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 may be sensitive to the local environment or that
149 produce a lot of false positives without uncovering many problems, set
150 RRA_MAINTAINER_TESTS to a true value.
152 To bootstrap from a Git checkout, or If you change the Automake files
153 and need to regenerate Makefile.in, you will need Automake 1.11 or
154 later. For bootstrap or if you change configure.ac or any of the m4
155 files it includes and need to regenerate configure or config.h.in, you
156 will need Autoconf 2.64 or later. You will also need Perl 5.010 or
157 later and the JSON, Perl6::Slurp, and Readonly modules (from CPAN) to
158 bootstrap the test suite data from a Git checkout.
160 COMPILING AND INSTALLING
162 You can build and install the plugin with the standard commands:
168 Pass --enable-silent-rules to configure for a quieter build (similar to
169 the Linux kernel). Use make warnings instead of make to build with full
170 GCC compiler warnings (requires a relatively current version of GCC).
172 The last step will probably have to be done as root. By default, the
173 plugin is installed as /usr/local/lib/krb5/plugins/pwqual/strength.so
174 and the Heimdal external password check function is installed as
175 /usr/local/bin/heimdal-strength. You can change these paths with the
176 --prefix, --libdir, and --bindir options to configure.
178 To build with the system version of CrackLib, pass --with-cracklib to
179 configure. You can optionally add a directory, giving the root
180 directory where CrackLib was installed, or separately set the include
181 and library path with --with-cracklib-include and --with-cracklib-lib.
183 krb5-strength will automatically build with TinyCDB if it is found. To
184 specify the installation path of TinyCDB, use --with-tinycdb. You can
185 also separately set the include and library path with
186 --with-tinycdb-include and --with-tinycdb-lib.
188 Similarly, krb5-strength will automatically build with SQLite if it is
189 found. To specify the installation path of SQLite, use --with-sqlite.
190 You can also separately set the include and library path with
191 --with-sqlite-include and --with-sqlite-lib.
193 Normally, configure will use krb5-config to determine the flags to use
194 to compile with your Kerberos libraries. If krb5-config isn't found, it
195 will look for the standard Kerberos libraries in locations already
196 searched by your compiler. If the the krb5-config script first in your
197 path is not the one corresponding to the Kerberos libraries you want to
198 use or if your Kerberos libraries and includes aren't in a location
199 searched by default by your compiler, you need to specify a different
200 Kerberos installation root via --with-krb5=PATH. For example:
202 ./configure --with-krb5=/usr/pubsw
204 You can also individually set the paths to the include directory and the
205 library directory with --with-krb5-include and --with-krb5-lib. You may
206 need to do this if Autoconf can't figure out whether to use lib, lib32,
207 or lib64 on your platform.
209 To specify a particular krb5-config script to use, either set the
210 PATH_KRB5_CONFIG environment variable or pass it to configure like:
212 ./configure PATH_KRB5_CONFIG=/path/to/krb5-config
214 To not use krb5-config and force library probing even if there is a
215 krb5-config script on your path, set PATH_KRB5_CONFIG to a nonexistent
218 ./configure PATH_KRB5_CONFIG=/nonexistent
220 krb5-config is not used and library probing is always done if either
221 --with-krb5-include or --with-krb5-lib are given.
223 You can pass the --enable-reduced-depends flag to configure to try to
224 minimize the shared library dependencies encoded in the binaries. This
225 omits from the link line all the libraries included solely because the
226 Kerberos libraries depend on them and instead links the programs only
227 against libraries whose APIs are called directly. This will only work
228 with shared Kerberos libraries and will only work on platforms where
229 shared libraries properly encode their own dependencies (such as Linux).
230 It is intended primarily for building packages for Linux distributions
231 to avoid encoding unnecessary shared library dependencies that make
232 shared library migrations more difficult. If none of the above made any
233 sense to you, don't bother with this flag.
237 First, build and install either a CrackLib dictionary as described in
238 REQUIREMENTS above, or build a CDB dictionary with cdbmake-wordlist.
239 (Or both.) The CrackLib dictionary will consist of three files, one
240 each ending in *.hwm, *.pwd, and *.pwi. The CDB dictionary will consist
241 of a single file ending in *.cdb. Install those files somewhere on your
242 system. Then, follow the relevant instructions below for either Heimdal
245 See "Other Settings" below for additional krb5.conf setting supported by
246 both Heimdal and MIT Kerberos.
250 There are two options: using an external password check program, or
251 using the plugin. I recommend the external password check program
252 unless you encounter speed problems with that approach that cause
255 For either approach, first add a stanza like the following to the
256 [appdefaults] section of your /etc/krb5.conf (or wherever your krb5.conf
260 password_dictionary = /path/to/cracklib/dictionary
261 password_dictionary_cdb = /path/to/cdb/dictionary.cdb
262 password_dictionary_sqlite = /path/to/sqlite/dictionary.sqlite
265 The first setting configures a CrackLib dictionary, the second a CDB
266 dictionary, and the third a SQLite dictionary. The provided path should
267 be the full path to the dictionary files, omitting the trailing *.hwm,
268 *.pwd, and *.pwi extensions for the CrackLib dictionary. You can use
269 any combination of the three settings. If you use more than one,
270 CrackLib will be checked first, then CDB, and then SQLite as
273 When checking against a CDB database, the password, the password with
274 the first character removed, the last character removed, the first and
275 last characters removed, the first two characters removed, and the last
276 two characters removed will all be checked against the dictionary.
278 When checking a SQLite database, the password will be rejected if it is
279 within edit distance one of any word in the dictionary, meaning that the
280 database word can be formed from the password by deleting, adding, or
281 changing a single character.
283 Then, for the external password checking program, add a new section (or
284 modify the existing [password_quality] section) to look like the
288 policies = external-check
289 external_program = /usr/local/bin/heimdal-strength
291 You can, of course, combine this policy with others. Replace the path
292 with the full path to wherever you have installed heimdal-strength. You
293 can put this section in your kdc.conf instead of krb5.conf if you
296 If you want to instead use the module, use the following section
300 policies = krb5-strength
301 policy_libraries = /usr/local/lib/krb5/plugins/pwqual/strength.so
303 in either krb5.conf or kdc.conf. Note that some older versions of
304 Heimdal have a bug in the support for loading modules when
305 policy_libraries is set. If you get an error like:
307 didn't find `kadm5_password_verifier' symbol in `(null)'
309 you may have to omit policy_libraries in your configuration and instead
310 pass the --check-library argument to kpasswdd specifying the library to
313 Additional configuration is required to use the history implementation.
314 Ensure that its dependencies are installed, and then examine the local
315 configuration settings at the top of the heimdal-history program. By
316 default, it requires a _history user and _history group be present on
317 the system, and all history information will be read and written as that
318 user and group. It also requires a nobody user and nogroup group to be
319 present, and all strength checking will be done as that user and group.
320 It uses various files in /var/lib/heimdal-history to store history and
321 statistical information by default, so if using the defaults, create
322 that directory and ensure it is writable by the _history user.
324 Once that setup is done, change your [password_quality] configuration
328 policies = external-check
329 external_program = /usr/local/bin/heimdal-history
331 The heimdal-history program will automatically also run heimdal-strength
332 as well, looking for it in /usr/local/bin, /usr/bin, and /bin. Change
333 the PATH setting at the top of the script if you have different
334 requirements. You should continue to configure heimdal-strength as if
335 you were running it directly.
339 To add this module to the list of password quality checks, add a section
340 to krb5.conf (or to a separate kdc.conf if you use that) like:
344 module = strength:/usr/local/lib/krb5/plugins/pwqual/strength.so
347 to register the plugin.
349 There are two ways to tell where the dictionary is. One option is to
350 use krb5.conf (and in this case you must use krb5.conf, even if you use
351 a separate kdc.conf file). For this approach, add the following to the
352 [appdefaults] section:
355 password_dictionary = /path/to/cracklib/dictionary
356 password_dictionary_cdb = /path/to/cdb/dictionary.cdb
357 password_dictionary_sqlite = /path/to/sqlite/dictionary.sqlite
360 The first setting configures a CrackLib dictionary, the second a CDB
361 dictionary, and the third a SQLite dictionary. The provided path should
362 be the full path to the dictionary files, omitting the trailing *.hwm,
363 *.pwd, and *.pwi extensions for the CrackLib dictionary. You can use
364 any combination of the three settings. If you use more than one,
365 CrackLib will be checked first, then CDB, and then SQLite as
368 When checking against a CDB database, the password, the password with
369 the first character removed, the last character removed, the first and
370 last characters removed, the first two characters removed, and the last
371 two characters removed will all be checked against the dictionary.
373 When checking a SQLite database, the password will be rejected if it is
374 within edit distance one of any word in the dictionary, meaning that the
375 database word can be formed from the password by deleting, adding, or
376 changing a single character.
378 The second option is to use the normal dict_path setting. In the
379 [realms] section of your krb5.conf kdc.conf, under the appropriate realm
380 or realms, specify the path to the dictionary:
382 dict_file = /path/to/cracklib/dictionary
384 This will be taken as a CrackLib dictionary path, the same as the
385 setting for password_dictionary above. The provided path should be the
386 full path to the dictionary files, omitting the trailing *.hwm, *.pwd,
387 or *.pwi extension. However, be aware that, if you use this approach,
388 you will probably want to disable the built-in standard dict pwqual
389 plugin by adding the line:
393 to the pwqual block of the [plugins] section as shown above. Otherwise,
394 it will also try to load a dictionary at the same path to do simple
397 You can also mix and match these settings, by using dict_path for the
398 CrackLib dictionary path and krb5.conf for the CDB or SQLite dictionary
399 paths. If both settings are used for the CrackLib path, krb5.conf
400 overrides the dict_path setting (so that dict_path can be used for other
401 password quality modules). There is no way to specify a CDB or SQLite
402 dictionary via the dict_path setting.
406 The following additional settings are supported in the [appdefaults]
407 section of krb5.conf when running under either Heimdal or MIT Kerberos.
411 If set to a numeric value, passwords with fewer than this number of
412 unique characters will be rejected. This can be used to reject, for
413 example, passwords that are long strings of the same character or
414 repetitions of small numbers of characters, which may be too easy to
419 If set to a numeric value, passwords with fewer than that number of
420 characters will be rejected, independent of any length restrictions
421 in CrackLib. Note that this setting does not bypass the minimum
422 length requirements in CrackLib itself (which, for the version
423 embedded in this package, is eight characters).
425 require_ascii_printable
427 If set to a true boolean value, rejects any password that contains
428 non-ASCII characters or ASCII control characters. Spaces are
429 allowed; tabs are not (at least assuming the POSIX C locale). No
430 canonicalization or character set is defined for Kerberos passwords
431 in general, so you may want to reject non-ASCII characters to avoid
432 interoperability problems with computers with different default
433 character sets or Unicode normalization forms.
437 This option allows specification of more complex character class
438 requirements. The value of this parameter should be one or more
439 whitespace-separated rule. Each rule has the syntax:
441 [<min>-<max>:]<class>[,<class>...]
443 where <class> is one of "upper", "lower", "digit", or "symbol"
444 (without the quote marks). The symbol class includes all characters
445 other than alphanumeric characters, including space. The listed
446 classes must appear in the password. Separate multiple required
447 classes with a comma (and no space).
449 The character class checks will be done in whatever locale the
450 plugin or password check program is run in, which will normally be
451 the POSIX C locale but may be different depending on local
456 require_classes = upper,lower,digit
458 This requires all passwords contain at least one uppercase letter,
459 at least one lowercase letter, and at least one digit.
461 If present, <min> and <max> specify the minimum password length and
462 maximum password length to which this rule applies. This allows one
463 to specify character class requirements that change with password
464 length. So, for example:
466 require_classes = 8-19:upper,lower 8-15:digit 8-11:symbol
468 requires all passwords from 8 to 11 characters long contain all four
469 character classes, passwords from 12 to 15 characters long contain
470 upper and lower case and a digit, and passwords from 16 to 19
471 characters long contain both upper and lower case. Passowrds longer
472 than 20 characters have no character class restrictions. (This
473 example is probably used in conjunction with minimum_length = 8.)
477 If set to a true boolean value, the password must contain at least
478 one character that is not a letter (uppercase or lowercase) or a
479 space. This may be helpful in combination with passphrases; users
480 may choose a stock English phrase, and this will force at least some
481 additional complexity.
483 You can omit any dictionary setting and only use the above settings, in
484 which case only the above checks and checks for passwords based on the
485 principal will be done, bypassing any dictionary check. (But for that
486 simple style of password strength checking, there are probably better
487 strength checking plugins already available.)
491 The krb5-strength web page at:
493 http://www.eyrie.org/~eagle/software/krb5-strength/
495 will always have the current version of this package, the current
496 documentation, and pointers to any additional resources.
498 I welcome bug reports and patches for this package at eagle@eyrie.org.
499 However, please be aware that I tend to be extremely busy and work
500 projects often take priority. I'll save your mail and get to it as soon
501 as I can, but it may take me a couple of months.
505 krb5-strength is maintained using Git. You can access the current
506 source by cloning the repository at:
508 git://git.eyrie.org/kerberos/krb5-strength.git
510 or view the repository via the web at:
512 http://git.eyrie.org/?p=kerberos/krb5-strength.git
514 When contributing modifications, either patches (possibly generated by
515 git format-patch) or Git pull requests are welcome.
519 The krb5-strength package as a whole is covered by the following
520 copyright statement and license:
522 Copyright 2006, 2007, 2009, 2010, 2012, 2013
523 The Board of Trustees of the Leland Stanford Junior University
525 Permission is hereby granted, free of charge, to any person obtaining
526 a copy of this software and associated documentation files (the
527 "Software"), to deal in the Software without restriction, including
528 without limitation the rights to use, copy, modify, merge, publish,
529 distribute, sublicense, and/or sell copies of the Software, and to
530 permit persons to whom the Software is furnished to do so, subject to
531 the following conditions:
533 The above copyright notice and this permission notice shall be
534 included in all copies or substantial portions of the Software.
536 THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
537 EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
538 MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
539 IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
540 CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
541 TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
542 SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
544 The embedded version of CrackLib (all files in the cracklib
545 subdirectory) is covered by the Artistic license. See the file
546 cracklib/LICENCE for more information. Combined derivative works that
547 include this code, such as binaries built with the embedded CrackLib,
548 will need to follow the terms of the Artistic license as well as the
551 All other individual files without an explicit exception below are
552 released under this license. Some files may have additional copyright
553 holders as noted in those files. There is detailed information about
554 the licensing of each file in the LICENSE file in this distribution.
556 Some files in this distribution are individually released under
557 different licenses, all of which are compatible with the above general
558 package license but which may require preservation of additional
559 notices. All required notices are preserved in the LICENSE file.