Pass-through Authentication (PTA)

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🔐 What is Pass-through Authentication (PTA)?

Pass-through Authentication (PTA) is a method of authenticating users in Azure Active Directory (Azure AD) where the authentication process happens on-premises instead of in the cloud.

With PTA, users login to Azure AD, but instead of Azure validating their credentials directly, the request is passed through to your on-premises Active Directory (AD) for validation.

🧠 How Does Pass-through Authentication Work?

  1. A user tries to log in to a cloud application (e.g., Office 365).

  2. Azure AD passes the authentication request to the on-prem AD via a secure agent.

  3. The on-prem AD validates the credentials.

  4. If valid, a token is sent back to Azure AD, and the user gains access.

This means Azure AD doesn’t store or validate the actual passwords; it's simply a proxy for passing the authentication request.

🧰 Key Features of PTA:

FeatureDescription
On-Prem AuthenticationPasswords are authenticated by the on-premises AD, not in the cloud.
No Password SyncingPasswords are not stored in Azure AD, so there's no risk of cloud storage of plain-text passwords.
Single Sign-On (SSO)Provides SSO to cloud apps using the same credentials as your on-premises AD.
High SecurityNo need to sync password hashes to the cloud; users’ passwords are verified on-premises.
Requires Azure AD ConnectYou need Azure AD Connect to set up PTA and the necessary agents on-premises.

🛠️ How to Set It Up:

  1. Install the Azure AD Connect Tool on your on-prem server.

  2. Choose the Pass-through Authentication option during the setup process.

  3. Install the PTA agents on one or more on-prem servers.

  4. Configure your Azure AD Connect to sync and pass requests between your on-premises AD and Azure AD.

  5. Users can now log in to cloud apps using their on-prem credentials.

🆚 PTA vs. Password Hash Synchronization (PHS)

FeaturePass-through Authentication (PTA)Password Hash Synchronization (PHS)
Authentication LocationOn-premises (via Azure AD Connect)Azure AD Cloud (using password hash)
Cloud Storage of PasswordNo password stored in the cloudPassword hashes are stored in the cloud
Need for On-Prem ServerYes (PTA agents required)No (only requires Azure AD Connect)
Backup AuthenticationWorks as long as the on-prem server is availableWorks even if the on-prem server is down (as passwords are cached in Azure AD)
User Experience (SSO)Yes, SSO for cloud appsYes, SSO for cloud apps
Best ForOrganizations with strict password control or those with existing on-prem systemsSimpler for cloud-first environments or those preferring a cloud-based approach

✅ Benefits of PTA:

  • No need for password synchronization.

  • Better control over on-premises credentials (since they never leave your on-prem AD).

  • Single sign-on (SSO) experience for users, without needing to manage separate credentials for cloud services.

  • Allows for hybrid authentication for organizations still dependent on on-prem AD.

🚨 Considerations:

  • Dependency on on-prem server availability: If your PTA agent or on-prem AD is down, users can't authenticate to cloud apps.

  • Requires more infrastructure (agents) compared to Password Hash Sync.

  • Not as resilient as Password Hash Sync (in case of on-prem failures).

🧪 Real-Life Scenario:

A company with a highly secure on-premises Active Directory setup prefers to keep password validation on-prem for added security and compliance, but also wants its users to access cloud-based Microsoft 365 apps without needing separate credentials. They set up Pass-through Authentication.

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