pam_cracklib module

pam_cracklib.so [...]

This module can be plugged into the password stack of a given application to provide some plug-in strength-checking for passwords.

The action of this module is to prompt the user for a password and check its strength against a system dictionary and a set of rules for identifying poor choices.

The first action is to prompt for a single password, check its strength and then, if it is considered strong, prompt for the password a second time (to verify that it was typed correctly on the first occasion). All being well, the password is passed on to subsequent modules to be installed as the new authentication token.

The strength checks works in the following manner: at first the Cracklib routine is called to check if the password is part of a dictionary; if this is not the case an additional set of strength checks is done. These checks are:

Palindrome

Is the new password a palindrome of the old one?

Case Change Only

Is the new password the the old one with only a change of case?

Similar

Is the new password too much like the old one? This is primarily controlled by one argument, difok which is a number of characters that if different between the old and new are enough to accept the new password, this defaults to 10 or 1/2 the size of the new password whichever is smaller.

To avoid the lockup associated with trying to change a long and complicated password, difignore is available. This argument can be used to specify the minimum length a new password needs to be before the difok value is ignored. The default value fordifignore is 23.

Simple

Is the new password too small? This is controlled by 5 arguments minlen, dcredit, ucredit, lcredit, and ocredit. See the section on the arguments for the details of how these work and there defaults.

Rotated

Is the new password a rotated version of the old password?

Already used

Was the password used in the past? Previously used passwords are to be found in /etc/security/opasswd.

This module with no arguments will work well for standard unix password encryption. With md5 encryption, passwords can be longer than 8 characters and the default settings for this module can make it hard for the user to choose a satisfactory new password. Notably, the requirement that the new password contain no more than 1/2 of the characters in the old password becomes a non-trivial constraint. For example, an old password of the form "the quick brown fox jumped over the lazy dogs" would be difficult to change... In addition, the default action is to allow passwords as small as 5 characters in length. For a md5 systems it can be a good idea to increase the required minimum size of a password. One can then allow more credit for different kinds of characters but accept that the new password may share most of these characters with the old password.

Options

debug

This option makes the module write information to syslog(3) indicating the behavior of the module (this option does not write password information to the log file).

type=XXX

The default action is for the module to use the following prompts when requesting passwords: "New UNIX password: " and "Retype UNIX password: ". The default word UNIX can be replaced with this option.

retry=N

Prompt user at most N times before returning with error. The default is 1

difok=N

This argument will change the default of 5 for the number of characters in the new password that must not be present in the old password. In addition, if 1/2 of the characters in the new password are different then the new password will be accepted anyway.

difignore=N

How many characters should the password have before difok will be ignored. The default is 23.

minlen=N

The minimum acceptable size for the new password (plus one if credits are not disabled which is the default). In addition to the number of characters in the new password, credit (of +1 in length) is given for each different kind of character (other, upper, lower and digit). The default for this parameter is 9 which is good for a old style UNIX password all of the same type of character but may be too low to exploit the added security of a md5 system. Note that there is a pair of length limits in Cracklib itself, a "way too short" limit of 4 which is hard coded in and a defined limit (6) that will be checked without reference to minlen. If you want to allow passwords as short as 5 characters you should not use this module.

dcredit=N

(N >= 0) This is the maximum credit for having digits in the new password. If you have less than or N digits, each digit will count +1 towards meeting the current minlen value. The default for dcredit is 1 which is the recommended value for minlen less than 10.

(N < 0) This is the minimum number of digits that must be met for a new password.

ucredit=N

(N >= 0) This is the maximum credit for having upper case letters in the new password. If you have less than or N upper case letters each letter will count +1 towards meeting the current minlen value. The default for ucredit is 1 which is the recommended value for minlen less than 10.

(N > 0) This is the minimum number of upper case letters that must be met for a new password.

lcredit=N

(N >= 0) This is the maximum credit for having lower case letters in the new password. If you have less than or N lower case letters, each letter will count +1 towards meeting the current minlen value. The default for lcredit is 1 which is the recommended value for minlen less than 10.

(N < 0) This is the minimum number of lower case letters that must be met for a new password.

ocredit=N

(N >= 0) This is the maximum credit for having other characters in the new password. If you have less than or N other characters, each character will count +1 towards meeting the current minlen value. The default for ocredit is 1 which is the recommended value for minlen less than 10.

(N < 0) This is the minimum number of other characters that must be met for a new password.

use_authtok

This argument is used to force the module to not prompt the user for a new password but use the one provided by the previously stacked passwordmodule.

dictpath=/path/to/dict

Path to the cracklib dictionaries.

Module Services Provided

Only he password service is supported.

Return Values

PAM_SUCCESS

The new password passes all checks.

PAM_AUTHTOK_ERR

No new password was entered, the username could not be determined or the new password fails the strength checks.

PAM_AUTHTOK_RECOVERY_ERR

The old password was not supplied by a previous stackked module or got not requested from the user. The first error can happen if use_authtok is specified.

PAM_SERVICE_ERR

A internal error occured.

Examples

For an example of the use of this module, we show how it may be stacked with the password component of pam_unix(8)

 

#
# These lines stack two password type modules. In this example the
# user is given 3 opportunities to enter a strong password. The
# "use_authtok" argument ensures that the pam_unix module does not
# prompt for a password, but instead uses the one provided by
# pam_cracklib.
#
passwd  password required       pam_cracklib.so retry=3
passwd  password required       pam_unix.so use_authtok

Another example (in the /etc/pam.d/passwd format) is for the case that you want to use md5 password encryption:

 

#%PAM-1.0
#
# These lines allow a md5 systems to support passwords of at least 14
# bytes with extra credit of 2 for digits and 2 for others the new
# password must have at least three bytes that are not present in the
# old password
#
password  required pam_cracklib.so \
               difok=3 minlen=15 dcredit= 2 ocredit=2
password  required pam_unix.so use_authtok nullok md5

And here is another example in case you don't want to use credits:

 

#%PAM-1.0
#
# These lines require the user to select a password with a minimum
# length of 8 and with at least 1 digit number, 1 upper case letter,
# and 1 other character
#
password  required pam_cracklib.so \
               dcredit=-1 ucredit=-1 ocredit=-1 lcredit=0 minlen=8
password  required pam_unix.so use_authtok nullok md5

Old News

Linux Password Security with pam_cracklib

Enabling pam_cracklib

The pam_cracklib module is enabled via the system's standard PAM configuration interface. On Debian systems, this is the /etc/pam.d/common-password file (but it's/etc/pam.d/system-auth on RedHat-derived systems--can't we all just get along?). The typical configuration looks something like this:

password required pam_cracklib.so retry=3 minlen=6 difok=3

password required pam_unix.so md5 use_authtok

The first line enables the pam_cracklib module and sets several module parameters. "retry=3" means that users get three chances to pick a good password before the passwd program aborts. Users can always re-run the passwd program and start over again, however. "minlen=6" sets the minimum number of characters in the password. Actually, since Linux systems generally use MD5 password hashes, which are not limited to 8 character passwords like the old DES56 hashes, you probably want to think about increasing the "minlen" parameter to something a bit longer. We'll come back to this notion a bit later in the article. "difok=3" sets the minimum number of characters that must be different from the previous password. If you increase "minlen", you may also want to increase this value as well.

The second line invokes the standard pam_unix module. The "md5" argument here is what enables standard Linux MD5 password hashes, though you have the option of using old-style DES56 hashes for backwards compatibility with legacy Unix systems. "use_authtok" tells pam_unix to not bother doing any of its own internal password checks, which duplicate many of the checks inpam_cracklib, but instead accept the password that the user inputs after it's been thoroughly checked by pam_cracklib.

Simple Checks

By default, pam_cracklib performs a number of basic checks on the new password:

 

• Is the new password just the old password with the letters reversed ("password" vs. "drowssap") or rotated ("password" vs. "asswordp")?
• Does the new password only differ from the old one due to change of case ("password" vs. "Password")?
• Are at least some minimum number of characters in the new password not present in the old password? This is where the "difok" parameter comes into play.

 

These are the same checks you get in the pam_unix module if you turn on the "obscure" flag, but since we're already using pam_cracklib we don't need to do this.

Length and Strength

While the "minlen" parameter controls the minimum password length, things are not as simple as they might appear. This is because pam_cracklib combines the notion of password length with password "strength" (the use of mixed-case and non-letter characters).

 

"minlen" is actually the minimum required length for a password consisting of all lower-case letters. But users get "length credits" for using upper- and lower-case letters, numbers, and non-alphanumeric characters. The default is normally that you can only get a maximum of "1 credit" for each type of character. So if the administrator sets "minlen=12", a user could still have an 8 character password if they used all four types of characters. Actually, since using a lower-case letter gets you a credit, the real minimum length for an all lower-case password is minlen-1.

 

The maximum credit for any particular class of characters is actually customizable. The four parameters "lcredit", "ucredit", "dcredit", and "ocredit" are used to set the maximum credit for lower-case, upper-case, numeric (digit), and non-alphanumeric (other) characters, respectively. For example, you could add the following parameters on the pam_cracklib line in the/etc/pam.d/common-password file:

 

lcredit=0 ucredit=1 dcredit=1 ocredit=2

 

In other words, lower-case characters aren't special at all, so you get no credit there. On the other hand we give extra credit if the user puts two or more non-alphanumeric characters in their password. One point is still the max credit for upper-case characters and numbers. Note that no matter what you set "minlen" to and no matter how many "credits" you give to your users, pam_cracklib will never let users pick passwords with less than six characters--this is a hard-coded internal minimum.

 

Play around with these values and find something that makes sense for your site, but as a starting point I might recommend "minlen=12 difok=4" for machines using MD5 password hashes. This means that the smallest password a user could have is 8 characters, and that's only if they use all four character sets.

Dictionary Checks

pam_cracklib also checks the user's password against it's own internal dictionaries of easily guessed passwords. On Debian systems, pam_cracklib's dictionaries live in/var/cache/cracklib and are rebuilt nightly by the 
/usr/sbin/update-cracklib script. Other Linux distros may have other mechanisms for updating the dictionaries (as far as I can tell, RedHat doesn't provide any tools for doing this).

 

The update-cracklib script searches a number of directories for input files, including /usr/local/dict and /usr/local/share/dict. So adding your own words is as easy as putting them in a file in one of these directories and running update-cracklib or waiting for cron to do it for you. Note that if you want to add other directories to update-cracklib's search path, you can do this by modifying the /etc/cracklib/cracklib.conf file (at least on Debian systems).

Password "History"

pam_cracklib is capable of consulting a user's password "history" and not allowing them to re-use old passwords. However, the functionality for actually storing the user's old passwords is enabled via the pam_unix module.

 

The first step is to make sure to create an empty /etc/security/opasswd file for storing old user passwords. If you forget to do this before enabling the history feature in the PAM configuration file, then all user password updates will fail because the pam_unix module will constantly be returning errors from the password history code due to the file being missing.

 

Treat your opasswd file like your /etc/shadow file because it will end up containing user password hashes (albeit for old user passwords that are no longer in use):

 

touch /etc/security/opasswd

chown root:root /etc/security/opasswd

chmod 600 /etc/security/opasswd

 

Once you've got the opasswd file set up, enable password history checking by adding the option "remember=<x>" to the pam_unix configuration line in the /etc/pam.d/common-password file. Here's how I have things set up on my Knoppix machine:

 

password required pam_cracklib.so retry=3 minlen=12 difok=4

password required pam_unix.so md5 remember=12 use_authtok

 

The value of the "remember" parameter is the number of old passwords you want to store for a user. It turns out that there's an internal maximum of 400 previous passwords, so values higher than 400 are all equivalent to 400. Before you complain about this limit, consider that even if your site forces users to change passwords every 30 days, 400 previous passwords represents over 30 years of password history. This is probably sufficient for even the oldest of legacy systems.

 

Once you've enabled password history, the opasswd file starts filling up with user entries that look like this:

 

hal:1000:<n>:<hash1>,<hash2>,...,<hashn>

 

The first two fields are the username and user ID. The <n> in the third field represents the number of old passwords currently being stored for the user--this value is incremented by one every time a new hash is added to the user's password history until <n> ultimately equals the value of the "remember" parameter set on the pam_unix configuration line. <hash1>,<hash2>,...,<hashn> are actually the MD5 password hashes for the user's old passwords.

Password Expiration

At this point you may be wondering how to get the system to automatically force users to change their password after some period of time. This is not actually the job of pam_cracklib. Instead, these parameters are set in the /etc/login.defs file on most Linux systems. PASS_MAX_DAYS is how often users have to change their passwords. PASS_MIN_DAYS is how long a user is forced to live with their new password before their allowed to change it again. PASS_WARN_AGE is the number of days before the password expiration date that the user is warned that their password is about to expire. The choice of values for these parameters is entirely dependent on site policy.

 

Note that these parameters are only applied to new accounts created with the default system useradd program. If you use some other mechanism for creating accounts on the system, then you'll have to use the chage command (this is not a typo) to manually set these parameters on your user accounts. And if you use a naming service such as LDAP or NIS for account management, then you're completely on your own.

 

By the way, if you've ever wondered what all those extra fields in the /etc/shadow file were for, the answer is that they store the password expiration/aging information for the user.

Linux check passwords against a dictionary attack

Dictionary attack is used for detecting password.

Wikipedia defines:
A dictionary attack is a technique for defeating a cipher or authentication mechanism by trying to determine its decryption key or passphrase by searching a large number of possibilities.

A dictionary attack also exploits the tendency of people to choose weak passwords, and is related to the previous attack. Password cracking programs usually come equipped with "dictionaries", or word lists, with thousands or even millions of entries of several kinds, including:
=> Words in various languages
=> names of people
=> Places
=> Commonly used passwords etc

However you can use the existence of these dictionary attack tools demonstrates the relative strengths of different password choices against such attacks.

Check user passwords against a dictionary attack

You can be configured to verify that passwords (read as weak password) cannot be guessed easily using Linux PAM module called pam_cracklib.so. It will check the passwd against dictionary words. User is not allowed to set new password until and unless conditions satisfied (i.e. weak password is not allowed).

Open password configuration file according to your Linux distribution. And make modification as follows.

Redhat/Fedora/CentOS Linux

cracklib PAM module is installed by default so no need to install anything. Just open config file:
# vi /etc/pam.d/system-auth
Append/modify as follows:
password required /lib/security/pam_cracklib.so retry=2 minlen=10 difok=6

Debian or Ubentu Linux

First install libpam-cracklib PAM module to enable cracklib support.
# apt-get install libpam-cracklib
or
$ sudo install libpam-cracklib
Now open config file:
# vi /etc/pam.d/system-auth
Append/modify as follows:
password required pam_cracklib.so retry=2 minlen=10 difok=6

Save and close the file

Where,

• retry=2 : Prompt user at most 2 times before returning with error
• minlen=10 : minimum length allowed for an account password is set to 10 characters. This is the minimum simplicity count for a good password. And you are allowed only 2 times using retry option.
• difok=6: How many characters can be the same in the new password relative to the old. User will see error - BAD PASSWORD: is too similar to the old one
• You can also apply following options to compute the 'unsimplicity' of the password.
  • dcredit=N : Digits characters
  • ucredit=N : Upper characters
  • lcredit=N : Lower characters
  • ocredit=N : Other characters

Please note that restrictions are only applied to normal users (not to root user).