2 (Kerberos password strength checking plugin)
4 Maintained by Russ Allbery <eagle@eyrie.org>
6 Copyright 2006, 2007, 2009, 2010, 2012, 2013 The Board of Trustees of
7 the Leland Stanford Junior University. Portions copyright 1993 Alec
8 Muffett. Developed by Derrick Brashear and Ken Hornstein of Sine Nomine
9 Associates, on behalf of Stanford University. This software is
10 distributed under a BSD-style license and under the Artistic License.
11 Please see the section LICENSE for more information.
15 krb5-strength provides a password quality plugin for the MIT Kerberos
16 KDC (specifically the kadmind server), an external password quality
17 program for use with Heimdal, and a password history implementation for
18 use with Heimdal. Passwords can be tested with CrackLib, checked
19 against a CDB database of known weak passwords, checked for length,
20 checked for non-printable or non-ASCII characters that may be difficult
21 to enter reproducibly, required to contain particular character classes,
22 or any combination of these tests. It supports both Heimdal and MIT
23 Kerberos (1.9 or later).
27 Heimdal includes a capability to plug in external password quality
28 checks and comes with an example that checks passwords against CrackLib.
29 However, in testing at Stanford, we found that CrackLib with its default
30 transform rules does not catch passwords that can be guessed using the
31 same dictionary with other tools, such as Jack the Ripper. We then
32 discovered other issues with CrackLib with longer passwords, such as
33 some bad assumptions about how certain measures of complexity will
34 scale, and wanted to impose other limitations that it didn't support.
36 This plugin provides the ability to check password quality against the
37 standard version of CrackLib, or against a modified version of CrackLib
38 that only passes passwords that resist attacks from both Crack and Jack
39 the Ripper using the same rule sets. It also supports doing simpler
40 dictionary checks against a CDB database, which is fast with very large
41 dictionaries, and imposing other programmatic checks on passwords such
42 as character class requirements.
44 For Heimdal, it includes both a program usable as an external password
45 quality check and a plugin that implements the dynamic module API. For
46 MIT Kerberos (1.9 or later), it includes a plugin for the password
47 quality (pwqual) plugin API.
49 krb5-strength can be built with either the system CrackLib or with the
50 modified version of CrackLib included in this package. Note, however,
51 that if you're building against the system CrackLib, Heimdal includes in
52 the distribution a strength-checking plugin and an external password
53 check program that use the system CrackLib. With Heimdal, it would
54 probably be easier to use that plugin or program than build this package
55 unless you want the modified CrackLib.
57 For information about the changes to the CrackLib included in this
58 toolkit, see cracklib/HISTORY. The primary changes are tighter rules,
59 which are more aggressive at finding dictionary words with characters
60 appended and prepended, which tighten the requirements for password
61 entropy, and which add stricter rules for longer passwords. They are
62 also minor changes to fix portability issues and remove some code that
63 doesn't make sense in the kadmind context.
65 Ideally, the changes to CrackLib should be added to the standard
66 CrackLib distribution by adding an additional interface to configure its
67 behavior, at which point this package can likely wither away in favor of
68 much simpler plugins that link to the standard CrackLib library.
70 krb5-strength also includes a password history implementation for
71 Heimdal. This is separate from the password strength implementation but
72 can be stacked with it so that both strength and history checks are
73 performed. This history implementation is available only via the
74 Heimdal external password quality interface. MIT Kerberos includes its
75 own password history implementation.
79 For Heimdal, you may use either the external password quality check
80 tool, installed as heimdal-strength, or the plugin as you choose. It
81 has been tested with Heimdal 1.2.1 and later, but has not recently been
82 tested with versions prior to 1.5.
84 For MIT Kerberos, version 1.9 or higher is required for the password
85 quality plugin interface. MIT Kerberos does not support an external
86 password quality check tool directly, so you will need to install the
89 You can optionally build against the system CrackLib library. Any
90 version should be supported, but note that some versions, particularly
91 older versions close to the original code, do things like printing
92 diagnostics to stderr, calling exit, and otherwise not being
93 well-behaved for use inside plugins or libraries. If using a system
94 CrackLib library, use version 2.8.22 or later to avoid these problems.
96 You can also optionally build against the TinyCDB library, which
97 provides support for simpler and faster password checking against a CDB
98 dictionary file, and the SQLite library (a version new enough to support
99 the sqlite3_open_v2 API; 3.7 should be more than sufficient), which
100 provides support for checking whether passwords are within edit distance
101 one of a dictionary word.
103 For this module to be effective for either Heimdal or MIT Kerberos, you
104 will also need to construct a dictionary. The mkdict and packer
105 utilities to build a CrackLib dictionary from a word list are included
106 in this toolkit but not installed by default. You can run them out of
107 the cracklib directory after building. You can also use the utilities
108 that come with the stock CrackLib package (often already packaged in a
109 Linux distribution); the database format is compatible.
111 For building a CDB or SQLite dictionary, use the provided
112 krb5-strength-wordlist program. For CDB dictionries, the cdb utility
113 must be on your PATH. For SQLite, the DBI and DBD::SQLite Perl modules
114 are required. krb5-strength-wordlist requires Perl 5.006 or later.
116 For a word list to use as source for the dictionary, you can use
117 /usr/share/dict/words if it's available on your system, but it would be
118 better to find a more comprehensive word list. Since word lists are
119 bulky, often covered by murky copyrights, and easily locatable on the
120 Internet with a modicum of searching, none are included in this toolkit.
122 The password history program, heimdal-history, requires Perl 5.010 or
123 later plus the following CPAN modules:
127 Getopt::Long::Descriptive
132 and their dependencies.
134 To run the test suite, you will need Perl 5.010 or later and the
135 dependencies of the heimdal-history program. The following additional
136 Perl modules will be used by the test suite if present:
145 All are available on CPAN. Those tests will be skipped if the modules
148 To enable tests that don't detect functionality problems but are used to
149 sanity-check the release, set the environment variable RELEASE_TESTING
150 to a true value. To enable tests that may be sensitive to the local
151 environment or that produce a lot of false positives without uncovering
152 many problems, set the environment variable AUTHOR_TESTING to a true
155 To bootstrap from a Git checkout, or If you change the Automake files
156 and need to regenerate Makefile.in, you will need Automake 1.11 or
157 later. For bootstrap or if you change configure.ac or any of the m4
158 files it includes and need to regenerate configure or config.h.in, you
159 will need Autoconf 2.64 or later. You will also need Perl 5.010 or
160 later and the DBI, DBD::SQLite, JSON, Perl6::Slurp, and Readonly modules
161 (from CPAN) to bootstrap the test suite data from a Git checkout.
163 COMPILING AND INSTALLING
165 You can build and install the plugin with the standard commands:
171 Pass --enable-silent-rules to configure for a quieter build (similar to
172 the Linux kernel). Use make warnings instead of make to build with full
173 GCC compiler warnings (requires a relatively current version of GCC).
175 The last step will probably have to be done as root. By default, the
176 plugin is installed as /usr/local/lib/krb5/plugins/pwqual/strength.so
177 and the Heimdal external password check function is installed as
178 /usr/local/bin/heimdal-strength. You can change these paths with the
179 --prefix, --libdir, and --bindir options to configure.
181 To build with the system version of CrackLib, pass --with-cracklib to
182 configure. You can optionally add a directory, giving the root
183 directory where CrackLib was installed, or separately set the include
184 and library path with --with-cracklib-include and --with-cracklib-lib.
186 krb5-strength will automatically build with TinyCDB if it is found. To
187 specify the installation path of TinyCDB, use --with-tinycdb. You can
188 also separately set the include and library path with
189 --with-tinycdb-include and --with-tinycdb-lib.
191 Similarly, krb5-strength will automatically build with SQLite if it is
192 found. To specify the installation path of SQLite, use --with-sqlite.
193 You can also separately set the include and library path with
194 --with-sqlite-include and --with-sqlite-lib.
196 Normally, configure will use krb5-config to determine the flags to use
197 to compile with your Kerberos libraries. If krb5-config isn't found, it
198 will look for the standard Kerberos libraries in locations already
199 searched by your compiler. If the the krb5-config script first in your
200 path is not the one corresponding to the Kerberos libraries you want to
201 use or if your Kerberos libraries and includes aren't in a location
202 searched by default by your compiler, you need to specify a different
203 Kerberos installation root via --with-krb5=PATH. For example:
205 ./configure --with-krb5=/usr/pubsw
207 You can also individually set the paths to the include directory and the
208 library directory with --with-krb5-include and --with-krb5-lib. You may
209 need to do this if Autoconf can't figure out whether to use lib, lib32,
210 or lib64 on your platform.
212 To specify a particular krb5-config script to use, either set the
213 PATH_KRB5_CONFIG environment variable or pass it to configure like:
215 ./configure PATH_KRB5_CONFIG=/path/to/krb5-config
217 To not use krb5-config and force library probing even if there is a
218 krb5-config script on your path, set PATH_KRB5_CONFIG to a nonexistent
221 ./configure PATH_KRB5_CONFIG=/nonexistent
223 krb5-config is not used and library probing is always done if either
224 --with-krb5-include or --with-krb5-lib are given.
226 You can pass the --enable-reduced-depends flag to configure to try to
227 minimize the shared library dependencies encoded in the binaries. This
228 omits from the link line all the libraries included solely because the
229 Kerberos libraries depend on them and instead links the programs only
230 against libraries whose APIs are called directly. This will only work
231 with shared Kerberos libraries and will only work on platforms where
232 shared libraries properly encode their own dependencies (such as Linux).
233 It is intended primarily for building packages for Linux distributions
234 to avoid encoding unnecessary shared library dependencies that make
235 shared library migrations more difficult. If none of the above made any
236 sense to you, don't bother with this flag.
240 First, build and install either a CrackLib dictionary as described in
241 REQUIREMENTS above, or build a CDB or SQLite dictionary with
242 krb5-strength-wordlist. (Or any combination thereof.) The CrackLib
243 dictionary will consist of three files, one each ending in *.hwm, *.pwd,
244 and *.pwi. The CDB and SQLite dictionaries will be single files,
245 conventionally ending in *.cdb and *.sqlite respectively. Install those
246 files somewhere on your system. Then, follow the relevant instructions
247 below for either Heimdal or MIT Kerberos.
249 See "Other Settings" below for additional krb5.conf setting supported by
250 both Heimdal and MIT Kerberos.
254 There are two options: using an external password check program, or
255 using the plugin. I recommend the external password check program
256 unless you encounter speed problems with that approach that cause
259 For either approach, first add a stanza like the following to the
260 [appdefaults] section of your /etc/krb5.conf (or wherever your krb5.conf
264 password_dictionary = /path/to/cracklib/dictionary
265 password_dictionary_cdb = /path/to/cdb/dictionary.cdb
266 password_dictionary_sqlite = /path/to/sqlite/dictionary.sqlite
269 The first setting configures a CrackLib dictionary, the second a CDB
270 dictionary, and the third a SQLite dictionary. The provided path should
271 be the full path to the dictionary files, omitting the trailing *.hwm,
272 *.pwd, and *.pwi extensions for the CrackLib dictionary. You can use
273 any combination of the three settings. If you use more than one,
274 CrackLib will be checked first, then CDB, and then SQLite as
277 When checking against a CDB database, the password, the password with
278 the first character removed, the last character removed, the first and
279 last characters removed, the first two characters removed, and the last
280 two characters removed will all be checked against the dictionary.
282 When checking a SQLite database, the password will be rejected if it is
283 within edit distance one of any word in the dictionary, meaning that the
284 database word can be formed from the password by deleting, adding, or
285 changing a single character.
287 Then, for the external password checking program, add a new section (or
288 modify the existing [password_quality] section) to look like the
292 policies = external-check
293 external_program = /usr/local/bin/heimdal-strength
295 You can, of course, combine this policy with others. Replace the path
296 with the full path to wherever you have installed heimdal-strength. You
297 can put this section in your kdc.conf instead of krb5.conf if you
300 If you want to instead use the module, use the following section
304 policies = krb5-strength
305 policy_libraries = /usr/local/lib/krb5/plugins/pwqual/strength.so
307 in either krb5.conf or kdc.conf. Note that some older versions of
308 Heimdal have a bug in the support for loading modules when
309 policy_libraries is set. If you get an error like:
311 didn't find `kadm5_password_verifier' symbol in `(null)'
313 you may have to omit policy_libraries in your configuration and instead
314 pass the --check-library argument to kpasswdd specifying the library to
317 Additional configuration is required to use the history implementation.
318 Ensure that its dependencies are installed, and then examine the local
319 configuration settings at the top of the heimdal-history program. By
320 default, it requires a _history user and _history group be present on
321 the system, and all history information will be read and written as that
322 user and group. It also requires a nobody user and nogroup group to be
323 present, and all strength checking will be done as that user and group.
324 It uses various files in /var/lib/heimdal-history to store history and
325 statistical information by default, so if using the defaults, create
326 that directory and ensure it is writable by the _history user.
328 Once that setup is done, change your [password_quality] configuration
332 policies = external-check
333 external_program = /usr/local/bin/heimdal-history
335 The heimdal-history program will automatically also run heimdal-strength
336 as well, looking for it in /usr/local/bin, /usr/bin, and /bin. Change
337 the PATH setting at the top of the script if you have different
338 requirements. You should continue to configure heimdal-strength as if
339 you were running it directly.
343 To add this module to the list of password quality checks, add a section
344 to krb5.conf (or to a separate kdc.conf if you use that) like:
348 module = strength:/usr/local/lib/krb5/plugins/pwqual/strength.so
351 to register the plugin.
353 There are two ways to tell where the dictionary is. One option is to
354 use krb5.conf (and in this case you must use krb5.conf, even if you use
355 a separate kdc.conf file). For this approach, add the following to the
356 [appdefaults] section:
359 password_dictionary = /path/to/cracklib/dictionary
360 password_dictionary_cdb = /path/to/cdb/dictionary.cdb
361 password_dictionary_sqlite = /path/to/sqlite/dictionary.sqlite
364 The first setting configures a CrackLib dictionary, the second a CDB
365 dictionary, and the third a SQLite dictionary. The provided path should
366 be the full path to the dictionary files, omitting the trailing *.hwm,
367 *.pwd, and *.pwi extensions for the CrackLib dictionary. You can use
368 any combination of the three settings. If you use more than one,
369 CrackLib will be checked first, then CDB, and then SQLite as
372 When checking against a CDB database, the password, the password with
373 the first character removed, the last character removed, the first and
374 last characters removed, the first two characters removed, and the last
375 two characters removed will all be checked against the dictionary.
377 When checking a SQLite database, the password will be rejected if it is
378 within edit distance one of any word in the dictionary, meaning that the
379 database word can be formed from the password by deleting, adding, or
380 changing a single character.
382 The second option is to use the normal dict_path setting. In the
383 [realms] section of your krb5.conf kdc.conf, under the appropriate realm
384 or realms, specify the path to the dictionary:
386 dict_file = /path/to/cracklib/dictionary
388 This will be taken as a CrackLib dictionary path, the same as the
389 setting for password_dictionary above. The provided path should be the
390 full path to the dictionary files, omitting the trailing *.hwm, *.pwd,
391 or *.pwi extension. However, be aware that, if you use this approach,
392 you will probably want to disable the built-in standard dict pwqual
393 plugin by adding the line:
397 to the pwqual block of the [plugins] section as shown above. Otherwise,
398 it will also try to load a dictionary at the same path to do simple
401 You can also mix and match these settings, by using dict_path for the
402 CrackLib dictionary path and krb5.conf for the CDB or SQLite dictionary
403 paths. If both settings are used for the CrackLib path, krb5.conf
404 overrides the dict_path setting (so that dict_path can be used for other
405 password quality modules). There is no way to specify a CDB or SQLite
406 dictionary via the dict_path setting.
410 The following additional settings are supported in the [appdefaults]
411 section of krb5.conf when running under either Heimdal or MIT Kerberos.
415 If set to a numeric value, passwords with fewer than this number of
416 unique characters will be rejected. This can be used to reject, for
417 example, passwords that are long strings of the same character or
418 repetitions of small numbers of characters, which may be too easy to
423 If set to a numeric value, passwords with fewer than that number of
424 characters will be rejected, independent of any length restrictions
425 in CrackLib. Note that this setting does not bypass the minimum
426 length requirements in CrackLib itself (which, for the version
427 embedded in this package, is eight characters).
429 require_ascii_printable
431 If set to a true boolean value, rejects any password that contains
432 non-ASCII characters or ASCII control characters. Spaces are
433 allowed; tabs are not (at least assuming the POSIX C locale). No
434 canonicalization or character set is defined for Kerberos passwords
435 in general, so you may want to reject non-ASCII characters to avoid
436 interoperability problems with computers with different default
437 character sets or Unicode normalization forms.
441 This option allows specification of more complex character class
442 requirements. The value of this parameter should be one or more
443 whitespace-separated rule. Each rule has the syntax:
445 [<min>-<max>:]<class>[,<class>...]
447 where <class> is one of "upper", "lower", "digit", or "symbol"
448 (without the quote marks). The symbol class includes all characters
449 other than alphanumeric characters, including space. The listed
450 classes must appear in the password. Separate multiple required
451 classes with a comma (and no space).
453 The character class checks will be done in whatever locale the
454 plugin or password check program is run in, which will normally be
455 the POSIX C locale but may be different depending on local
460 require_classes = upper,lower,digit
462 This requires all passwords contain at least one uppercase letter,
463 at least one lowercase letter, and at least one digit.
465 If present, <min> and <max> specify the minimum password length and
466 maximum password length to which this rule applies. This allows one
467 to specify character class requirements that change with password
468 length. So, for example:
470 require_classes = 8-19:upper,lower 8-15:digit 8-11:symbol
472 requires all passwords from 8 to 11 characters long contain all four
473 character classes, passwords from 12 to 15 characters long contain
474 upper and lower case and a digit, and passwords from 16 to 19
475 characters long contain both upper and lower case. Passowrds longer
476 than 20 characters have no character class restrictions. (This
477 example is probably used in conjunction with minimum_length = 8.)
481 If set to a true boolean value, the password must contain at least
482 one character that is not a letter (uppercase or lowercase) or a
483 space. This may be helpful in combination with passphrases; users
484 may choose a stock English phrase, and this will force at least some
485 additional complexity.
487 You can omit any dictionary setting and only use the above settings, in
488 which case only the above checks and checks for passwords based on the
489 principal will be done, bypassing any dictionary check. (But for that
490 simple style of password strength checking, there are probably better
491 strength checking plugins already available.)
495 The krb5-strength web page at:
497 http://www.eyrie.org/~eagle/software/krb5-strength/
499 will always have the current version of this package, the current
500 documentation, and pointers to any additional resources.
502 I welcome bug reports and patches for this package at eagle@eyrie.org.
503 However, please be aware that I tend to be extremely busy and work
504 projects often take priority. I'll save your mail and get to it as soon
505 as I can, but it may take me a couple of months.
509 krb5-strength is maintained using Git. You can access the current
510 source by cloning the repository at:
512 git://git.eyrie.org/kerberos/krb5-strength.git
514 or view the repository via the web at:
516 http://git.eyrie.org/?p=kerberos/krb5-strength.git
518 When contributing modifications, either patches (possibly generated by
519 git format-patch) or Git pull requests are welcome.
523 The krb5-strength package as a whole is covered by the following
524 copyright statement and license:
526 Copyright 2006, 2007, 2009, 2010, 2012, 2013
527 The Board of Trustees of the Leland Stanford Junior University
529 Permission is hereby granted, free of charge, to any person obtaining
530 a copy of this software and associated documentation files (the
531 "Software"), to deal in the Software without restriction, including
532 without limitation the rights to use, copy, modify, merge, publish,
533 distribute, sublicense, and/or sell copies of the Software, and to
534 permit persons to whom the Software is furnished to do so, subject to
535 the following conditions:
537 The above copyright notice and this permission notice shall be
538 included in all copies or substantial portions of the Software.
540 THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
541 EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
542 MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
543 IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
544 CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
545 TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
546 SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
548 The embedded version of CrackLib (all files in the cracklib
549 subdirectory) is covered by the Artistic license. See the file
550 cracklib/LICENCE for more information. Combined derivative works that
551 include this code, such as binaries built with the embedded CrackLib,
552 will need to follow the terms of the Artistic license as well as the
555 All other individual files without an explicit exception below are
556 released under this license. Some files may have additional copyright
557 holders as noted in those files. There is detailed information about
558 the licensing of each file in the LICENSE file in this distribution.
560 Some files in this distribution are individually released under
561 different licenses, all of which are compatible with the above general
562 package license but which may require preservation of additional
563 notices. All required notices are preserved in the LICENSE file.