Posts Active Directory Fundamentals (Part 3)- Group Policies
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Active Directory Fundamentals (Part 3)- Group Policies

Introduction

In this blogpost, we’ll discuss about the Group Policies in Active directory, which is one of the important elements of AD. We’ll cover concepts like :

  • What are Group Policies in Active Directory
  • Why Group Policies are required
  • What can be accomplised using Group policies
  • How group policies can be enumerated and compromised

Group policies are created to centrally manage the operating system, users and computers in the whole Active Directory domain. These policies allow us to manage user and computer settings from a centralized console called as GPMC (Group Policy Management Console).


The shortcut to open Group Policy Management Console is gpmc.msc

Group Policy Objects

A GPO (Group policy object) contains the Group Policy settings and represents the files related to policy settings in the file system and in the Active Directory. As you can see in the above screenshot, there are only two GPOs under Group policy Objects container. These two are the core GPOs which are created automatically when an domain is created.

Default Domain Policy

This default policy defines a set of basic settings for all users and computers in a domain in terms of password policy, Kerberos policy and account lockout policy.


Default Domain Controllers Policy

The Default Domain Controllers Policy defines a set of basic security and auditing settings for all domain controllers in a domain.

Default Domain Controller

Group policy objects are a set of policies that are grouped together and applied to either computer or user objects.

Group Policy Management Editor

Group Policy Management Editor

The shortcut to open Group Policy Management Editor is gpedit.msc. This is the user interface where you can edit the group policies. As you can see in the above screenshot, the policy settings are divided into two categories: computer configuration and user configuration.

Computer Configuration

Let’s say if the settings of a GPO is specified under computer configuration, and it is applied on a particular computer object, then that computer object will have that specified security settings, system behavior, application settings, and computer startup and shutdown scripts as specified by the GPO irrespective of the user who logs on to the computer on which GPO is applied.

User Configuration

In this case, the policies defined under user configuration defines the user specific system behavior, application settings, security settings, assigned and published applications, user logon and logoff scripts, and folder redirection. If a policy is defined for a particular user, it will be applicable on any system where that user logs in.

Note:- The policies defined under the computer-configuration override the ones which are defined under user configuration.

Once a GPO is created, it needs to be linked to one or more AD containers, like to the whole domain or some particular OU (organizational unit) in order to take effect. For example, if you want to create a Group Policy to grant all members of DB Admins team access to required shared folders for SQL, then this GPO has to be linked to just the DBAdmins OU.

Common GPOs

Some common type of group policies that are used to enable different kind of tasks like setting up:

  • Security policies like disabling NTLM authentication, and custom policies like restricting access to the command prompt for normal users
  • IT policies like restricting access to USB drives, configuring logon banner to display legal notices to end users etc.
  • Policies for audit and compliance like enabling the advanced windows auditing policies to enable audit for important objects and logging the recommended important windows events IDs in the events viewer etc.
  • Complex password policies in a domain to deal with password spraying attacks
  • Automation policies like deploying standard tools on multiple computers/servers, mapping network drives, automating logon/logoff tasks and much more.

Group Policy Refresh Interval

There is a refresh interval which defines when the time when a particular domain computer or server requests the group policy updates from a domain controller to make sure to replicate the changes in any of the Group policies. The daefault refresh interval is 90 minutes with a random offset interval added to every computer/server to prevent all the domain machines from requesting group policy updates from DC at the same time. Will discuss more about GPOs later in this series.

SYSVOL

One of the most important network share in AD is SYSVOL since it stores the Group policy templates and is shared by default on domain controllers. All the domain computers access this share to check the domain policies. It’s default location is %SYSTEMROOT%\SYSVOL\sysvol in the domain controller. And the location on network would be \\<domain_name>\SYSVOL

A sysvol folder is made up of following components:

Scripts (i.e. startup scripts and files like .bat, vbs etc. that are referenced in GPOs)  

Group Policies folder and templates (replicated throughout the domain)

Junction Points (work like a shortcut. One directory can point to a different directory)

SYSVOL Policies folder

The SYSVOL policies folder contains all the GPOs. And folder name for each GPO is same as that GPO’s GUID.

SYSVOL

Below are the components of a Policy folder in SYSVOL.

Machine – (stores GPO’s machine specific config)

User – (stores GPO’s user specific config)

GPT.INI – (stores GPO config)

Policy

SYSVOL replication is done by DFSR (Distributed File System Replication) which is the default replication mechanism for SYSVOL folder replication domain-wide.

Group Policy Preferences

Microsoft introduced an important feature called as “Group Policy Preferences” which helped to store and use credentials in scenarios like creating scheduled tasks, changing local admin password on all workstations etc. It was really helpful for administrators since it provides an automated mechanism (instead of doing it with the help of scripts) where the passwords are not saved in cleartext like in case of scripts, but were stored as AES-256 bit encrypted in the xml file in SYSVOL directory. But later Microsoft published the private key on their MSDN portal which allowed to decrypt any password stored in the xml files created like this.

aes key

Since everyone has read access to SYSVOL, it is just a matter of finding the cpassword in the XML files and decrypting the passwords.

PowerSploit’s GPP-Password searches xml in sysvol policies directory for cpassword field and decrypts it using the key that Microsoft published earlier.

Though Microsoft released a patch later and disabled the option to enter password like this but the XML files are still there if not removed by admins and a lot of times the password works and is a good practice to hunt for these files. And then later Microsoft a way to change local Administrator password by using LAPS (Local Administrator Password Solution).

Best practices:

  • For system administrators is to delete the existing GPP xml files in the SYSVOL folder which have passwords and make sure there are no such files there. And
  • For pentesters, the best practice is to enumerate the SYSVOL policies folder to hunt for xml files containing cpassword.
  • Best practice for defenders is to create AD decoys to have xml files with passwords in it and enable auditing to get alerts when someone access those folders which are not linked to actual GPOs. I’ll write a detailed post on this later in this series.

Finding the GPOs in a Domain

To check all the GPOs in a domain, we can use below command:

PS C:\> Get-GPO -All | Select DisplayName

DisplayName
-----------
Remove OneDrive
Default Domain Policy
Bginfo
Default Domain Controllers Policy
Remove windows features
No auto-restart (wupdates)
No Sleep

Viewing the Settings of a GPO

To view the settings of a GPO, we can run

PS C:\> Get-GPOReport -Name "<GPO Name>" -Path <Path With File Name> -ReportType "<HTML|XML>"

gporeport

We can actually see the definition of a policy in xml format, like what is GPO doing if it is enabling a firewall rule or running a script etc:

  <Computer>
    <VersionDirectory>1</VersionDirectory>
    <VersionSysvol>1</VersionSysvol>
    <Enabled>true</Enabled>
    <ExtensionData>
      <Extension xmlns:q1="http://www.microsoft.com/GroupPolicy/Settings/WindowsFirewall" xsi:type="q1:WindowsFirewallSettings">
        <q1:GlobalSettings>
          <q1:PolicyVersion>
            <q1:Value>541</q1:Value>
          </q1:PolicyVersion>
        </q1:GlobalSettings>
        <q1:InboundFirewallRules>
          <q1:Version>2.29</q1:Version>
          <q1:Action>Allow</q1:Action>
          <q1:Name>Allow WinRM port 5985 </q1:Name>
          <q1:Dir>In</q1:Dir>
          <q1:LPort>5985</q1:LPort>
          <q1:Protocol>6</q1:Protocol>
          <q1:Active>true</q1:Active>
        </q1:InboundFirewallRules>
      </Extension>
      <Name>Windows Firewall</Name>
    </ExtensionData>
    <ExtensionData>
      <Extension xmlns:q2="http://www.microsoft.com/GroupPolicy/Settings/Registry" xsi:type="q2:RegistrySettings">
        <q2:Blocked>false</q2:Blocked>
      </Extension>
      <Name>Registry</Name>
    </ExtensionData>
  </Computer>
  <User>
    <VersionDirectory>1</VersionDirectory>
    <VersionSysvol>1</VersionSysvol>
    <Enabled>true</Enabled>
  </User>
</GPO>

In the above config, we can see it is enabling a rule in Windows Firewall.

Finding misconfigured Group policies

To demonstrate this, I created a GPO and provided a user ghost permissions to edit the GPO.

gpo_perms

Now, we’ll get the cn of the GPO “Local Admin Password”

gpo_acl

And do an ACL check on this GPO and see the permissions that user ghost has that we configured earlier.

gpo_badacl

This is how a misconfigured Group policy object looks like. Since user ghost has the permissions to edit the GPO, the settings can be edited and some other task be accomplished from this GPO without domain admin privileges.