3 # Password history via Heimdal external strength checking.
5 # This script is meant to be called via the Heimdal external password strength
6 # checking interface and maintains per-user password history that also rejects
7 # one-character permutations. Password history is stored as Crypt::PBKDF2
8 # hashes with random salt for each password.
10 # Written by Russ Allbery <eagle@eyrie.org>
11 # Copyright 2013, 2014
12 # The Board of Trustees of the Leland Stanford Junior University
14 ##############################################################################
15 # Declarations and configuration
16 ##############################################################################
25 use Fcntl qw(O_CREAT O_RDWR);
26 use File::Basename qw(basename);
27 use Getopt::Long::Descriptive qw(describe_options);
29 use JSON qw(encode_json decode_json);
30 use POSIX qw(setgid setuid);
32 use Sys::Syslog qw(openlog syslog LOG_AUTH LOG_INFO LOG_WARNING);
34 # The most convenient interface to Berkeley DB files is ties.
35 ## no critic (Miscellanea::ProhibitTies)
37 # The number of PBKDF2 iterations to use when hashing passwords. This number
38 # should be chosen so as to force the hash operation to take approximately 0.1
39 # seconds on current hardware.
40 Readonly my $HASH_ITERATIONS => 65536;
42 # Path to the history database. Currently, this must be a Berkeley DB file in
43 # the old DB_HASH format. Keys will be principal names, and values will be a
44 # JSON array of hashes. Each hash will have two keys: timestamp, which holds
45 # the seconds since UNIX epoch at which the history entry was stored, and
46 # hash, which holds the Crypt::PBKDF2 LDAP-style password hash.
47 Readonly my $HISTORY_PATH => '/var/lib/heimdal-history/history.db';
49 # User and group used to do all password history lookups and writes, assuming
50 # that this program is invoked as root and can therefore change UID and GID.
51 Readonly my $HISTORY_USER => '_history';
52 Readonly my $HISTORY_GROUP => '_history';
54 # Path to the Berkeley DB file (DB_HASH format) that stores statistics on
55 # password length of accepted passwords. Each successful password validation
56 # will increase the counter for that length. This is read and written with
57 # $HISTORY_USER and $HISTORY_GROUP.
58 Readonly my $LENGTH_STATS_PATH => '/var/lib/heimdal-history/lengths.db';
60 # The message to return to the user if we reject the password because it was
61 # found in the user's history.
62 Readonly my $REJECT_MESSAGE => 'password was previously used';
64 # The path to the external strength checking program to run. This is done
65 # first before checking history, and if it fails, that failure is returned as
66 # the failure for this program.
67 Readonly my $STRENGTH_PROGRAM => '/usr/bin/heimdal-strength';
69 # User and group used to do password strength checking. Generally, this
70 # doesn't require any privileges since the strength dictionary is
72 Readonly my $STRENGTH_USER => 'nobody';
73 Readonly my $STRENGTH_GROUP => 'nogroup';
75 ##############################################################################
77 ##############################################################################
79 # Change real and effective UID and GID to those for the given user and group.
80 # Does nothing if not running as root.
82 # $user - User to change the UID to
83 # $group - Group to change the GID to (and clear all supplemental groups)
86 # Throws: Text exception on any failure
88 my ($user, $group) = @_;
90 # If running as root, drop privileges. Fail if we can't get the UID and
91 # GID corresponding to our users.
92 if ($> == 0 || $< == 0) {
93 my $uid = getpwnam($user)
94 or die "$0: cannot get UID for $user\n";
95 my $gid = getgrnam($group)
96 or die "$0: cannot get GID for $group\n";
97 setgid($gid) or die "$0: cannot setgid to $gid: $!\n";
98 setuid($uid) or die "$0: cannot setuid to $uid: $!\n";
99 if ($> == 0 || $< == 0) {
100 die "$0: failed to drop permissions\n";
106 ##############################################################################
108 ##############################################################################
110 # Given a list of keys and values for a log message as a hash reference,
111 # return in encoded format following our logging protocol. The log format is
112 # a set of <key>=<value> parameters separated by a space. Values containing
113 # whitespace are quoted with double quotes, with any internal double quotes
116 # Here also is defined a custom sort order for the encoded key/value pairs to
117 # keep them in a reasonable order for a human to read.
119 # $params_ref - Reference to a hash of key/value pairs
121 # Returns: The encoded format as a string
122 sub encode_log_message {
123 my ($params_ref) = @_;
125 # Define the custom sort order for keys.
128 = map { $_ => $order++ } qw(action principal error result reason);
130 # Build the message from the parameters.
132 for my $key (sort { $order{$a} <=> $order{$b} } keys %{$params_ref}) {
133 my $value = $params_ref->{$key};
134 $value =~ s{ \" }{\"\"}xmsg;
135 if ($value =~ m{ [ \"] }xms) {
136 $value = qq{"$value"};
138 $message .= qq{$key=$value };
144 # Log a non-fatal error encountered while trying to check or store password
145 # history. This is used for errors where the password is accepted, but we ran
146 # into some anomalous event such as corrupted history data that should be
147 # drawn to the attention of an administrator. The log format is a set of
148 # <key>=<value> parameters, with the following keys:
150 # - action: the action performed (currently always "check")
151 # - principal: the principal to check a password for
152 # - error: an error message explaining the anomalous situation
154 # Values containing whitespace are quoted with double quotes, with any
155 # internal double quotes doubled.
157 # $principal - Principal for which we checked a password
158 # $error - The error message
162 my ($principal, $error) = @_;
163 my $message = encode_log_message(
165 principal => $principal,
168 syslog(LOG_WARNING, '%s', $message);
172 # Log the disposition of a particular password strength checking request. All
173 # log messages are logged through syslog at class info. The log format is a
174 # set of <key>=<value> parameters, with the following keys:
176 # - action: the action performed (currently always "check")
177 # - principal: the principal to check a password for
178 # - result: either "accepted" or "rejected"
179 # - reason: the reason for a rejection
181 # Values containing whitespace are quoted with double quotes, with any
182 # internal double quotes doubled.
184 # $principal - Principal for which we checked a password
185 # $result - "accepted" or "rejected" per above
186 # $reason - On rejection, the reason
190 my ($principal, $result, $reason) = @_;
192 # Create the message.
195 principal => $principal,
198 if ($result eq 'rejected' && defined($reason)) {
199 $message{reason} = $reason;
201 my $message = encode_log_message(\%message);
204 syslog(LOG_INFO, '%s', $message);
208 ##############################################################################
210 ##############################################################################
212 # Given a password, return the hash for that password. Hashing is done with
213 # PBKDF2 using SHA-2 as the underlying hash function. As of version 0.133330,
216 # $password - Password to hash
218 # Returns: Hash encoded in the LDAP-compatible Crypt::PBKDF2 format
221 my $hasher = Crypt::PBKDF2->new(
222 hash_class => 'HMACSHA2',
223 iterations => $HASH_ITERATIONS,
225 return $hasher->generate($password);
228 # Given a password and the password history for the user as a reference to a
229 # array, check whether that password is found in the history. The history
230 # array is expected to contain anonymous hashes. The only key of interest is
231 # the "hash" key, whose value is expected to be a hash in the LDAP-compatible
232 # Crypt::PBKDF2 format.
234 # Invalid history entries are ignored for the purposes of this check and
235 # treated as if the entry did not exist.
237 # $principal - Principal to check (solely for logging purposes)
238 # $password - Password to check
239 # $history_ref - Reference to array of anonymous hashes with "hash" keys
241 # Returns: True if the password matches one of the history hashes, false
244 my ($principal, $password, $history_ref) = @_;
245 my $hasher = Crypt::PBKDF2->new(hash_class => 'HMACSHA2');
247 # Walk the history looking at each hash key.
248 for my $entry (@{$history_ref}) {
249 my $hash = $entry->{hash};
250 next if !defined($hash);
252 # validate throws an exception if the hash is in an invalid format.
253 # Treat that case the same as a miss, but log it.
254 if (eval { $hasher->validate($hash, $password) }) {
257 log_error($principal, "hash validate failed: $@");
265 ##############################################################################
267 ##############################################################################
269 # Given a principal and a password, determine whether the password was found
270 # in the password history for that user.
272 # $path - Path to the history file
273 # $principal - Principal for which to check history
274 # $password - Check history for this password
276 # Returns: True if $password is found in history, false otherwise
277 # Throws: On failure to open, lock, or tie the database
279 my ($path, $principal, $password) = @_;
281 # Open and lock the database and retrieve the history for the user.
282 # We have to lock for write so that we can create the database if it
283 # doesn't already exist. Password change should be infrequent enough
284 # and our window is fast enough that it shouldn't matter. We do this
285 # in a separate scope so that the history hash goes out of scope and
286 # is freed and unlocked.
290 my $mode = O_CREAT | O_RDWR;
291 tie(%history, 'DB_File::Lock', [$path, $mode, oct(600)], 'write')
292 or die "$0: cannot open $path: $!\n";
293 $history_json = $history{$principal};
296 # If there is no history for the user, return the trivial false.
297 if (!defined($history_json)) {
301 # Decode history from JSON. If this fails (corrupt history), treat it as
302 # if the user has no history, but log the error message.
303 my $history_ref = eval { decode_json($history_json) };
304 if (!defined($history_ref)) {
305 log_error($principal, "history JSON decoding failed: $@");
309 # Finally, check the password against the hashes in history.
310 return is_in_history($principal, $password, $history_ref);
313 # Write a new history entry to the database given the principal and the
314 # password to record. History records are stored as JSON arrays of objects,
315 # with keys "timestamp" and "hash".
317 # $path - Path to the history file
318 # $principal - Principal for which to check history
319 # $password - Check history for this password
322 # Throws: On failure to open, lock, or tie the database
324 my ($path, $principal, $password) = @_;
326 # Open and lock the database for write.
328 my $mode = O_CREAT | O_RDWR;
329 tie(%history, 'DB_File::Lock', [$path, $mode, oct(600)], 'write')
330 or die "$0: cannot open $path: $!\n";
332 # Read the existing history. If the existing history is corrupt, treat
333 # that as equivalent to not having any history, but log an error.
334 my $history_json = $history{$principal};
336 if (defined($history_json)) {
337 $history_ref = eval { decode_json($history_json) };
339 log_error($principal, "history JSON decoding failed: $@");
342 if (!defined($history_ref)) {
346 # Add a new history entry.
347 my $entry = { timestamp => time(), hash => password_hash($password) };
348 unshift(@{$history_ref}, $entry);
350 # Store the encoded data back in the history database.
351 $history{$principal} = encode_json($history_ref);
353 # The database is closed and unlocked when %history goes out of scope.
354 # Unfortunately, we lose on error detection here, since there doesn't
355 # appear to be a way to determine whether all the writes succeeded. But
356 # losing a bit of history in the rare error case of failing to write to
357 # local disk is probably not a big deal.
361 # Write statistics about password length. Given the length of the password
362 # and the path to the length statistics database, increments the counter for
363 # that password length.
365 # Any failure to open or write to the database is ignored, since this is
366 # considered optional logging and should not block the password change.
368 # $path - Path to the length statistics file
369 # $length - Length of the accepted password
372 sub update_length_counts {
373 my ($path, $length) = @_;
375 # Open and lock the database for write.
377 my $mode = O_CREAT | O_RDWR;
378 tie(%lengths, 'DB_File::Lock', [$path, $mode, oct(600)], 'write')
381 # Write each of the hashes.
384 # The database is closed and unlocked when %lengths goes out of scope.
388 ##############################################################################
389 # Heimdal password strength protocol
390 ##############################################################################
392 # Run another external password strength checker and return the results. This
393 # allows us to chain to another program that handles the actual strength
394 # checking prior to handling history.
396 # $principal - Principal attempting to change their password
397 # $password - The new password
399 # Returns: Scalar context: true if the password was accepted, false otherwise
400 # List context: whether the password is okay, the exit status of the
401 # strength checking program, and the error message if the first
403 # Throws: Text exception on failure to execute the program, or read or write
404 # from it or to it, or if it fails without an error
406 my ($principal, $password) = @_;
408 # Run the external strength checking program. If we're root, we'll run it
409 # as the strength checking user and group.
410 my $in = "principal: $principal\nnew-password: $password\nend\n";
411 my $init = sub { drop_privileges($STRENGTH_USER, $STRENGTH_GROUP) };
413 run([$STRENGTH_PROGRAM, $principal], \$in, \$out, \$err, init => $init);
414 my $status = ($? >> 8);
417 my $okay = ($status == 0 && $out eq "APPROVED\n");
419 # If the program failed, collect the error message.
422 $err =~ s{ \n .* }{}xms;
424 die "$0: password strength checking failed without an error\n";
428 # Return the results.
429 return wantarray ? ($okay, $err, $status) : $okay;
432 # Read a Heimdal external password strength checking request from the provided
433 # file handle and return the principal (ignored for our application) and the
436 # The protocol expects the following data (without leading whitespace) on
437 # standard input, in precisely this order:
439 # principal: <principal>
440 # new-password: <password>
443 # There is one and only one space after the colon, and any subsequent spaces
444 # are part of the value (such as leading spaces in the password).
446 # $fh - File handle from which to read
448 # Returns: Scalar context: the password
449 # List context: a list of the password and the principal
450 # Throws: Text exception on any protocol violations or IO errors
451 sub read_change_data {
453 my @keys = qw(principal new-password);
456 # Read the data elements we expect. Verify that they come in the correct
457 # order and the correct format.
459 for my $key (@keys) {
460 my $line = readline($fh);
461 if (!defined($line)) {
462 die "$0: truncated input before $key: $!\n";
465 if ($line =~ s{ \A \Q$key\E : [ ] }{}xms) {
468 die "$0: unrecognized input line before $key\n";
472 # The final line of input must be a literal "end\n";
473 my $line = readline($fh);
474 if (!defined($line)) {
475 die "$0: truncated input before end: $!\n";
476 } elsif ($line ne "end\n") {
477 die "$0: unrecognized input line before end\n";
480 # Return the results.
481 my $password = $data{'new-password'};
482 my $principal = $data{principal};
483 return wantarray ? ($password, $principal) : $password;
486 ##############################################################################
488 ##############################################################################
490 # Always flush output.
493 # Clean up the script name for error reporting.
495 local $0 = basename($0);
497 # Parse the argument list.
498 my ($opt, $usage) = describe_options(
500 ['database|d=s', 'Path to the history database, overriding the default'],
501 ['help|h', 'Print usage message and exit'],
502 ['manual|man|m', 'Print full manual and exit'],
503 ['stats|S=s', 'Path to hash of length statistics'],
504 ['strength|s=s', 'Path to strength checking program to run'],
507 print {*STDOUT} $usage->text
508 or die "$0: cannot write to standard output: $!\n";
510 } elsif ($opt->manual) {
511 say {*STDOUT} 'Feeding myself to perldoc, please wait...'
512 or die "$0: cannot write to standard output: $!\n";
513 exec('perldoc', '-t', $fullpath);
515 my $database = $opt->database || $HISTORY_PATH;
516 my $stats_db = $opt->stats || $LENGTH_STATS_PATH;
518 # Open syslog for result reporting.
519 openlog($0, 'pid', LOG_AUTH);
521 # Read the principal and password that we're supposed to check.
522 my ($password, $principal) = read_change_data(\*STDIN);
524 # Delegate to the external strength checking program.
525 my ($okay, $error, $status) = strength_check($principal, $password);
527 log_result($principal, 'rejected', $error);
532 # Drop privileges for the rest of the program.
533 drop_privileges($HISTORY_USER, $HISTORY_GROUP);
535 # Hash the password and check history. Exit if a hash is in history.
536 if (check_history($database, $principal, $password)) {
537 log_result($principal, 'rejected', $REJECT_MESSAGE);
538 warn "$REJECT_MESSAGE\n";
542 # The password is accepted. Record it, update the length counter, and return
544 log_result($principal, 'accepted');
545 write_history($database, $principal, $password);
546 say {*STDOUT} 'APPROVED'
547 or die "$0: cannot write to standard output: $!\n";
548 update_length_counts($stats_db, length($password));
553 ##############################################################################
555 ##############################################################################
558 heimdal-history heimdal-strength Heimdal -hm BerkeleyDB timestamps POSIX
559 whitespace API Allbery sublicense MERCHANTABILITY NONINFRINGEMENT syslog
560 pseudorandom JSON LDAP-compatible PBKDF2 SHA-256
564 heimdal-history - Password history via Heimdal external strength checking
568 B<heimdal-history> [B<-hm>] [B<-d> I<database>] [B<-S> I<length-stats-db>]
569 [B<-s> I<strength-program>] [B<principal>]
573 B<heimdal-history> is an implementation of password history via the
574 Heimdal external password strength checking interface. It stores separate
575 history for each principal, hashed using Crypt::PBKDF2 with
576 randomly-generated salt. (The randomness is from a weak pseudorandom
577 number generator, not strongly random.)
579 Password history is stored in a BerkeleyDB DB_HASH file. The key is the
580 principal. The value is a JSON array of objects, each of which has two
581 keys. C<timestamp> contains the time when the history entry was added (in
582 POSIX seconds since UNIX epoch), and C<hash> contains the hash of a
583 previously-used password in the Crypt::PBKDF2 LDAP-compatible format.
584 Passwords are hashed using PBKDF2 (from PKCS#5) with SHA-256 as the
585 underlying hash function using a number of rounds configured in this
586 script. See L<Crypt::PBKDF2> for more information.
588 B<heimdal-history> also checks password strength before checking history.
589 It does so by invoking another program that also uses the Heimdal external
590 password strength checking interface. By default, it runs
591 B</usr/bin/heimdal-strength>. Only if that program approves the password
592 does it hash it and check history.
594 As with any implementation of the Heimdal external password strength
595 checking protocol, B<heimdal-history> expects, on standard input:
597 principal: <principal>
598 new-password: <password>
601 (with no leading whitespace). <principal> is the principal changing its
602 password (passed to the other password strength checking program but
603 otherwise unused here), and <password> is the new password. There must
604 be exactly one space after the colon. Any subsequent spaces are taken to
605 be part of the principal or password.
607 If invoked as root, B<heimdal-history> will run the external strength
608 checking program as user C<nobody> and group C<nogroup>, and will check
609 and write to the history database as user C<_history> and group
610 C<_history>. These users must exist on the system if it is run as root.
612 The result of each password check will be logged to syslog (priority
613 LOG_INFO, facility LOG_AUTH). Each log line will be a set of key/value
614 pairs in the format C<< I<key>=I<value> >>. The keys are:
620 The action performed (currently always C<check>).
624 The principal for which a password was checked.
628 An internal error message that did not stop the history check, but which
629 may indicate that something is wrong with the history database (such as
630 corrupted entries or invalid hashes). If this key is present, neither
631 C<result> nor C<reason> will be present. There will be a subsequent log
632 message from the same invocation giving the final result of the history
633 check (assuming B<heimdal-history> doesn't exit with a fatal error).
637 Either C<accepted> or C<rejected>.
641 If the password was rejected, the reason for the rejection.
645 The value will be surrounded with double quotes if it contains a double
646 quote or space. Any double quotes in the value will be doubled, so C<">
653 =item B<-d> I<database>, B<--database>=I<database>
655 Use I<database> as the history database file instead of the default
656 (F</var/lib/heimdal-history/history.db>). Primarily used for testing,
657 since Heimdal won't pass this argument.
659 =item B<-h>, B<--help>
661 Print a short usage message and exit.
663 =item B<-m>, B<--manual>, B<--man>
665 Display this manual and exit.
667 =item B<-S> I<length-stats-db>, B<--stats>=I<length-stats-db>
669 Use I<length-stats-db> as the database file for password length statistics
670 instead of the default (F</var/lib/heimdal-history/lengths.db>).
671 Primarily used for testing, since Heimdal won't pass this argument.
673 =item B<-s> I<strength-program>, B<--strength>=I<strength-program>
675 Run I<strength-program> as the external strength-checking program instead
676 of the default (F</usr/bin/heimdal-strength>). Primarily used for
677 testing, since Heimdal won't pass this argument.
683 On approval of the password, B<heimdal-history> will print C<APPROVED> and
684 a newline to standard output and exit with status 0.
686 If the password is rejected by the strength checking program or if it (or
687 a version with a single character removed) matches one of the hashes stored
688 in the password history, B<heimdal-history> will print the reason for
689 rejection to standard error and exit with status 0.
691 On any internal error, B<heimdal-history> will print the error to standard
692 error and exit with a non-zero status.
698 =item F</usr/bin/heimdal-strength>
700 The default password strength checking program. This program must follow
701 the Heimdal external password strength checking API.
703 =item F</var/lib/heimdal-history/history.db>
705 The default database path. If B<heimdal-strength> is run as root, this
706 file needs to be readable and writable by user C<_history> and group
707 C<_history>. If it doesn't exist, it will be created with mode 0600.
709 =item F</var/lib/heimdal-history/history.db.lock>
711 The lock file used to synchronize access to the history database. As with
712 the history database, if B<heimdal-strength> is run as root, this file
713 needs to be readable and writable by user C<_history> and group
716 =item F</var/lib/heimdal-history/lengths.db>
718 The default length statistics path, which will be a BerkeleyDB DB_HASH
719 file of password lengths to counts of passwords with that length. If
720 B<heimdal-strength> is run as root, this file needs to be readable and
721 writable by user C<_history> and group C<_history>. If it doesn't exist,
722 it will be created with mode 0600.
724 =item F</var/lib/heimdal-history/lengths.db.lock>
726 The lock file used to synchronize access to the length statistics
727 database. As with the length statistics database, if B<heimdal-strength>
728 is run as root, this file needs to be readable and writable by user
729 C<_history> and group C<_history>.
735 Russ Allbery <eagle@eyrie.org>
737 =head1 COPYRIGHT AND LICENSE
739 Copyright 2013, 2014 The Board of Trustees of the Leland Stanford Junior
742 Permission is hereby granted, free of charge, to any person obtaining a
743 copy of this software and associated documentation files (the "Software"),
744 to deal in the Software without restriction, including without limitation
745 the rights to use, copy, modify, merge, publish, distribute, sublicense,
746 and/or sell copies of the Software, and to permit persons to whom the
747 Software is furnished to do so, subject to the following conditions:
749 The above copyright notice and this permission notice shall be included in
750 all copies or substantial portions of the Software.
752 THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
753 IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
754 FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
755 THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
756 LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
757 FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
758 DEALINGS IN THE SOFTWARE.
762 L<Crypt::PBKDF2>, L<heimdal-strength(1)>