This page is part of the Smart App Launch Implementation Guide (v2.2.0-ballot: STU 2.2 Ballot 1) based on FHIR (HL7® FHIR® Standard) R4. The current version which supersedes this version is 2.1.0. For a full list of available versions, see the Directory of published versions

Backend Services

Profile Audience and Scope

This profile is intended to be used by developers of backend services (clients) that autonomously (or semi-autonomously) need to access resources from FHIR servers that have pre-authorized defined scopes of access. This specification handles use cases complementary to the SMART App Launch protocol. Specifically, this profile describes the runtime process by which the client acquires an access token that can be used to retrieve FHIR resources. This specification is designed to work with FHIR Bulk Data Access, but is not restricted to use for retrieving bulk data; it may be used to connect to any FHIR API endpoint, including both synchronous and asynchronous access.

Use this profile when the following conditions all apply:

• The target FHIR authorization server can register the client and pre-authorize access to a defined set of FHIR resources.
• The client may run autonomously, or with user interaction that does not include access authorization.
• The client supports client-confidential-asymmetric authentication
• No compelling need exists for a user to authorize the access at runtime.

Note that the FHIR specification includes a set of security considerations including security, privacy, and access control. These considerations apply to diverse use cases and provide general guidance for choosing among security specifications for particular use cases.

Examples

• An analytics platform or data warehouse that periodically performs a bulk data import from an electronic health record system for analysis of a population of patients.

• A lab monitoring service that determines which patients are currently admitted to the hospital, reviews incoming laboratory results, and generates clinical alerts when specific trigger conditions are met. Note that in this example, the monitoring service may be a backend client to multiple servers.

• A data integration service that periodically queries the EHR for newly registered patients and synchronizes these with an external database

• A utilization tracking system that queries an EHR every minute for bed and room usage and displays statistics on a wall monitor.

• Public health surveillance studies that do not require real-time exchange of data.

Underlying Standards

See section References.

Top-level steps for Backend Services Authorization

1. Register Backend Service (one-time step, can be out-of-band)
2. Retrieve .well-known/smart-configuration
3. Obtain access token
4. Access FHIR API

Register SMART Backend Service (communicating public keys)

Before a SMART client can run against a FHIR server, the client SHALL register with the server by following the registration steps described in client-confidential-asymmetric authentication.

Retrieve .well-known/smart-configuration

In order to request authorization to access FHIR resources, the app discovers the EHR FHIR server’s SMART configuration metadata, including OAuth token endpoint URL.

Request

The app issues an HTTP GET with an Accept header supporting application/json to retrieve the SMART configuration file.

Response

Servers respond with a discovery response that meets discovery requirements described in client-confidential-asymmetric authentication.

Example Request and Response

For a full example, see example request and response.

Obtain access token

By the time a client has been registered with the FHIR authorization server, the key elements of organizational trust will have been established. That is, the client will be considered “pre-authorized” to access FHIR resources. Then, at runtime, the client will need to obtain an access token in order to retrieve FHIR resources as pre-authorized. Such access tokens are issued by the FHIR authorization server, in accordance with the OAuth 2.0 Authorization Framework, RFC6749.

Because the authorization scope is limited to protected resources previously arranged with the FHIR authorization server, the client credentials grant flow, as defined in Section 4.4 of RFC6749, may be used to request authorization. Use of the client credentials grant type requires that the client SHALL be a “confidential” client capable of protecting its authentication credential.

This specification describes requirements for requesting an access token through the use of an OAuth 2.0 client credentials flow, with a JWT assertion as the client’s authentication mechanism. The exchange, as depicted above, allows the client to authenticate itself to the FHIR authorization server and to request a short-lived access token in a single exchange.

Request

To begin the exchange, the client SHALL use the Transport Layer Security (TLS) Protocol Version 1.2 (RFC5246) or a more recent version of TLS to authenticate the identity of the FHIR authorization server and to establish an encrypted, integrity-protected link for securing all exchanges between the client and the FHIR authorization server’s token endpoint. All exchanges described herein between the client and the FHIR server SHALL be secured using TLS V1.2 or a more recent version of TLS .

Before a client can request an access token, it generates a one-time-use authentication JWT as described in client-confidential-symmetric authentication. After generating this authentication JWT, the client requests an access token via HTTP POST to the FHIR authorization server’s token endpoint URL, using content-type application/x-www-form-urlencoded with the following parameters:

Scopes

The client is pre-authorized by the server. In other words, by the time a client initiates an access token request, the server has already associated the client with the authority to access certain data. The client then includes a set of scopes in the access token request, which causes the server to apply additional access restrictions following the SMART Scopes syntax. For Backend Services, requested scopes will be system/ scopes (for example system/Observation.rs, which requests an access token capable of reading all Observations that the client has been pre-authorized to access). The use of Backend Services with user/ and patient/ scopes is not prohibited, but would required out-of-band coordination to establish context (e.g., to establish which user or patient applies).

Response

Enforce Authorization

There are several cases where a client might ask for data that the server cannot or will not return:

• Client explicitly asks for data that it is not authorized to see (e.g., a client asks for Observation resources but has scopes that only permit access to Patient resources). In this case a server SHOULD respond with a failure to the initial request.
• Client explicitly asks for data that the server does not support (e.g., a client asks for Practitioner resources but the server does not support FHIR access to Practitioner data). In this case a server SHOULD respond with a failure to the initial request.
• Client explicitly asks for data that the server supports and that appears consistent with its access scopes – but some additional out-of-band rules/policies/restrictions prevents the client from being authorized to see these data. In this case, the server MAY withhold certain results from the response, and MAY indicate to the client that results were withheld by including OperationOutcome information in the “error” array for the response as a partial success.

Rules regarding circumstances under which a client is required to obtain and present an access token along with a request are based on risk-management decisions that each FHIR resource service needs to make, considering the workflows involved, perceived risks, and the organization’s risk-management policies. Refresh tokens SHOULD NOT be issued.

Validate Authentication JWS

The FHIR authorization server validates a client’s authentication JWT according to the client-confidential-asymmetric authentication profile. See JWT validation rules.

Evaluate Requested Access

Once the client has been authenticated, the FHIR authorization server SHALL mediate the request to assure that the scope requested is within the scope pre-authorized to the client.

Issue Access Token

If an error is encountered during the authorization process, the FHIR authorization server SHALL respond with the appropriate error message defined in Section 5.2 of the OAuth 2.0 specification. The FHIR authorization server SHOULD include an error_uri or error_description as defined in OAuth 2.0.

If the access token request is valid and authorized, the FHIR authorization server SHALL issue an access token in response. The access token response SHALL be a JSON object with the following properties:

To minimize risks associated with token redirection, the scope of each access token SHOULD encompass, and be limited to, the resources requested. Access tokens issued under this profile SHALL be short-lived; the expires_in value SHOULD NOT exceed 300, which represents an expiration-time of five minutes. To establish longer-term access, clients can request new access tokens as needed.

Example Token Request and Response

For a full example, see example token request and response.

Access FHIR API

With a valid access token, the app can access protected FHIR data by issuing a FHIR API call to the FHIR endpoint on the FHIR resource server.

Request

From the access token response, an app has received an OAuth2 bearer-type access token (access_token property) that can be used to fetch clinical data. The app issues a request that includes an Authorization header that presents the access_token as a “Bearer” token:

Authorization: Bearer {{access_token}}

(Note that in a real request, {{access_token}} is replaced with the actual token value.)

Response

The resource server SHALL validate the access token and ensure that it has not expired and that its scope covers the requested resource. The method used by the EHR to validate the access token is beyond the scope of this specification but generally involves an interaction or coordination between the EHR’s resource server and the authorization server.

On occasion, an Backend Service may receive a FHIR resource that contains a “reference” to a resource hosted on a different resource server. The Backend Service SHOULD NOT blindly follow such references and send along its access_token, as the token may be subject to potential theft. The Backend Service SHOULD either ignore the reference, or initiate a new request for access to that resource.

Example Request and Response

For a full example, see example FHIR API request and response.