The password filter architecture is useful to check that a password is compliant with the system security policy. It will typically check that when a user changes his password, it follows the required complexity.

Microsoft opened the API so that users can extend the functionality with their own filters.

Mubix diverted this API by developing a password logger: the DLL just logs the password both on the disk and a remote server,  and does nothing else.

A perfect way to maintain a persistent access… I tested it:

Evilpassfilter exploitation process

1. Evilpassfilter.dll is loaded into lsass.exe
2. A user updates his password
3. The password goes through the Evilpassfilter password filter, which notifies the attacker through HTTP and also logs it locally.

Here is what I did to get it work (Windows 7 x64):

• Make sure the local password security policy is enabled on the target
• Create a new Win32 project in Visual Studio (2012)
• Eventually delete unnecessary files, to start with an empty project (stadfx.h and cie)
• Import the source code
• Create a Evilpassfilter.def file, which defines the exports:

  LIBRARY Evilpassfilter
  EXPORTS
     InitializeChangeNotify
     PasswordFilter
     PasswordChangeNotify

• In the project properties, make sure to select the appropriate architecture, matching with the one of your target.
  

  Selecting the compilation target architecture (win32/x64)

• In the input settings of the link editor, add wininet.lib as additional dependancy.
• Also add Evilpassfilter.def as module definition file.
  

  Evilpassfilter Visual Studio settings

• In the source code, fix line 72: return; –> return 1;
• Now you should be able to compile the library. You may want to make sure that the DLL is valid and integrated the exports (open it with IDA or a PE tool):
  

  Evilpassfilter.dll exports seen in IDA

• Copy the resulting DLL to the system32 folder.
• Open regedit HKLM\System\CurrentControlSet\Control\Lsa
  and add Evilpassfilter to the Notification Packages

Reboot and… now you should know what to do next :-)