PAM_CRACKLIB(8)

📦 version 1.4.0, in ubuntu jammy

✨ PAM_CRACKLIB ✨

okay bestie, welcome to pam_cracklib(8) from the Linux-PAM Manual 💖 this is the little PAM module that looks at your new password and goes "hmm... that's literally a dictionary word, try again" 🫶

🌸 NAME 🌸

pam_cracklib — a PAM module that checks your password against dictionary words 💅

🎀 SYNOPSIS 🎀

pam_cracklib.so [...]

💖 DESCRIPTION 💖

So here's the vibe: this module plugs right into the password stack of whatever application you want, and it gives you some plug-in strength-checking for passwords. It's the friend who tells you the truth about your outfit, except the outfit is your password. ✨

What it actually does: it prompts the user for a password, then checks how strong it is against a system dictionary and a whole set of rules for spotting bad choices.

The first move is to prompt for a single password, check its strength, and — if it's giving strong — prompt for the password a second time to make sure it was typed right the first time. All being well, that password gets passed on to the subsequent modules so it can be installed as the new authentication token. slay. 👑

The strength checks work like this: first the Cracklib routine gets called to see if the password is part of a dictionary. If it's not in there, then an extra set of strength checks runs. Those checks are:

  • Palindrome — is the new password a palindrome?
  • Case Change Only — is the new password just the old one with only a change of case?
  • Similar — is the new password too much like the old one? This one is primarily controlled by one argument, difok, which is a number of character changes (inserts, removals, or replacements) between the old and new password that are enough to accept the new password. This defaults to 5 changes.
  • Simple — is the new password too small? This is controlled by 6 arguments: minlen, maxclassrepeat, dcredit, ucredit, lcredit, and ocredit. See the section on the arguments for the details of how these work and their defaults.
  • Rotated — is the new password a rotated version of the old password?
  • Same consecutive characters — optional check for same consecutive characters.
  • Too long monotonic character sequence — optional check for too long monotonic character sequence.
  • Contains user name — optional check whether the password contains the user's name in some form.

With no arguments at all, this module will work well for standard unix password encryption 🦋 But heads up: 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 system it can be a good idea to increase the required minimum size of a password. Then you can 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. (And no, bestie, this option does not write password information to the log file. we would never 💀)
  • 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 the user at most N times before returning with error. The default is 1.
  • difok=N — this argument changes the default of 5 for the number of character changes in the new password that differentiate it from the old password.
  • 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 an 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 from the credit check is that a specific class 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.
  • maxsequence=N — reject passwords which contain monotonic character sequences longer than N. The default is 0, which means that this check is disabled. Examples of such sequences are '12345' or 'fedcb'. Note that most such passwords will not pass the simplicity check unless the sequence is only a minor part of the password.
  • maxclassrepeat=N — reject passwords which contain more than N consecutive characters of the same class. 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. 🚫
  • gecoscheck — check whether the words from the GECOS field (usually the full name of the user) longer than 3 characters in straight or reversed form are contained in the new password. If any such word is found, the new password is rejected.
  • enforce_for_root — the module will return error on a failed check also if the user changing the password is root. This option is off by default, which means that just the message about the failed check is printed but root can change the password anyway. Note that root is not asked for an old password, so the checks that compare the old and new password are not performed.
  • 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. that's it, that's the whole list. one job, done flawlessly 💅

🌈 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 — an internal error occurred.

🧚 EXAMPLES 🧚

For an example of the use of this module, here's 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

🦋 SEE ALSO 🦋

pam.conf(5), pam.d(5), pam(7)

💜 AUTHOR 💜

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

and that's the whole page bestie!! ✨💖🎀

this came from the Linux-PAM Manual, dated 06/08/2020 — no cap, that's the version and date, and pam_cracklib(8) is the page 🫶 go forth and pick a password that isn't literally a dictionary word, i'm begging 🙈💅

📦 where this page comes from