PAM_CRACKLIB(8)

Linux-PAM Manual

PAM_CRACKLIB(8)

NAME¶

pam_cracklib - PAM module to check the password against dictionary words

SYNOPSIS¶

pam_cracklib.so [ ...]

DESCRIPTION¶

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?

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 for difignore 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?

Same consecutive characters

Optional check for same consecutive characters.

Contains user name

Optional check whether the password contains the user's name in some form.

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).

authtok_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 example word UNIX can be replaced with this option, by default it is empty.

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.

minclass=N

The minimum number of required classes of characters for the new password. The default number is zero. The four classes are digits, upper and lower letters and other characters. The difference to the credit check is that a specific class if of characters is not required. Instead N out of four of the classes are required.

maxrepeat=N

Reject passwords which contain more than N same consecutive characters. The default is 0 which means that this check is disabled.

reject_username

Check whether the name of the user in straight or reversed form is contained in the new password. If it is found the new password is rejected.

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 password module.

dictpath=/path/to/dict

Path to the cracklib dictionaries.

MODULE TYPES PROVIDED¶

Only the password module type is provided.

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 stacked module or got not requested from the user. The first error can happen if use_authtok is specified.

PAM_SERVICE_ERR

A internal error occurred.

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

AUTHOR¶

pam_cracklib was written by Cristian Gafton <gafton@redhat.com>

06/04/2011