Enforce Azure AD password protection for Windows Server Active Directory
Azure AD password protection is a feature that enhances password policies in an organization. On-premises deployment of password protection uses both the global and custom banned-password lists that are stored in Azure AD. It does the same checks on-premises as Azure AD for cloud-based changes.
Azure AD password protection is designed with these principles in mind:
- Domain controllers never have to communicate directly with the internet.
- No new network ports are opened on domain controllers.
- No Active Directory schema changes are required. The software uses the existing Active Directory container and serviceConnectionPoint schema objects.
- No minimum Active Directory domain or forest functional level (DFL/FFL) is required.
- The software doesn't create or require accounts in the Active Directory domains that it protects.
- User clear-text passwords never leave the domain controller, either during password validation operations or at any other time.
- The software is not dependent on other Azure AD features; for example Azure AD password hash sync is not related and is not required in order for Azure AD password protection to function.
- Incremental deployment is supported, however the password policy is only enforced where the Domain Controller Agent (DC Agent) is installed. See next topic for more details.
Azure AD password protection supports incremental deployment across domain controllers in an Active Directory domain but it's important to understand what this really means and what the tradeoffs are.
The Azure AD password protection DC agent software can only validate passwords when it is installed on a domain controller, and only for password changes that are sent to that domain controller. It is not possible to control which domain controllers are chosen by Windows client machines for processing user password changes. In order to guarantee consistent behavior and universal password protection security enforcement, the DC agent software MUST be installed on all domain controllers in a domain.
Many organizations will want to do careful testing of Azure AD password protection on a subset of their domain controllers prior to doing a full deployment. Azure AD password protection does support partial deployment, ie the DC agent software on a given DC will actively validate passwords even when other DCs in the domain do not have the DC agent software installed. Partial deployments of this type are NOT secure and are NOT recommended other than for testing purposes.
It's important to understand the underlying design and function concepts before you deploy Azure AD password protection in an on-premises Active Directory environment. The following diagram shows how the components of password protection work together:
- The Azure AD Password Protection Proxy service runs on any domain-joined machine in the current Active Directory forest. Its primary purpose is to forward password policy download requests from domain controllers to Azure AD. It then returns the responses from Azure AD to the domain controller.
- The password filter DLL of the DC Agent receives user password-validation requests from the operating system. It forwards them to the DC Agent service that's running locally on the domain controller.
- The DC Agent service of password protection receives password-validation requests from the password filter DLL of the DC Agent. It processes them by using the current (locally available) password policy and returns the result: pass or fail.
How password protection works
Each Azure AD Password Protection Proxy service instance advertises itself to the domain controllers in the forest by creating a serviceConnectionPoint object in Active Directory.
Each DC Agent service for password protection also creates a serviceConnectionPoint object in Active Directory. This object is used primarily for reporting and diagnostics.
The DC Agent service is responsible for initiating the download of a new password policy from Azure AD. The first step is to locate an Azure AD Password Protection Proxy service by querying the forest for proxy serviceConnectionPoint objects. When an available proxy service is found, the DC Agent sends a password policy download request to the proxy service. The proxy service in turn sends the request to Azure AD. The proxy service then returns the response to the DC Agent service.
After the DC Agent service receives a new password policy from Azure AD, the service stores the policy in a dedicated folder at the root of its domain sysvol folder share. The DC Agent service also monitors this folder in case newer policies replicate in from other DC Agent services in the domain.
The DC Agent service always requests a new policy at service startup. After the DC Agent service is started, it checks the age of the current locally available policy hourly. If the policy is older than one hour, the DC Agent requests a new policy from Azure AD via the proxy service, as described previously. If the current policy isn't older than one hour, the DC Agent continues to use that policy.
Whenever an Azure AD password protection password policy is downloaded, that policy is specific to a tenant. In other words, password policies are always a combination of the Microsoft global banned-password list and the per-tenant custom banned-password list.
The DC Agent communicates with the proxy service via RPC over TCP. The proxy service listens for these calls on a dynamic or static RPC port, depending on the configuration.
The DC Agent never listens on a network-available port.
The proxy service never calls the DC Agent service.
The proxy service is stateless. It never caches policies or any other state downloaded from Azure.
The DC Agent service always uses the most recent locally available password policy to evaluate a user's password. If no password policy is available on the local DC, the password is automatically accepted. When that happens, an event message is logged to warn the administrator.
Azure AD password protection isn't a real-time policy application engine. There can be a delay between when a password policy configuration change is made in Azure AD and when that change reaches and is enforced on all domain controllers.
Azure AD password protection acts as a supplement to the existing Active Directory password policies, not a replacement. This includes any other 3rd-party password filter dlls that may be installed. Active Directory always requires that all password validation components agree before accepting a password.
Forest/tenant binding for password protection
Deployment of Azure AD password protection in an Active Directory forest requires registration of that forest with Azure AD. Each proxy service that is deployed must also be registered with Azure AD. These forest and proxy registrations are associated with a specific Azure AD tenant, which is identified implicitly by the credentials that are used during registration.
The Active Directory forest and all deployed proxy services within a forest must be registered with the same tenant. It is not supported to have an Active Directory forest or any proxy services in that forest being registered to different Azure AD tenants. Symptoms of such a mis-configured deployment include the inability to download password policies.
The two required agent installers for Azure AD password protection are available from the Microsoft Download Center.
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