pwdb Module Type password

in Example 5-3 on page 86? It causes users to be prompted for their old password prior to being prompted (by pam_cracklib) for their new password! Cool, huh? The root user is excepted from this requirement.

Module type password. When the pam_pwdb module is used as module type password, it performs the task of updating the password. This means that, when a user invokes the passwd command, upon successfully entering a new password (as determined by pam_cracklib), pam_pwdb will update the new password with the pwdb library. The acceptable argument types are debug; nullok; not_set_pass; use_authtok; try_first_pass; use_first_pass; md5; ⁴ bigcrypt; shadow; radius; unix. Those arguments not already discussed are summarized in Table 5.4.

Notice in Example 5-3 on page 86, the use_authtok argument is specified. This means that pam_pwdb will use the new password it receives from pam_cracklib. Essentially, pam_cracklib controls the choice of the new password, but pam_pwdb actually does the updating.

Module type account. When using pam_pwdb as module type account, its purpose is to verify account information of the user. This includes validating that the user has an account, what password aging parameters, if any, are associated with the user, and whether or not the user needs to be warned about a pending password expiration or offered advice on the choice of a new password. As this module type, pam_pwdb recognizes only the debug argument.

Module type session. When using pam_pwdb as module type session, its sole purpose is to log the username and service type to syslog, once at login and then subsequently at logout. It recognizes no arguments.

The pam_cracklib Module The pam_cracklib module is intended to work only with the password module type. It's purpose is to check a password for strength and for length, both elements being configurable with arguments described below. This module functions only in a stack, since it has no updating capabilities. It requires the libcrack library and the cracklib_dict Crack dictionary, both of which are found in /usr/lib of the Red Hat 5.2/6.0 distribution. As you can see, this module depends heavily on elements of the Crack utility, which is discussed in Chapter 12.

The flexibility of PAM is evidenced by the fact that this is not the only password strength checking PAM module. Another is pam_passwd+, which is available at

http://www.us.kernel.org/pub/linux/libs/pam/modules.html

The arguments available to pam_cracklib are described in Table 5.5.

Table 5.5 Arguments for pam_cracklib

In addition to the configurable options in Table 5.5, the pam_cracklib checks the new password for strength by

1. Verifying that the new password is not the reverse of the old password.

2. Verifying that the new password is not a simple case change of some characters of the old password.

3. Checking if the new password is in the cracklib_dict. If it is, it warns the user but does not force another password choice.

The Effect of Stacking pam_pwdb and pam_cracklib For Module Type password In this section, we will take a look at how pam_cracklib interacts with pam_pwdb in the stack shown in Example 5-3 on page 86.

Recall that in Example 5-3 the two stacked entries appeared in /etc/ pam.d/passwd.

Password  required  /lib/security/pam_cracklib.so retry=3
Password  required  /lib/security/pam_pwdb.so use_authtok

The first entry invokes pam_cracklib and prompts the user for his or her new password (remember that the auth module type pam_pwdb entry is responsible for prompting the user for his or her old password, for authentication). After the user has supplied the new password, pam_cracklib requests that it be repeated for verification. Once completed, pam_cracklib performs its checks to see if the password is acceptable. If so, it passes the new password to pam_pwdb which has the use_authtok argument meaning it will accept this new password and request the pwdb library to update the appropriate database.

Let's take a look at the power and flexibility of these modules by considering an example. Suppose that we would like to use md5 instead of the standard UNIX crypt(3) mechanism for hashing purposes. This is a good idea, because popular password-guessing tools like Crack require significantly more CPU resources to guess passwords (see The White Hat Use of Crack on page 337). The major benefit of using md5 is that you can require longer passwords—20, 30, or even more characters. Let's look at an example requiring 20-character passwords. We'll also set the type argument to see if our users are paying attention. Example 5-5 shows what the stack might look like if we impose these changes in /etc/pam.d/passwd.

Example 5-5 Using md5 and minlen in /etc/pam.d/passwd

Password  required  /lib/security/pam_cracklib.so minlen=20\
     retry=3 type=SECRET
password  required  /lib/security/pam_pwdb.so md5 use_authtok

Now that we have made these changes, let's see what happens to the user, mary, when she tries to change her password in Example 5-6. She is offered three opportunities to select a password. This is due to the retry=3 argument to pam_cracklib (see Example 5-5 on page 91). Actually, it appears that Mary is attempting to make good password choices. Unfortunately she doesn't know about the changes to PAM and therefore doesn't know that she needs to choose a longer password. So you, being the responsive administrator, inform her that she needs to use a 20-character password. "What?!" she replies. And you gently tell her that she can use a passphrase. Happy now, she goes about her task (Example 5-7).

Example 5-6 Unsuccessful Password Change

$passwd
Changing password for mary
(current)UNIX password:j3n#Ky
New SECRET password:Rt!72g
BAD PASSWORD:is too simple
New SECRET password:8x@$iI
BAD PASSWORD:is too simple
New SECRET password:P5-+yh
BAD PASSWORD:is too simple
New SECRET password:8x@$iI
passwd:Authentication token manipulation error
$

Example 5-7 Successful Password Change

$passwd
Changing password for mary
(current)UNIX password:j3n#Ky
New SECRET password:I need a #%$3+raise
Retype new SECRET password:I need a #%$3+raise
passwd:all authentication tokens updated successfully
$

Notice that the message displayed by pam_cracklib contains our type entry, New SECRET password:. This change does not appear in the message from pam_pwdb—(current) UNIX password:—because pam_pwdb does not support the type argument.

While she chose a password of 20 characters (spaces count!), she wouldn't have been required to, because the default values of dcredit, ucredit, lcredit, and ocredit (see Table 5.5 on page 89) are 1 each. Because of her password choice, she would have a credit of 4, which would have allowed her to choose a password as short as 16 characters in length.