- Dictionary attack
- Brute-force attack
- Rainbow tables: A pre-computed list of password hashes (derived from commonly set passwords) is compared against an existing data dump to find the correct password in its plaintext form.It should be noted that this approach will be ineffective when password hashes are salted and salt values are too large, all of which increases the overall complexity.
- Single crack mode: JtR creators recommend running this mode first as it’s considered the quickest. Single crack mode uses information from UNIX passwd files -- users' full names, usernames, etc. -- as present in GECOS fields within UNIX passwd/shadow files to “guess” passwords. This can be helpful in cases when a user has set a password for an account based on commonly available information or phrase in the username (e.g. admin:password, john:john123).
- Wordlist mode: This mode relies on the user providing a text file with a list of passwords, ideally one per line and no duplicates.
- Incremental mode: JtR’s equivalent of brute-force is the most powerful cracking mode, but is so time consuming that for a password complex enough, it may never be able to complete in practical course of time and never terminate.
john mypasswd
This will try "single crack" mode first, then use a wordlist with rules,
and finally go for "incremental" mode.
john --show mypasswd
If the account list gets large and doesn't fit on the screen, you should, of course, use your shell's output redirection.
You might notice that many accounts have a disabled shell. You can make John skip those in the report. Assuming that the disabled shell is called "/etc/expired", the command would be:
john --show --shells=-/etc/expired mypasswd
or shorter, but will also match "/any/path/expired":
john --show --shells=-expired mypasswd
or if you also want to ignore some other shell, say "/etc/newuser":
john --show --shells=-expired,newuser mypasswd
To check if any root (UID 0) accounts got cracked:
john --show --users=0 mypasswd
or to check for cracked root (UID 0) accounts in multiple files:
john --show --users=0 *passwd* *.pwd
To display the root (username "root") account only:
john --show --users=root mypasswd
And finally, to check for privileged groups:
john --show --groups=0,1 mypasswd
4. You might prefer to manage the cracking modes manually. It is wise to start with "single crack" mode:
john --single mypasswd
or since the GNU-style double dashes are optional and since option names can be abbreviated for as long as they remain unambiguous:
john -si mypasswd
You should not abbreviate options in scripts which you would want to work with future versions of John since what is unambiguous now might become ambiguous with the addition of more options.
If you have more files to crack, it is preferable to load them at the same time:
john --single passwd1 passwd2
or even:
john --single *passwd* *.pwd
This way, John will run faster and might even crack more passwords than it would if you ran it on each password file separately.
5. To catch weak passwords not derived from readily available users' personal information, you should proceed with cracking modes demanding more processor time. First, let's try a tiny wordlist with word mangling rules enabled:
john --wordlist=password.lst --rules mypasswd
or abbreviating the options:
john -w=password.lst -ru mypasswd
Then proceed with a larger wordlist, also applying the mangling rules:
john --wordlist=all.lst --rules mypasswd
If you've got a lot of spare disk space to trade for performance and the hash type of your password files is relatively slow, you may use John's "unique" utility to eliminate any duplicate candidate passwords:
john --wordlist=all.lst --rules --stdout | unique mangled.lst
john --wordlist=mangled.lst mypasswd
If you know that your target hash type truncates passwords at a given length, you may optimize this even further:
john --wordlist=all.lst --rules --stdout=8 | unique mangled8.lst
john --wordlist=mangled8.lst mypasswd
Alternatively, you may simply use huge.lst available on Openwall wordlist collection CDs. It has word mangling rules pre-applied for the most common languages and it has any duplicates purged.
Depending on target hash type, the number of different salts (if applicable), the size of your wordlist, rules, and processor performance, wordlist-based cracking may take anywhere from under a second to many days.
You do not have to leave John running on a (pseudo-)terminal. If running John on a Unix-like system, you can simply disconnect from the server, close your xterm, etc. John will catch the SIGHUP ("hangup" signal) and continue running. Alternatively, you may prefer to start it in the background right away:
john --wordlist=all.lst --rules mypasswd &
Obviously, the "&" is specific to Unix shells and will not work on most other platforms.
You may further enhance this by specifying a session name:
john --session=allrules --wordlist=all.lst --rules mypasswd &
This ensures that you won't accidentally interfere with the instance of John running in the background if you proceed to start other sessions.
To view the status of a running session, use:
john --status
for the default session or:
john --status=allrules
for any other session. This works for both interrupted and running sessions. To obtain the most up-to-date information from a running session on a Unix-like system, send a SIGHUP to the appropriate "john" process.
Any interrupted sessions may be continued with:
john --restore
or:
john --restore=allrules
Finally, to make John have less impact on other processes, you should set the option "Idle = Y" in the configuration file.
To only crack accounts with a "good" shell (in general, the shell, user, and group filters described above work for all cracking modes as well):
john --wordlist=all.lst --rules --shells=sh,csh,tcsh,bash mypasswd
Like with all other cracking modes, it is faster to crack all the files you need cracked simultaneously:
john --wordlist=all.lst --rules passwd1 passwd2
You can crack some passwords only. This will try cracking all root (UID 0) accounts in all the password files:
john --wordlist=all.lst --rules --users=0 *passwd*
Alternatively, you may wish to not waste time cracking your very own passwords, if you're sure they're uncrackable:
john --wordlist=all.lst --rules --users=-root,solar *passwd*
Sometimes it is useful to split your password hashes into two sets which you crack separately, like:
john --wordlist=all.lst --rules --salts=2 *passwd*
john --wordlist=all.lst --rules --salts=-2 *passwd*
This will make John try salts used on two or more password hashes first and then try the rest. Total cracking time will be almost the same, but you will get some passwords cracked earlier, which is useful, for example, for penetration testing and demonstrations to management. Similarly, you may check all password hashes with a small wordlist, but only those that you can check faster (with "--salts=2") with a larger one. With large numbers of password hashes and/or with a highly non-uniform distribution of salts, it may be appropriate to use a threshold larger than 2 with "--salts" (sometimes even values as high as 1000 will do).
Note that the default wordlist rules include ":" (a no-op - try words as they are in the list) on the first line. If you already ran through a wordlist without using rules, and then decided to also try the same wordlist with rules, you'd better comment this line out.
6. The most powerful cracking mode in John is called "incremental" (not a proper name, but kept for historical reasons). You can simply run:
john --incremental mypasswd
or:
john -i mypasswd
This will use the default "incremental" mode parameters, which are defined in the configuration file's section named either [Incremental:ASCII] (for most hash types) or [Incremental:LM_ASCII] (for Windows LM hashes). By default, the [Incremental:ASCII] parameters are set to use the full printable ASCII character set (95 characters) and to try all possible password lengths from 0 to 13 (if the current hash type has a lower maximum password length, incremental mode's length limit is reduced accordingly). [Incremental:LM_ASCII] is similar, except that it takes advantage of LM hashes being case-insensitive and of their halves being limited to 7 characters each.
In some cases it is faster to use some other pre-defined incremental mode parameters and only crack simpler passwords, from a limited character set. The following command will try 10 different characters only, passwords from "0" to "99999999999999999999" (in an optimal order):
john -i=digits mypasswd
Of course, you can use most of the additional features demonstrated above for wordlist mode with "incremental" mode as well. For example, on a large-scale penetration test, you may have John crack only root (UID 0) accounts in a set of password files:
john -i -u=0 *.pwd
7. If you've got a password file for which you already have a lot of passwords cracked or obtained by other means, and the passwords are unusual, then you may want to generate a new charset file, based on character frequencies from that password file only:
john --make-charset=custom.chr mypasswd
Then use that new file with "incremental" mode.
If you've got many password files from a particular country, organization, etc., it might be useful to use all of them for the charset file that you then use to crack even more passwords from these files or from some other password files from the same place:
john --make-charset=custom.chr passwd1 passwd2
[ Configure your custom "incremental" mode now. See below. ]
john -i=custom passwd3
You can use some pre-defined or custom word filters when generating the charset file to have John consider some simpler passwords only:
john --make-charset=my_alpha.chr --external=filter_alpha mypasswd
If your "pot file" got large enough (or if you don't have any charset files at all), you might want to use it to generate a new set of main charset files:
makechr
where "makechr" is a script that invokes "john --make-charset=..." with varying filenames, for all of the external mode word filters defined in the configuration file. In this example, John will overwrite the charset files with new ones that are based on your entire $JOHN/john.pot (John uses the entire "pot file" if you don't specify any password files).
8. Finally, you might want to e-mail all users with weak passwords to tell them to change their passwords. (This is not always a good idea, though, since lots of people do not check their e-mail or ignore such messages, and the messages can be a hint for crackers.) Edit the "mailer" script supplied with John: the message it sends and possibly the mail command (especially if the password file is from a different machine). Then run:
mailer mypasswd
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