Category Archives: Active Directory

Applying group policy preferences based on Citrix delivery group or machine catalog membership

We’ve slowly been transitioning our Citrix XenApp environment from static VMs to Machine Creation Services (MCS) based VMs. My goal was to have two master (fat) images and two machine catalogs. Because of policy and application segregation requirements those two catalogs translated into more than two delivery groups. With these delivery groups came the requirement to apply different group policies to different machines. One option would be to move the corresponding AD object into a different OU and apply policy that way. While that would work due to AD objects not being automatically moved/re-created after machine creation it still requires some administrative overhead. It was clear that dynamically adjusting certain policies based on delivery group membership would be ideal.

After a little digging I found where both the delivery group and machine catalog memberships were written to in the registry by the VDA. Below is an example of how we applied a user GPP item dynamically based on the delivery group of the machine.

Registry Key Path: HKEY_LOCAL_MACHINE\SOFTWARE\Citrix\VirtualDesktopAgent\State
Delivery Group Value Name: DesktopGroupName
Machine Catalog Value Name: DesktopCatalogName

Deploying a RODC for Palo Alto PAN-OS Credential Phishing Prevention

I was recently tasked with setting up the AD side of PAN-OS Credential Phishing Prevention. For some technical reason that I haven’t been able to find it requires a read-only domain controller (I attempted putting the credential agent on a regular DC just to see if it would work and it seemed to read credentials without issue. If anyone has information about RODC requirement I’d love to hear it.) We don’t have or use any read-only domain controllers currently, so I had to deploy one for each domain we needed to protect. This brought up a few questions to mind…

  • How would I decide/maintain what users have their passwords replicated to the RODC?
  • How do these passwords get replicated to the RODC? By design passwords are only replicated to an RODC after an initial authentication attempt when they are configured for password replication.
  • Since the sole reason this domain controller is being deployed is for PAN-OS I don’t want it to handle logons and I want to make it very lightweight. How do I prevent user logons/authentication from occurring on this DC?
  • How are usernames identified? Will it handle all formats (samAccountName, explicit UPN, implicit UPN, and email address)?

How would I decide/maintain what users have their passwords replicated to the RODC?

This one is pretty easy for me. I don’t see any reason to exclude any accounts from credential detection, so I will use ‘Domain Users’. I usually stay away from using default groups, but this is one of the few cases where it makes sense to do so.

How do these passwords get replicated to the RODC?

I turned the logging level up to verbose (HKEY_LOCAL_MACHINE\SOFTWARE\Palo Alto Networks\User-ID Credential Agent\Log | DebugLevel=5) on the credential agent after full configuration and saw that the agent enumerates all the objects within the ‘msDS-Reveal-OnDemandGroup‘ attribute of the RODC computer object (and DNs manually specified in the user-id agent seen in the screenshot below) and executes ‘repadmin‘ against each object to force replication. As password changes are detected it re-replicates passwords using the same method.

How do I prevent user logons/authentication?

Clients discover domain controllers using DC Locator. I decided to prevent the domain controller from registering all SRV records except for the two necessary for replication (LdapIpAddress + DsaCname). To do this I set a local policy under ‘Computer Settings → Administrative Templates → System → NetLogon → DC Locator DNS records‘ called ‘DC Locator DNS Records not registered by the DCs‘. The value I set for this policy was:

Ldap LdapAtSite Pdc Gc GcAtSite GcIpAddress DcByGuid Kdc KdcAtSite Dc DcAtSite Rfc1510Kdc Rfc1510KdcAtSite GenericGc GenericGcAtSite Rfc1510UdpKdc Rfc1510Kpwd Rfc1510UdpKpwd

How are usernames identified?

After experimentation it is clear that when using the domain credential filter method PAN-OS is getting the user from the IP<->user relationship and only looks for that user’s password in web site submissions. No matter what username I put in a form the submission triggered a detection as long as the password matched my password. Another user’s credentials under my session did not trigger a detection. I was happy with this because I do not have to worry about certain username formats not being detected.

After all of these questions/concerns were addressed came the actual implementation. You are required to install both the ‘User-ID Agent’ and the ‘User-ID Credential Agent’ on the RODC. According to the documentation this instance of the user-id agent should not be used for IP<->user relationship gathering and should only be pulling credentials. The credential agent creates the ‘bloom filter’ and sends it over to the user-id agent. PAN-OS connects to the user-id agent receives the newest version of the bloom filter. One issue I ran into was around permissioning and service accounts. Normally you would assign a domain account with limited permissions to the user-id agent, but the thing to consider here is that credential agent and user-id agent communicate using named pipes. According to the documentation on named pipes if no ACL is specified when creating a named pipe the default ACL is:

  • LocalSystem – Full Control
  • Administrators – Full Control
  • Creator Owner – Full Control
  • Everyone – Read
  • Anonymous – Read

The issue here is that the credential agent only runs under LocalSystem and assigning a non-administrator account to the user-id agent service prevents the user-id agent from communicating to the credential agent’s named pipe. Leaving the user-id agent service running under LocalSystem worked, but created another problem. When running under LocalSystem for some reason it was unable to enumerate the ‘msDS-Reveal-OnDemandGroup‘ attribute (seen in the UaDebug.log file) for the RODC meaning it couldn’t determine what user accounts were allowed to sync to this RODC. I found that if I manually specified a group DN in the user-id agent it would work under LocalSystem. The only other option would be switching to a ‘DOMAIN\Administrators’ service account (since this a domain controller) which I did not want to do. Since I’m only using ‘Domain Users’ this was easy enough to configure.

UPDATE: There seems to be a discrepancy between how the User-ID agent worked previously, the current documentation, and how it works now. In the past the User-ID agent configuration utility would adjust the ‘Log on as’ value for the ‘User-ID Agent’ service to the account you specified in the agent setup ‘Authentication’ tab. It seems now the service continues to run as LocalSystem, but uses the account specified in the configuration to actually probe the DCs and AD. I was able to leave it running as LocalSystem, specify an account with the proper rights in the ‘Authentication’ tab, and leave the group DN blank under the ‘Credentials’ tab in the user-id agent configuration utility. I verified the agent was using the account via logon events in the security event log on the RODC.

After configuring this you can monitor both log files to verify proper operation and then later verify PAN-OS is properly receiving the bloom filters. Be sure to restart the user-id agent after making any changes.

Credential agent log (UaCredDebug.log) sending bloom filter:

02/08/19 12:43:46:593 [ Info  667]: Sent BF to UaService. 21edc031f4891d2c42c133acded980ba

User-ID Agent log (UaDebug.log) receiving bloom filter from credential agent:

02/08/19 12:43:46:593[ Info 2896]: Received BF Push. Different from current one.
02/08/19 12:43:46:593[ Info 2897]: 0829f71740aab1ab98b33eae21dee122->21edc031f4891d2c42c133acded980ba

Azure AD Connect mail-enabled public folder synchronization issues – The cause of the error is not clear

We recently went through some Exchange Online Protection (EOP) cleanup and part of that involved turning on Directory Based Edge Blocking. We already went through the exercise of syncing all objects (especially ones part of Exchange), but the only ones that weren’t being synced were mail-enabled public folders. After turning on Directory Based Edge Blocking we realized there were a few public folders that needed to receive mail from the Internet. After syncing mail-enabled public folders (this is a newer feature in AD Connect) we received synchronization errors for four objects. The only thing these objects had in common was that they referenced a mail-enabled public folder by either having that object as a group member or having it as a forwarding object on a mailbox.

The errors we receiving were:

  • The cause of the error is not clear. This operation will be retried during the next synchronization. If the issue persists, contact Technical Support.
  • IdentityDataValidationFailed

The workaround is to create a mail contact object that has the same targetAddress as the mail-enabled public folder object and use that object in place of the public folder object in something like a group membership. The issue with this is that by design a mail contact’s targetAddress is also part of its proxyAddresses attribute and the mail-enabled public folder object of course already has the email address as part of its proxyAddresses attribute. This duplicate is not allowed. The way around this is to modify the mail contact object so that the targetAddress is not part of proxyAddresses. To create this special mail contact you do the following:

  • Create a mail contact in Exchange with a fake external address
  • Disable e-mail address policy for the object
  • Use ADSIEdit to:
    • Change the targetAddress to the email address of the mail-enabled public folder
    • Remove the fake external address you specified earlier from proxyAddresses

After the object has been created you can now use it in lieu of the mail-enabled public folder in group memberships and other attributes.

The replication operation failed because of a schema mismatch between the servers involved

Last week I had deploy a new domain controller to the root domain in a forest (it happened to be an RODC for a unique use case, but that is irrelevant). The domain only partially replicated before failing and showing errors on the new DC.

The errors were:

Log Name:      Directory Service
Source: Microsoft-Windows-ActiveDirectory_DomainService
Date: 1/4/2019 11:19:18 AM
Event ID: 1791
Task Category: Replication
Level: Error
Keywords: Classic
User: ANONYMOUS LOGON
Computer: rodc1.domain.com
Description:
Replication of application directory partition DC=domain,DC=com from source 24c77a2c-6da0-41a1-95cf-e0542bca5b89 (dc1.domain.com) has been aborted. Replication requires consistent schema but last attempt to synchronize the schema had failed. It is crucial that schema replication functions properly. See previous errors for more diagnostics. If this issue persists, please contact Microsoft Product Support Services for assistance. Error 8418: The replication operation failed because of a schema mismatch between the servers involved..
Log Name:      Directory Service
Source: Microsoft-Windows-ActiveDirectory_DomainService
Date: 1/4/2019 11:19:31 AM
Event ID: 1203
Task Category: Replication
Level: Warning
Keywords: Classic
User: ANONYMOUS LOGON
Computer: rodc1.domain.com
Description:
The directory service could not replicate the following object from the source directory service at the following network address because of an Active Directory Domain Services schema mismatch.
Object:
CN=Bob Smith,OU=Users,OU=All Users,DC=domain,DC=com
Network address:
24c77a2c-6da0-41a1-95cf-e0542bca5b89._msdcs.domain.com

It was obvious that the object referenced in the second event was causing the issue, but this object was in use and I couldn’t just remove it. When looking for related errors on the source DC I found this:

Log Name:      Directory Service
Source: Microsoft-Windows-ActiveDirectory_DomainService
Date: 1/4/2019 11:04:33 AM
Event ID: 1450
Task Category: Internal Processing
Level: Error
Keywords: Classic
User: ANONYMOUS LOGON
Computer: dc1.domain.com
Description:
The security descriptor propagation task could not calculate a new security descriptor for the following object.
Object:
CN=Bob Smith,OU=Users,OU=All Users,DC=domain,DC=com
This operation will be tried again later.
User Action
If this condition continues, attempt to view the status of this object and manually change the security descriptor.

Additional Data
Error value:
1340 The inherited access control list (ACL) or access control entry (ACE) could not be built.

This was much more specific and showed there was an issue with the ACL of the object. I tried making one small change to the security ACL on the object to verify there was an issue and received an error. This ACL was either corrupt or too large. I decided to try repairing the ACL on the object by using ADSI Edit (adsiedit.msc) to remove everything from the ACL, add only ‘Domain Admins’ and ‘SYSTEM’ with Full Control, and then resetting it using DSACLS. I also had to do this for the ‘ExchangeActiveSyncDevices’ child object and the leaf objects under that since this user had Exchange ActiveSync devices. I verified the child and leaf objects were inheriting from the user object and proceeded to reset the ACL using the DSACLS:

dsacls "CN=Bob Smith,OU=Users,OU=All Users,DC=domain,DC=com" /S /T

After resetting the ACL replication to this domain controller completed with this event:

Log Name:      Directory Service
Source: Microsoft-Windows-ActiveDirectory_DomainService
Date: 1/4/2019 11:49:51 AM
Event ID: 1394
Task Category: Service Control
Level: Information
Keywords: Classic
User: ANONYMOUS LOGON
Computer: rodc1.domain.com
Description:
All problems preventing updates to the Active Directory Domain Services database have been cleared. New updates to the Active Directory Domain Services database are succeeding. The Net Logon service has restarted.