| Russ Allbery > Software | kstart > |
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 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. Perl is also required to generate manual pages from a fresh Git checkout. You will also need Perl 5.010 or later and the Const::Fast, DBI, DBD::SQLite, JSON::MaybeXS, and Perl6::Slurp modules (from CPAN) to bootstrap the test suite data from a Git checkout.
The distribution:
| krb5-strength 3.3 | 2023-12-26 | tar.gz (PGP signature) | tar.xz (PGP signature) |
An archive of older releases is also available.
A Debian package is included in Debian 8.0 (jessie) and later releases. See the Debian package tracker for more information.
krb5-strength is maintained using the Git version control system. To check out the current development tree, see GitHub or clone:
https://git.eyrie.org/git/kerberos/krb5-strength.git
Pull requests on GitHub are welcome. You can also browse the current development source.
User documentation:
Developer documentation:
The krb5-strength package as a whole is covered by the following copyright and license:
Copyright 2016, 2020, 2023 Russ Allbery <eagle@eyrie.org>
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.
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.
Developed by Daria Phoebe Brashear and Ken Hornstein of Sine Nomine Associates, on behalf of Stanford University.
The embedded version of CrackLib (all files in the
cracklibsubdirectory) is covered by the Artistic license. See the filecracklib/LICENCEfor 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 individual source files are covered by other, compatible licenses. For complete copyright and license information, see the file LICENSE in the krb5-strength source distribution.
| Russ Allbery > Software | kstart > |