What is a Primary Refresh Token?
A Primary Refresh Token (PRT) is a key artifact of Azure AD authentication on Windows 10, iOS, and Android devices. It is a JSON Web Token (JWT) specially issued to Microsoft first party token brokers to enable single sign-on (SSO) across the applications used on those devices. In this article, we will provide details on how a PRT is issued, used, and protected on Windows 10 devices.
This article assumes that you already understand the different device states available in Azure AD and how single sign-on works in Windows 10. For more information about devices in Azure AD, see the article What is device management in Azure Active Directory?
Key terminology and components
The following Windows components play a key role in requesting and using a PRT:
- Cloud Authentication Provider (CloudAP): CloudAP is the modern authentication provider for Windows sign in, that verifies users logging to a Windows 10 device. CloudAP provides a plugin framework that identity providers can build on to enable authentication to Windows using that identity provider’s credentials.
- Web Account Manager (WAM): WAM is the default token broker on Windows 10 devices. WAM also provides a plugin framework that identity providers can build on and enable SSO to their applications relying on that identity provider.
- Azure AD CloudAP plugin: An Azure AD specific plugin built on the CloudAP framework, that verifies user credentials with Azure AD during Windows sign in.
- Azure AD WAM plugin: An Azure AD specific plugin built on the WAM framework, that enables SSO to applications that rely on Azure AD for authentication.
- Dsreg: An Azure AD specific component on Windows 10, that handles the device registration process for all device states.
- Trusted Platform Module (TPM): A TPM is a hardware component built into a device, that provides hardware-based security functions for user and device secrets. More details can be found in the article Trusted Platform Module Technology Overview.
What does the PRT contain?
A PRT contains claims generally contained in any Azure AD refresh token. In addition, there are some device-specific claims included in the PRT. They are as follows:
- Device ID: A PRT is issued to a user on a specific device. The device ID claim
deviceIDdetermines the device the PRT was issued to the user on. This claim is later issued to tokens obtained via the PRT. The device ID claim is used to determine authorization for Conditional Access based on device state or compliance.
- Session key: The session key is an encrypted symmetric key, generated by the Azure AD authentication service, issued as part of the PRT. The session key acts as the proof of possession when a PRT is used to obtain tokens for other applications.
Can I see what’s in a PRT?
A PRT is an opaque blob sent from Azure AD whose contents are not known to any client components. You cannot see what’s inside a PRT.
How is a PRT issued?
Device registration is a prerequisite for device based authentication in Azure AD. A PRT is issued to users only on registered devices. For more in-depth details on device registration, see the article Windows Hello for Business and Device Registration. During device registration, the dsreg component generates two sets of cryptographic key pairs:
- Device key (dkpub/dkpriv)
- Transport key (tkpub/tkpriv)
The private keys are bound to the device’s TPM if the device has a valid and functioning TPM, while the public keys are sent to Azure AD during the device registration process. These keys are used to validate the device state during PRT requests.
The PRT is issued during user authentication on a Windows 10 device in two scenarios:
- Azure AD joined or Hybrid Azure AD joined: A PRT is issued during Windows logon when a user signs in with their organization credentials. A PRT is issued with all Windows 10 supported credentials, for example, password and Windows Hello for Business. In this scenario, Azure AD CloudAP plugin is the primary authority for the PRT.
- Azure AD registered device: A PRT is issued when a user adds a secondary work account to their Windows 10 device. Users can add an account to Windows 10 in two different ways -
- Adding an account via the Use this account everywhere on this device prompt after signing in to an app (for example, Outlook)
- Adding an account from Settings > Accounts > Access Work or School > Connect
In Azure AD registered device scenarios, the Azure AD WAM plugin is the primary authority for the PRT since Windows logon is not happening with this Azure AD account.
3rd party identity providers need to support the WS-Trust protocol to enable PRT issuance on Windows 10 devices. Without WS-Trust, PRT cannot be issued to users on Hybrid Azure AD joined or Azure AD joined devices
What is the lifetime of a PRT?
Once issued, a PRT is valid for 14 days and is continuously renewed as long as the user actively uses the device.
How is a PRT used?
A PRT is used by two key components in Windows:
- Azure AD CloudAP plugin: During Windows sign in, the Azure AD CloudAP plugin requests a PRT from Azure AD using the credentials provided by the user. It also caches the PRT to enable cached sign in when the user does not have access to an internet connection.
- Azure AD WAM plugin: When users try to access applications, the Azure AD WAM plugin uses the PRT to enable SSO on Windows 10. Azure AD WAM plugin uses the PRT to request refresh and access tokens for applications that rely on WAM for token requests. It also enables SSO on browsers by injecting the PRT into browser requests. Browser SSO in Windows 10 is supported on Microsoft Edge (natively) and Chrome (via the Windows 10 Accounts or Office Online extension).
How is a PRT renewed?
A PRT is renewed in two different methods:
- Azure AD CloudAP plugin every 4 hours: The CloudAP plugin renews the PRT every 4 hours during Windows sign in. If the user does not have internet connection during that time, CloudAP plugin will renew the PRT after the device is connected to the internet.
- Azure AD WAM plugin during app token requests: The WAM plugin enables SSO on Windows 10 devices by enabling silent token requests for applications. The WAM plugin can renew the PRT during these token requests in two different ways:
- An app requests WAM for an access token silently but there’s no refresh token available for that app. In this case, WAM uses the PRT to request a token for the app and gets back a new PRT in the response.
- An app requests WAM for an access token but the PRT is invalid or Azure AD requires additional authorization (for example, Azure Multi-Factor Authentication). In this scenario, WAM initiates an interactive logon requiring the user to reauthenticate or provide additional verification and a new PRT is issued on successful authentication.
- A PRT is only issued and renewed during native app authentication. A PRT is not renewed or issued during a browser session.
- In Azure AD joined and hybrid Azure AD joined devices, the CloudAP plugin is the primary authority for a PRT. If a PRT is renewed during a WAM-based token request, the PRT is sent back to CloudAP plugin, which verifies the validity of the PRT with Azure AD before accepting it.
How is the PRT protected?
A PRT is protected by binding it to the device the user has signed in to. Azure AD and Windows 10 enable PRT protection through the following methods:
- During first sign in: During first sign in, a PRT is issued by signing requests using the device key cryptographically generated during device registration. On a device with a valid and functioning TPM, the device key is secured by the TPM preventing any malicious access. A PRT is not issued if the corresponding device key signature cannot be validated.
- During token requests and renewal: When a PRT is issued, Azure AD also issues an encrypted session key to the device. It is encrypted with the public transport key (tkpub) generated and sent to Azure AD as part of device registration. This session key can only be decrypted by the private transport key (tkpriv) secured by the TPM. The session key is the Proof-of-Possession (POP) key for any requests sent to Azure AD. The session key is also protected by the TPM and no other OS component can access it. Token requests or PRT renewal requests are securely signed by this session key through the TPM and hence, cannot be tampered with. Azure AD will invalidate any requests from the device that are not signed by the corresponding session key.
By securing these keys with the TPM, malicious actors cannot steal the keys nor replay the PRT elsewhere as the TPM is inaccessible even if an attacker has physical possession of the device. Thus, using a TPM greatly enhances the security of Azure AD Joined, Hybrid Azure AD joined, and Azure AD registered devices against credential theft. For performance and reliability, TPM 2.0 is the recommended version for all Azure AD device registration scenarios on Windows 10.
How are app tokens and browser cookies protected?
App tokens: When an app requests token through WAM, Azure AD issues a refresh token and an access token. However, WAM only returns the access token to the app and secures the refresh token in its cache by encrypting it with the user’s data protection application programming interface (DPAPI) key. WAM securely uses the refresh token by signing requests with the session key to issue further access tokens. The DPAPI key is secured by an Azure AD based symmetric key in Azure AD itself. When the device needs to decrypt the user profile with the DPAPI key, Azure AD provides the DPAPI key encrypted by the session key, which CloudAP plugin requests TPM to decrypt. This functionality ensures consistency in securing refresh tokens and avoids applications implementing their own protection mechanisms.
Browser cookies: In Windows 10, Azure AD supports browser SSO in Internet Explorer and Microsoft Edge natively or in Google Chrome via the Windows 10 accounts extension. The security is built not only to protect the cookies but also the endpoints to which the cookies are sent. Browser cookies are protected the same way a PRT is, by utilizing the session key to sign and protect the cookies.
When a user initiates a browser interaction, the browser (or extension) invokes a COM native client host. The native client host ensures that the page is from one of the allowed domains. The browser could send other parameters to the native client host, including a nonce, however the native client host guarantees validation of the hostname. The native client host requests a PRT-cookie from CloudAP plugin, which creates and signs it with the TPM-protected session key. As the PRT-cookie is signed by the session key, it cannot be tampered with. This PRT-cookie is included in the request header for Azure AD to validate the device it is originating from. If using the Chrome browser, only the extension explicitly defined in the native client host’s manifest can invoke it preventing arbitrary extensions from making these requests. Once Azure AD validates the PRT cookie, it issues a session cookie to the browser. This session cookie also contains the same session key issued with a PRT. During subsequent requests, the session key is validated effectively binding the cookie to the device and preventing replays from elsewhere.
When does a PRT get an MFA claim?
A PRT can get a multi-factor authentication (MFA) claim in specific scenarios. When an MFA-based PRT is used to request tokens for applications, the MFA claim is transferred to those app tokens. This functionality provides a seamless experience to users by preventing MFA challenge for every app that requires it. A PRT can get an MFA claim in the following ways:
- Sign in with Windows Hello for Business: Windows Hello for Business replaces passwords and uses cryptographic keys to provide strong two-factor authentication. Windows Hello for Business is specific to a user on a device, and itself requires MFA to provision. When a user logs in with Windows Hello for Business, the user’s PRT gets an MFA claim. This scenario also applies to users logging in with smartcards if smartcard authentication produces an MFA claim from ADFS.
- As Windows Hello for Business is considered multi-factor authentication, the MFA claim is updated when the PRT itself is refreshed, so the MFA duration will continually extend when users sign in with WIndows Hello for Business
- MFA during WAM interactive sign in: During a token request through WAM, if a user is required to do MFA to access the app, the PRT that is renewed during this interaction is imprinted with an MFA claim.
- In this case, the MFA claim is not updated continuously, so the MFA duration is based on the lifetime set on the directory.
- MFA during device registration: If an admin has configured their device settings in Azure AD to require MFA to register devices, the user needs to do MFA to complete the registration. During this process, the PRT that is issued to the user has the MFA claim obtained during the registration. This capability only applies to the user who did the join operation, not to other users who sign in to that device.
- Similar to the WAM interactive sign in, the MFA claim is not updated continuously, so the MFA duration is based on the lifetime set on the directory.
Windows 10 maintains a partitioned list of PRTs for each credential. So, there’s a PRT for each of Windows Hello for Business, password, or smartcard. This partitioning ensures that MFA claims are isolated based on the credential used, and not mixed up during token requests.
How is a PRT invalidated?
A PRT is invalidated in the following scenarios:
- Invalid user: If a user is deleted or disabled in Azure AD, their PRT is invalidated and cannot be used to obtain tokens for applications. If a deleted or disabled user already signed in to a device before, cached sign-in would log them in, until CloudAP is aware of their invalid state. Once CloudAP determines that the user is invalid, it blocks subsequent logons. An invalid user is automatically blocked from sign in to new devices that don’t have their credentials cached.
- Invalid device: If a device is deleted or disabled in Azure AD, the PRT obtained on that device is invalidated and cannot be used to obtain tokens for other applications. If a user is already signed in to an invalid device, they can continue to do so. But all tokens on the device are invalidated and the user does not have SSO to any resources from that device.
- Password change: After a user changes their password, the PRT obtained with the previous password is invalidated by Azure AD. Password change results in the user getting a new PRT. This invalidation can happen in two different ways:
- If user signs in to Windows with their new password, CloudAP discards the old PRT and requests Azure AD to issue a new PRT with their new password. If user does not have an internet connection, the new password cannot be validated, Windows may require the user to enter their old password.
- If a user has logged in with their old password or changed their password after signing into Windows, the old PRT is used for any WAM-based token requests. In this scenario, the user is prompted to reauthenticate during the WAM token request and a new PRT is issued.
- TPM issues: Sometimes, a device’s TPM can falter or fail, leading to inaccessibility of keys secured by the TPM. In this case, the device is incapable of getting a PRT or requesting tokens using an existing PRT as it cannot prove possession of the cryptographic keys. As a result, any existing PRT is invalidated by Azure AD. When Windows 10 detects a failure, it initiates a recovery flow to re-register the device with new cryptographic keys. With Hybrid Azure Ad join, just like the initial registration, the recovery happens silently without user input. For Azure AD joined or Azure AD registered devices, the recovery needs to be performed by a user who has administrator privileges on the device. In this scenario, the recovery flow is initiated by a Windows prompt that guides the user to successfully recover the device.
The following diagrams illustrate the underlying details in issuing, renewing, and using a PRT to request an access token for an application. In addition, these steps also describe how the aforementioned security mechanisms are applied during these interactions.
PRT issuance during first sign in
In Azure AD joined devices, this exchange happens synchronously to issue a PRT before the user can logon to Windows. In hybrid Azure AD joined devices, on-premises Active Directory is the primary authority. So, the user is only waiting until they can acquire a TGT to login, while the PRT issuance happens asynchronously. This scenario does not apply to Azure AD registered devices as logon does not use Azure AD credentials.
|A||User enters their password in the sign in UI. LogonUI passes the credentials in an auth buffer to LSA, which in turns passes it internally to CloudAP. CloudAP forwards this request to the CloudAP plugin.|
|B||CloudAP plugin initiates a realm discovery request to identify the identity provider for the user. If user’s tenant has a federation provider setup, Azure AD returns the federation provider’s Metadata Exchange endpoint (MEX) endpoint. If not, Azure AD returns that the user is managed indicating that user can authenticate with Azure AD.|
|C||If the user is managed, CloudAP will get the nonce from Azure AD. If the user is federated, CloudAP plugin requests a SAML token from the federation provider with the user’s credentials. Once it receives, the SAML token, it requests a nonce from Azure AD.|
|D||CloudAP plugin constructs the authentication request with the user’s credentials, nonce, and a broker scope, signs the request with the Device key (dkpriv) and sends it to Azure AD. In a federated environment, CloudAP plugin uses the SAML token returned by the federation provider instead of the user’ credentials.|
|E||Azure AD validates the user credentials, the nonce, and device signature, verifies that the device is valid in the tenant and issues the encrypted PRT. Along with the PRT, Azure AD also issues a symmetric key, called the Session key encrypted by Azure AD using the Transport key (tkpub). In addition, the Session key is also embedded in the PRT. This Session key acts as the Proof-of-possession (PoP) key for subsequent requests with the PRT.|
|F||CloudAP plugin passes the encrypted PRT and Session key to CloudAP. CloudAP request the TPM to decrypt the Session key using the Transport key (tkpriv) and re-encrypt it using the TPM’s own key. CloudAP stores the encrypted Session key in its cache along with the PRT.|
PRT renewal in subsequent logons
|A||User enters their password in the sign in UI. LogonUI passes the credentials in an auth buffer to LSA, which in turns passes it internally to CloudAP. CloudAP forwards this request to the CloudAP plugin.|
|B||If the user has previously logged on to the user, Windows initiates cached sign in and validates credentials to log the user in. Every 4 hours, the CloudAP plugin initiates PRT renewal asynchronously.|
|C||CloudAP plugin initiates a realm discovery request to identify the identity provider for the user. If user’s tenant has a federation provider setup, Azure AD returns the federation provider’s Metadata Exchange endpoint (MEX) endpoint. If not, Azure AD returns that the user is managed indicating that user can authenticate with Azure AD.|
|D||If the user is federated, CloudAP plugin requests a SAML token from the federation provider with the user’s credentials. Once it receives, the SAML token, it requests a nonce from Azure AD. If the user is managed, CloudAP will directly get the nonce from Azure AD.|
|E||CloudAP plugin constructs the authentication request with the user’s credentials, nonce, and the existing PRT, signs the request with the Session key and sends it to Azure AD. In a federated environment, CloudAP plugin uses the SAML token returned by the federation provider instead of the user’ credentials.|
|F||Azure AD validates the Session key signature by comparing it against the Session key embedded in the PRT, validates the nonce and verifies that the device is valid in the tenant and issues a new PRT. As seen before, the PRT is again accompanied with the Session key encrypted by Transport key (tkpub).|
|G||CloudAP plugin passes the encrypted PRT and Session key to CloudAP. CloudAP requests the TPM to decrypt the Session key using the Transport key (tkpriv) and re-encrypt it using the TPM’s own key. CloudAP stores the encrypted Session key in its cache along with the PRT.|
PRT usage during app token requests
|A||An application (for example, Outlook, OneNote etc.) initiates a token request to WAM. WAM, in turn, asks the Azure AD WAM plugin to service the token request.|
|B||If a Refresh token for the application is already available, Azure AD WAM plugin uses it to request an access token. To provide proof of device binding, WAM plugin signs the request with the Session key. Azure AD validates the Session key and issues an access token and a new refresh token for the app, encrypted by the Session key. WAM plugin requests Cloud AP plugin to decrypt the tokens, which, in turn, requests the TPM to decrypt using the Session key, resulting in WAM plugin getting both the tokens. Next, WAM plugin provides only the access token to the application, while it re-encrypts the refresh token with DPAPI and stores it in its own cache|
|C||If a Refresh token for the application is not available, Azure AD WAM plugin uses the PRT to request an access token. To provide proof of possession, WAM plugin signs the request containing the PRT with the Session key. Azure AD validates the Session key signature by comparing it against the Session key embedded in the PRT, verifies that the device is valid and issues an access token and a refresh token for the application. in addition, Azure AD can issue a new PRT (based on refresh cycle), all of them encrypted by the Session key.|
|D||WAM plugin requests Cloud AP plugin to decrypt the tokens, which, in turn, requests the TPM to decrypt using the Session key, resulting in WAM plugin getting both the tokens. Next, WAM plugin provides only the access token to the application, while it re-encrypts the refresh token with DPAPI and stores it in its own cache. WAM plugin will use the refresh token going forward for this application. WAM plugin also gives back the new PRT to Cloud AP plugin, which validates the PRT with Azure AD before updating it in its own cache. Cloud AP plugin will use the new PRT going forward.|
|E||WAM provides the newly issued access token to WAM, which in turn, provides it back to the calling application|
Browser SSO using PRT
|A||User logs in to Windows with their credentials to get a PRT. Once user opens the browser, browser (or extension) loads the URLs from the registry.|
|B||When a user opens an Azure AD login URL, the browser or extension validates the URL with the ones obtained from the registry. If they match, the browser invokes the native client host for getting a token.|
|C||The native client host validates that the URLs belong to the Microsoft identity providers (Microsoft account or Azure AD), extracts a nonce sent from the URL and makes a call to CloudAP plugin to get a PRT cookie.|
|D||The CloudAP plugin will create the PRT cookie, sign in with the TPM-bound session key and send it back to the native client host. As the cookie is signed by the session key, it cannot be tampered with.|
|E||The native client host will return this PRT cookie to the browser, which will include it as part of the request header called x-ms-RefreshTokenCredential and request tokens from Azure AD.|
|F||Azure AD validates the Session key signature on the PRT cookie, validates the nonce, verifies that the device is valid in the tenant, and issues an ID token for the web page and an encrypted session cookie for the browser.|
For more information on troubleshooting PRT-related issues, see the article Troubleshooting hybrid Azure Active Directory joined Windows 10 and Windows Server 2016 devices.