DLLPasswordFilterImplant – DLL Password Filter Implant With Exfiltration Capabilities

DLLPasswordFilterImplant is a custom password filter DLL that allows the capture of a user’s credentials. Each password change event on a domain will trigger the registered DLL in order to exfiltrate the username and new password value prior successfully changing it in the Active Directory (AD).
For more information about password filters consult Microsoft’s documentation.

Installing

1. To install the password filter on a system:

• Create the DLL for the targeted architecture. Compile in 32-bit for a 32-bit system and in 64-bit for a 64-bit system.
• Copy the DLL to the Windows installation directory. (Default folder: WindowsSystem32)
• Register the password filter by updating the following registry key:

  HKEY_LOCAL_MACHINESYSTEMCurrentControlSetControlLsa

  If the Notification Packages subkey exists, add the name of the DLL (“DLLPasswordFilterImplant” if you didn’t rename it) to the existing value data. Do not overwrite the existing values. If the subkey does not exist, create it and add the name of the DLL (“DLLPasswordFilterImplant” if you didn’t rename it) to the value data. NOTE: Do not include the .dll extension when adding the name of the DLL in the Notification Packages subkey.

• Configure the public key to use for encrypting credentials.

  KEY=key.pem
  # Generate an RSA key and dump its public key. Keep the private key around for decryption
  openssl genrsa -out $KEY 2048
  
  # Prepare the Windows registry key entry.
  echo 'Windows Registry Editor Version 5.00' > addKey.reg
  echo >> addKey.reg
  echo '[HKEY_LOCAL_MACHINESYSTEMCurrentControlSetControlLsa]' >> addKey.reg
  # If python2 does not exist, use `python` instead.
  echo "Key=hex:$(openssl rsa -in $KEY -pubout | sed -E '/^-/d' | base64 -d | python2 -c 'import sys; print(",".join(["{:02x}".format(ord(b)) for b in sys.stdin.read()]))')" >> addKey.reg

  You can then run addKey.reg file to append the raw public key to the registry. Note that using asymmetric encryption significantly increases the size of the data to exfiltrate due to message padding. There are possible improvements to be made to reduce the data overhead.

• Restart the system Source

2. To register the key and the domain for DNS exfiltration:

• Go to the following registry key:

  HKEY_LOCAL_MACHINESYSTEMCurrentControlSetControlLsa

• Create a string type subkey named “Domain”. Specify your domain in the value of that subkey. Your domain must start with a “.” . (Example value: “.yourdomain.com”)

Decrypting
The encrypted data is padded using OAEP and can be decrypted as follows:

# Convert the stitched hex string to raw bytes.
xxd -r -p exfiltrated.hex > raw.bin

# Decrypt using the private key.
openssl rsautl -decrypt -oaep -inkey $KEY -in raw.bin -out decrypted.txt

Uninstalling
To completely remove the password filter of a system:

• Unregister the password filter by updating the following registry key:

  HKEY_LOCAL_MACHINE SYSTEMCurrentControlSetControlLsa

  In the Notification Packages subkey remove the name of the DLL of the existing value data. Do not remove other existing values.

• Restart the system
• In the Windows installation directory (Default folder: WindowsSystem32), find the password filter DLL (“DLLPasswordFilterImplant.DLL” if you didn’t rename it) and delete the file.

Compatibility
Works on:

• Windows 7 Hosts (x64)
• Windows 10 Hosts (x64)
• Windows Server 2008 DCs (x64)
• Windows Server 2012 DCs (x64)
• Windows Server 2016 DCs (x64)

The password filter was tested exclusively on systems listed above.

Debug
Here are some tool that may help you debug the DLL (if necessary):

• Process Explorer
• Dependency Walker