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.

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. 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 very ALPHA! 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 real life.

Installation

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

OPF-alpha.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

Finally, create two dictionary files in the same directory where you placed opfservice.exe named 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

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.

Or you can skip all this and use one of the installers.

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

https://github.com/brockrob/OpenPasswordFilter/raw/master/OPFInstaller_x86.zip

The filter DLL bitness must match the OS, so choose correctly. .Net 3.5 is still required and the installer won't handle installing it for you because Visual Studio packaging a bootstrap package for that version has been broken since 2008 and I didn't have the patience to roll a custom action to test the OS version and go down the appropriate installation path (DISM vs .exe). I also can't set the reboot flag in the MSI with Visual Studio, so you'll have to manually do that as well, but it still saves some significant legwork.

The installers include lists. The match list is rockyou.txt with every line less than ten characters stripped out, lowered, sorted, and de-duped. The other 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).

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.