krb5-strength 3.2

(Kerberos password strength checking plugin)
Maintained by Russ Allbery <>

Copyright 2016, 2020 Russ Allbery <>. Copyright 2006-2007, 2009-2010, 2012-2014 The Board of Trustees of the Leland Stanford Junior University. Copyright 1993 Alec Muffett. This software is distributed under a BSD-style license. Please see the section LICENSE below for more information.


krb5-strength provides a password quality plugin for the MIT Kerberos KDC (specifically the kadmind server) and Heimdal KDC, an external password quality program for use with Heimdal, and a per-principal password history implementation for Heimdal. Passwords can be tested with CrackLib, checked against a CDB or SQLite database of known weak passwords with some transformations, 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.


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, or a SQLite database, which can reject all passwords within edit distance one of a dictionary word. It can also impose other programmatic checks on passwords such as character class requirements.

If you're just now starting with password checking, I recommend using the SQLite database with a large wordlist and minimum password lengths. We found this produced the best results with the least user frustration.

For Heimdal, krb5-strength 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, one of the other dictionary types, or the additional character class and length checks.

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, remove some code that doesn't make sense in the kadmind context, and close a few security issues. The standard CrackLib distribution on at least some Linux distributions now supports an additional interface to configure its behavior, and krb5-strength should change in the future to use that interface and drop the embedded copy.

krb5-strength also includes a password history implementation for Heimdal. This is separate from the password strength implementation but can be stacked with it so that both strength and history checks are performed. This history implementation is available only via the Heimdal external password quality interface. MIT Kerberos includes its own password history implementation.


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 7.0.

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. They also have known security vulnerabilities. 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, and the SQLite library (a version new enough to support the sqlite3_open_v2 API; 3.7 should be more than sufficient), which provides support for checking whether passwords are within edit distance one of a dictionary word.

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 or SQLite dictionary, use the provided krb5-strength-wordlist program. For CDB dictionries, the cdb utility must be on your PATH. For SQLite, the DBI and DBD::SQLite Perl modules are required. krb5-strength-wordlist requires Perl 5.010 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.

The password history program, heimdal-history, requires Perl 5.010 or later plus the following CPAN modules:

and their dependencies.

To bootstrap from a Git checkout, or if you change the Automake files and need to regenerate, you will need Automake 1.11 or later. For bootstrap or if you change or any of the m4 files it includes and need to regenerate configure or, you will need Autoconf 2.64 or later. You will also need Perl 5.010 or later and the DBI, DBD::SQLite, JSON, Perl6::Slurp, and Readonly modules (from CPAN) to generate man pages and bootstrap the test suite data from a Git checkout.


You can build and install krb5-strength with the standard commands:

    make install

If you are building from a Git clone, first run ./bootstrap in the source directory to generate the build files. make install will probably have to be done as root. Building outside of the source directory is also supported, if you wish, by creating an empty directory and then running configure with the correct relative path.

By default, the Heimdal external password check function is installed as /usr/local/bin/heimdal-strength, and the plugin is installed as /usr/local/lib/krb5/plugins/pwqual/ You can change these paths with the --prefix, --libdir, and --bindir options to configure.

By default, the embedded version of CrackLib will be used. 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. You can also build without any CrackLib support by passing --without-cracklib to configure.

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.

Similarly, krb5-strength will automatically build with SQLite if it is found. To specify the installation path of SQLite, use --with-sqlite. You can also separately set the include and library path with --with-sqlite-include and --with-sqlite-lib.

Normally, configure will use krb5-config to determine the flags to use to compile with your Kerberos libraries. 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

If krb5-config isn't found, configure 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 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.

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 compiler warnings (requires either GCC or Clang and may require a relatively current version of the compiler).

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 other libraries depend on them and instead links the programs only against libraries whose APIs are called directly. This will only work with shared libraries and will only work on platforms where shared libraries properly encode their own dependencies (this includes most modern platforms such as all 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.

After installing this software, see the man pages for krb5-strength, heimdal-strength, and heimdal-history for configuration information.


krb5-strength comes with a test suite, which you can run after building with:

    make check

If a test fails, you can run a single test with verbose output via:

    tests/runtests -o <name-of-test>

Do this instead of running the test program directly since it will ensure that necessary environment variables are set up.

To run the test suite, you will need Perl 5.010 or later and the dependencies of the heimdal-history program. The following additional Perl modules will also be used by the test suite if present:

All are available on CPAN. Some tests will be skipped if the modules are not available.

To enable tests that don't detect functionality problems but are used to sanity-check the release, set the environment variable RELEASE_TESTING to a true value. To enable tests that may be sensitive to the local environment or that produce a lot of false positives without uncovering many problems, set the environment variable AUTHOR_TESTING to a true value.


The krb5-strength web page at:

will always have the current version of this package, the current documentation, and pointers to any additional resources.

For bug tracking, use the issue tracker on GitHub:

However, please be aware that I tend to be extremely busy and work projects often take priority. I'll save your report and get to it as soon as I can, but it may take me a couple of months.


krb5-strength is maintained using Git. You can access the current source on GitHub at:

or by cloning the repository at:

or view the repository via the web at:

The repository is the canonical one, maintained by the author, but using GitHub is probably more convenient for most purposes. Pull requests are gratefully reviewed and normally accepted.


The krb5-strength package as a whole is covered by the following copyright statement and license:

  Copyright 2016, 2020 Russ Allbery <>
  Copyright 2006-2007, 2009-2010, 2012-2014
      The Board of Trustees of the Leland Stanford Junior University
  Copyright 1993 Alec Muffett
  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.
  Developed by Derrick Brashear and Ken Hornstein of Sine Nomine
  Associates, on behalf of Stanford University.
  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.

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, and detailed information about the licensing of each file, are recorded in the LICENSE file.

Files covered by a license with an assigned SPDX License Identifier include SPDX-License-Identifier tags to enable automated processing of license information. See for more information.

For any copyright range specified by files in this package as YYYY-ZZZZ, the range specifies every single year in that closed interval.

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