Introduction

OpenPasswordFilter is an open source custom password filter DLL and userspace service to better protect / control Active Directory domain passwords.

The genesis of this idea comes from conducting many penetration tests where organizations have users who choose common passwords and the ultimate difficulty of controlling this behavior. The fact is that any domain of size will have some user who chose Password1 or Summer2015 or Company123 as their password. Any intruder or low-privilege user who can guess or obtain usernames for the domain can easily run through these very common passwords and start expanding the level of access in the domain.

Microsoft provides a wonderful feature in Active Directory, which is the ability to create a custom password filter DLL. This DLL is loaded by LSASS on boot (if configured), and will be queried for each new password users attempt to set. The DLL simply replies with a TRUE or FALSE, as appropriate, to indicate that the password passes or fails the test.

There are some commercial options, but they are usually in the "call for pricing" category, and that makes it a little prohibitive for some organizations to implement truly effective preventive controls for this class of very common bad passwords.

This is where OpenPasswordFilter comes in -- an open source solution to add basic dictionary-based rejection of common passwords, as well as a check against haveibeenpwned.com's wonderful API.

OPF is comprised of two main parts:

1. OpenPasswordFilter.dll -- this is a custom password filter DLL that can be loaded by LSASS to vet incoming password changes.
2. OPFService.exe -- this is a C#-based service binary that provides a local user-space service for maintaining the dictionary and servicing requests.

The DLL communicates with the service on the loopback network interface to check passwords against the configured database of forbidden values, the pwnedpasswords API of haveibeenpwned.com (cheers to Troy Hunt for that - what a guy) as well as ensuring that the account's SAMAccountName, given name, surname, and display name are not in the password. This architecture is selected because it is difficult to reload the DLL after boot, and administrators are likely loathe to reboot their DCs when they want to add another forbidden password to the list. Just bear in mind how this architecture works so you understand what's going on.

NOTE The current version is pretty beta! I have tested it on some of my DCs, but your mileage may vary and you may wish to test in a safe location before using this in production.

Installation

You can download a precompiled 64-bit version of OPF from the following link:

OPF-beta.zip

You will want to configure the DLL so that Windows will load it for filtering passwords. Note that you will have to do this on all domain controllers, as any of them may end up servicing a password change request. Here is a link to Microsoft's documentation for setting up a password filter:

https://msdn.microsoft.com/en-us/library/windows/desktop/ms721766(v=vs.85).aspx

The bottom line is this:

1. Copy OpenPasswordFilter.dll to %WINDIR%\System32
2. Configure the HKLM\SYSTEM\CurrentControlSet\Control\Lsa\Notification Packages registry key with the DLL name

Note, you do not include the .dll extension in the registry key -- just OpenPasswordFilter.

Next, you will want to configure the OPF service. You can do so as follows:

> sc create OPF binPath= <full path to exe>\opfservice.exe start= boot

You can then start or stop the OPF service from the command line (as an administrator) with:

> NET START OPF

or

> NET STOP OPF

Finally, create several dictionary files in the the SYSVOL path '\127.0.0.1\sysvol\testdomain.com\OPF', obviously substituting your domain name. These are in SYSVOL so that they stay in sync across all domain controllers, and the service checks file modification time at the start of servicing a request and will read in the lists again if it has changed, so restarting the OPF service when modifying the lists is not necessary.

These are

• opfmatch.txt
• opfcont.txt
• opfregex.txt
• opfgroups.txt

Or you can skip all this and use the installer.

https://github.com/brockrob/OpenPasswordFilter/raw/master/OPFInstaller_x64.zip

opfmatch.txt and opfcont.txt

These should contain one forbidden password per line, such as:

Password1
Password2
Company123
Summer15
Summer2015
...

Passwords in opfmatch.txt will be tested for full matches, and those in opfcont.txt will be tested for a partial match. This is useful for rejecting any password containing poison strings such as password and welcome. I recommend constructing a list of bad seeds, then using hashcat rules to build opfcont.txt with the sort of leet mangling users are likely to try, like so:

hashcat -r /usr/share/hashcat/rules/Incisive-leetspeak.rule --stdout seedwordlist | tr A-Z a-z | sort | uniq > opfcont.txt

Bear in mind that if you use a unix like system to create your wordlists, the line terminators will need changing to Windows format:

unix2dos opfcont.txt

opfregex.txt

Similar to the opfmatch and opfconf files, include here regular expression - one per line - for invalid passwords. Example, including 'xx.*xx' will catch all passwords that have two x's followed by any text followed by two x's. Keep this list short as regular expression matching is more computationally expensive than simple matching or contains searches.

opfgroups.txt

This file contains zero or more Active-Directory group names - one per line. These can be security or distribution groups. A user is considered to be in a group if they are a descendent child of the group. A user's password will only be checked if the user is a member of any group listed in this file. If the file is present but contains no groups then every user will be checked.

Event Logging

The opfservice.exe application logs to the Application Event Log using codes 100, and 101. Searching the event log will identify what the opfservice is checking. If the service fails to start, it's likely an error ingesting the wordlists, and the line number of the problem entry will be written to the Application event log.

Production Installation Details

This requires a 64 bit OS as the password filter bitness must match that of the OS and I see no reason to target x86. If you build it yourself against x86, it will probably work. I cannot set the reboot flag in the MSI with Visual Studio, so you'll have to manually do that, but it still saves some significant legwork.

The installer includes lists. The match list is rockyou.txt with every line less than ten characters stripped out, lowered, sorted, and de-duped. The 'contains' list was made as described above with hashcat rules from a seed set containing some dumb words I've seen people base passwords on as well as some terms relevant to my environment (company names, industry terms, etc). The group and regex lists are empty.

If all has gone well, reboot your DC and test by using the normal GUI password reset function to choose a password that is on your forbidden list.