krb5-strength 2.1 (Kerberos password strength checking plugin) Maintained by Russ Allbery Copyright 2006, 2007, 2009, 2010, 2012, 2013 The Board of Trustees of the Leland Stanford Junior University. Portions copyright 1993 Alec Muffett. Developed by Derrick Brashear and Ken Hornstein of Sine Nomine Associates, on behalf of Stanford University. This software is distributed under a BSD-style license and under the Artistic License. Please see the section LICENSE for more information. BLURB krb5-strength provides a password quality plugin for the MIT Kerberos KDC (specifically the kadmind server) and an external password quality program for use with the Heimdal kpasswdd server. Passwords can be tested with CrackLib, checked against a CDB database of known weak passwords, checked for length, checked for non-printable or non-ASCII characters that may be difficult to enter reproducibly, required to contain particular character classes, or any combination of these tests. It supports both Heimdal and MIT Kerberos (1.9 or later). DESCRIPTION Heimdal includes a capability to plug in external password quality checks and comes with an example that checks passwords against CrackLib. However, in testing at Stanford, we found that CrackLib with its default transform rules does not catch passwords that can be guessed using the same dictionary with other tools, such as Jack the Ripper. We then discovered other issues with CrackLib with longer passwords, such as some bad assumptions about how certain measures of complexity will scale, and wanted to impose other limitations that it didn't support. This plugin provides the ability to check password quality against the standard version of CrackLib, or against a modified version of CrackLib that only passes passwords that resist attacks from both Crack and Jack the Ripper using the same rule sets. It also supports doing simpler dictionary checks against a CDB database, which is fast with very large dictionaries, and imposing other programmatic checks on passwords such as character class requirements. For Heimdal, it includes both a program usable as an external password quality check and a plugin that implements the dynamic module API. For MIT Kerberos (1.9 or later), it includes a plugin for the password quality (pwqual) plugin API. krb5-strength can be built with either the system CrackLib or with the modified version of CrackLib included in this package. Note, however, that if you're building against the system CrackLib, Heimdal includes in the distribution a strength-checking plugin and an external password check program that use the system CrackLib. With Heimdal, it would probably be easier to use that plugin or program than build this package unless you want the modified CrackLib. For information about the changes to the CrackLib included in this toolkit, see cracklib/HISTORY. The primary changes are tighter rules, which are more aggressive at finding dictionary words with characters appended and prepended, which tighten the requirements for password entropy, and which add stricter rules for longer passwords. They are also minor changes to fix portability issues and remove some code that doesn't make sense in the kadmind context. Ideally, the changes to CrackLib should be added to the standard CrackLib distribution by adding an additional interface to configure its behavior, at which point this package can likely wither away in favor of much simpler plugins that link to the standard CrackLib library. REQUIREMENTS For Heimdal, you may use either the external password quality check tool, installed as heimdal-strength, or the plugin as you choose. It has been tested with Heimdal 1.2.1 and later, but has not recently been tested with versions prior to 1.5. For MIT Kerberos, version 1.9 or higher is required for the password quality plugin interface. MIT Kerberos does not support an external password quality check tool directly, so you will need to install the plugin. You can optionally build against the system CrackLib library. Any version should be supported, but note that some versions, particularly older versions close to the original code, do things like printing diagnostics to stderr, calling exit, and otherwise not being well-behaved for use inside plugins or libraries. If using a system CrackLib library, use version 2.8.22 or later to avoid these problems. You can also optionally build against the TinyCDB library, which provides support for simpler and faster password checking against a CDB dictionary file. For this module to be effective for either Heimdal or MIT Kerberos, you will also need to construct a dictionary. The mkdict and packer utilities to build a CrackLib dictionary from a word list are included in this toolkit but not installed by default. You can run them out of the cracklib directory after building. You can also use the utilities that come with the stock CrackLib package (often already packaged in a Linux distribution); the database format is compatible. For building a CDB dictionary, use the provided cdbmake-wordlist program. The CDB utility must be on your PATH. cdbmake-wordlist requires Perl 5.006 or later. For a word list to use as source for the dictionary, you can use /usr/share/dict/words if it's available on your system, but it would be better to find a more comprehensive word list. Since word lists are bulky, often covered by murky copyrights, and easily locatable on the Internet with a modicum of searching, none are included in this toolkit. To run the test suite, you will also need Perl 5.006 or later. The following additional Perl modules will be used by the test suite if present: IPC::Run JSON Perl6::Slurp Test::MinimumVersion Test::Perl::Critic Test::Pod Test::Spelling Test::Strict All are available on CPAN. Those tests will be skipped if the modules are not available. To enable tests that may be sensitive to the local environment or that produce a lot of false positives without uncovering many problems, set RRA_MAINTAINER_TESTS to a true value. To bootstrap from a Git checkout, or If you change the Automake files and need to regenerate Makefile.in, you will need Automake 1.11 or later. For bootstrap or if you change configure.ac or any of the m4 files it includes and need to regenerate configure or config.h.in, you will need Autoconf 2.64 or later. You will also need Perl 5.010 or later and the JSON, Perl6::Slurp, and Readonly modules (from CPAN) to bootstrap the test suite data from a Git checkout. COMPILING AND INSTALLING You can build and install the plugin with the standard commands: ./configure make make install Pass --enable-silent-rules to configure for a quieter build (similar to the Linux kernel). Use make warnings instead of make to build with full GCC compiler warnings (requires a relatively current version of GCC). The last step will probably have to be done as root. By default, the plugin is installed as /usr/local/lib/krb5/plugins/pwqual/strength.so and the Heimdal external password check function is installed as /usr/local/bin/heimdal-strength. You can change these paths with the --prefix, --libdir, and --bindir options to configure. To build with the system version of CrackLib, pass --with-cracklib to configure. You can optionally add a directory, giving the root directory where CrackLib was installed, or separately set the include and library path with --with-cracklib-include and --with-cracklib-lib. krb5-strength will automatically build with TinyCDB if it is found. To specify the installation path of TinyCDB, use --with-tinycdb. You can also separately set the include and library path with --with-tinycdb-include and --with-tinycdb-lib. Normally, configure will use krb5-config to determine the flags to use to compile with your Kerberos libraries. If krb5-config isn't found, it will look for the standard Kerberos libraries in locations already searched by your compiler. If the the krb5-config script first in your path is not the one corresponding to the Kerberos libraries you want to use or if your Kerberos libraries and includes aren't in a location searched by default by your compiler, you need to specify a different Kerberos installation root via --with-krb5=PATH. For example: ./configure --with-krb5=/usr/pubsw You can also individually set the paths to the include directory and the library directory with --with-krb5-include and --with-krb5-lib. You may need to do this if Autoconf can't figure out whether to use lib, lib32, or lib64 on your platform. To specify a particular krb5-config script to use, either set the PATH_KRB5_CONFIG environment variable or pass it to configure like: ./configure PATH_KRB5_CONFIG=/path/to/krb5-config To not use krb5-config and force library probing even if there is a krb5-config script on your path, set PATH_KRB5_CONFIG to a nonexistent path: ./configure PATH_KRB5_CONFIG=/nonexistent krb5-config is not used and library probing is always done if either --with-krb5-include or --with-krb5-lib are given. You can pass the --enable-reduced-depends flag to configure to try to minimize the shared library dependencies encoded in the binaries. This omits from the link line all the libraries included solely because the Kerberos libraries depend on them and instead links the programs only against libraries whose APIs are called directly. This will only work with shared Kerberos libraries and will only work on platforms where shared libraries properly encode their own dependencies (such as Linux). It is intended primarily for building packages for Linux distributions to avoid encoding unnecessary shared library dependencies that make shared library migrations more difficult. If none of the above made any sense to you, don't bother with this flag. CONFIGURATION First, build and install either a CrackLib dictionary as described in REQUIREMENTS above, or build a CDB dictionary with cdbmake-wordlist. (Or both.) The CrackLib dictionary will consist of three files, one each ending in *.hwm, *.pwd, and *.pwi. The CDB dictionary will consist of a single file ending in *.cdb. Install those files somewhere on your system. Then, follow the relevant instructions below for either Heimdal or MIT Kerberos. See "Other Settings" below for additional krb5.conf setting supported by both Heimdal and MIT Kerberos. Heimdal There are two options: using an external password check program, or using the plugin. I recommend the external password check program unless you encounter speed problems with that approach that cause kpasswd to time out. For either approach, first add a stanza like the following to the [appdefaults] section of your /etc/krb5.conf (or wherever your krb5.conf file is located): krb5-strength = { password_dictionary = /path/to/cracklib/dictionary password_dictionary_cdb = /path/to/cdb/dictionary.cdb } The first setting configures a CrackLib dictionary and the second a CDB dictionary. The provided path should be the full path to the dictionary files, omitting the trailing *.hwm, *.pwd, and *.pwi extensions for the CrackLib dictionary. You can use either or both settings. If you use both, CrackLib will be checked first, and then CDB. When checking a CDB database, the password, the password with the first character removed, the last character removed, the first and last characters removed, the first two characters removed, and the last two characters removed will all be checked against the dictionary. Then, for the external password checking program, add a new section (or modify the existing [password_quality] section) to look like the following: [password_quality] policies = external-check external_program = /usr/local/bin/heimdal-strength You can, of course, combine this policy with others. Replace the path with the full path to wherever you have installed heimdal-strength. You can put this section in your kdc.conf instead of krb5.conf if you prefer. If you want to instead use the module, use the following section instead: [password_quality] policies = krb5-strength policy_libraries = /usr/local/lib/krb5/plugins/pwqual/strength.so in either krb5.conf or kdc.conf. Note that some older versions of Heimdal have a bug in the support for loading modules when policy_libraries is set. If you get an error like: didn't find `kadm5_password_verifier' symbol in `(null)' you may have to omit policy_libraries in your configuration and instead pass the --check-library argument to kpasswdd specifying the library to load. MIT Kerberos To add this module to the list of password quality checks, add a section to krb5.conf (or to a separate kdc.conf if you use that) like: [plugins] pwqual = { module = strength:/usr/local/lib/krb5/plugins/pwqual/strength.so } to register the plugin. There are two ways to tell where the dictionary is. One option is to use krb5.conf (and in this case you must use krb5.conf, even if you use a separate kdc.conf file). For this approach, add the following to the [appdefaults] section: krb5-strength = { password_dictionary = /path/to/cracklib/dictionary password_dictionary_cdb = /path/to/cdb/dictionary.cdb } The first setting configures a CrackLib dictionary and the second a CDB dictionary. The provided path should be the full path to the dictionary files, omitting the trailing *.hwm, *.pwd, and *.pwi extensions for the CrackLib dictionary. You can use either or both settings. If you use both, CrackLib will be checked first, and then CDB. When checking a CDB database, the password, the password with the first character removed, the last character removed, the first and last characters removed, the first two characters removed, and the last two characters removed will all be checked against the dictionary. The second option is to use the normal dict_path setting. In the [realms] section of your krb5.conf kdc.conf, under the appropriate realm or realms, specify the path to the dictionary: dict_file = /path/to/cracklib/dictionary This will be taken as a CrackLib dictionary path, the same as the setting for password_dictionary above. The provided path should be the full path to the dictionary files, omitting the trailing *.hwm, *.pwd, or *.pwi extension. However, be aware that, if you use this approach, you will probably want to disable the built-in standard dict pwqual plugin by adding the line: disable = dict to the pwqual block of the [plugins] section as shown above. Otherwise, it will also try to load a dictionary at the same path to do simple dictionary matching. You can also mix and match these settings, by using dict_path for the CrackLib dictionary path and krb5.conf for the CDB dictionary path. If both settings are used, krb5.conf overrides the dict_path setting (so that dict_path can be used for other password quality modules). There is no way to specify a CDB dictionary via the dict_path setting. Other Settings The following additional settings are supported in the [appdefaults] section of krb5.conf when running under either Heimdal or MIT Kerberos. minimum_length If set to a numeric value, passwords with fewer than that number of characters will be rejected, independent of any length restrictions in CrackLib. Note that this setting does not bypass the minimum length requirements in CrackLib itself (which, for the version embedded in this package, is eight characters). require_ascii_printable If set to a true boolean value, rejects any password that contains non-ASCII characters or ASCII control characters. Spaces are allowed; tabs are not (at least assuming the POSIX C locale). No canonicalization or character set is defined for Kerberos passwords in general, so you may want to reject non-ASCII characters to avoid interoperability problems with computers with different default character sets or Unicode normalization forms. require_classes This option allows specification of more complex character class requirements. The value of this parameter should be one or more whitespace-separated rule. Each rule has the syntax: [-:][,...] where is one of "upper", "lower", "digit", or "symbol" (without the quote marks). The symbol class includes all characters other than alphanumeric characters, including space. The listed classes must appear in the password. Separate multiple required classes with a comma (and no space). The character class checks will be done in whatever locale the plugin or password check program is run in, which will normally be the POSIX C locale but may be different depending on local configuration. A simple example: require_classes = upper,lower,digit This requires all passwords contain at least one uppercase letter, at least one lowercase letter, and at least one digit. If present, and specify the minimum password length and maximum password length to which this rule applies. This allows one to specify character class requirements that change with password length. So, for example: require_classes = 8-19:upper,lower 8-15:digit 8-11:symbol requires all passwords from 8 to 11 characters long contain all four character classes, passwords from 12 to 15 characters long contain upper and lower case and a digit, and passwords from 16 to 19 characters long contain both upper and lower case. Passowrds longer than 20 characters have no character class restrictions. (This example is probably used in conjunction with minimum_length = 8.) require_non_letter If set to a true boolean value, the password must contain at least one character that is not a letter (uppercase or lowercase) or a space. This may be helpful in combination with passphrases; users may choose a stock English phrase, and this will force at least some additional complexity. You can omit any dictionary setting and only use the above settings, in which case only the above checks and checks for passwords based on the principal will be done, bypassing any dictionary check. (But for that simple style of password strength checking, there are probably better strength checking plugins already available.) SUPPORT The krb5-strength web page at: http://www.eyrie.org/~eagle/software/krb5-strength/ will always have the current version of this package, the current documentation, and pointers to any additional resources. I welcome bug reports and patches for this package at eagle@eyrie.org. However, please be aware that I tend to be extremely busy and work projects often take priority. I'll save your mail and get to it as soon as I can, but it may take me a couple of months. SOURCE REPOSITORY krb5-strength is maintained using Git. You can access the current source by cloning the repository at: git://git.eyrie.org/kerberos/krb5-strength.git or view the repository via the web at: http://git.eyrie.org/?p=kerberos/krb5-strength.git When contributing modifications, either patches (possibly generated by git format-patch) or Git pull requests are welcome. LICENSE The krb5-strength package as a whole is covered by the following copyright statement and license: Copyright 2006, 2007, 2009, 2010, 2012, 2013 The Board of Trustees of the Leland Stanford Junior University Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions: The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software. THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE. The embedded version of CrackLib (all files in the cracklib subdirectory) is covered by the Artistic license. See the file cracklib/LICENCE for more information. Combined derivative works that include this code, such as binaries built with the embedded CrackLib, will need to follow the terms of the Artistic license as well as the above license. All other individual files without an explicit exception below are released under this license. Some files may have additional copyright holders as noted in those files. There is detailed information about the licensing of each file in the LICENSE file in this distribution. Some files in this distribution are individually released under different licenses, all of which are compatible with the above general package license but which may require preservation of additional notices. All required notices are preserved in the LICENSE file.