Best Practices in Authorization for SMART on FHIR EHRs

This page catalog best practices in developing secure SMART on FHIR EHR implementations. As such, these considerations don’t directly affect interoperability; rather, they describe pracical implications of security decisions. This page is a work in progress; we anticipate describing details such as the entropy required in generating access tokens.

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1. Best Practices for Organizational Policy

1.1. Transport Security

1.1.1 All transmissions that involve the exchange of sensitive information (e.g., end-user credential, client credential, authorization code, access token, refresh token, FHIR resource) are required to be conducted over links that have been secured using Transport Layer Security (TLS). For maximal security and interoperability, the latest, widely deployed version of TLS, configured with cipher suites recommended by NIST FIPS SP 140-2, Annex A, should be used. At this time, this version is RFC5246, The Transport Layer Security (TLS) Protocol, Version 1.2. The server involved in the exchange MUST authenticate its own identity to the client and set up the secure channel. Depending upon organizational policy, authentication of the client may also be required.
Although mutual TLS is an option, it may not be the most practical solution, given the lack of client libraries and the fact that a load balancer placed in front of a resource server (a common practice) will terminate the TLS link.
OAuth 2.0 Security: Going Beyond Bearer Tokens suggests that in such cases, application-level encryption may be the best approach.

2. Best Practices for Authorization Servers

2.1 OAuth Grant Models

2.1.1 RFC 6749, The OAuth 2.0 Authorization Framework, defines four types of “authorization grants” that a client can exchange for an access token. The Authorization Code Grant (section 4.1 of RFC6749) provides the distinct advantages of 1) enabling the end-user to authenticate directly to the authorization server, avoiding having to share credentials with the client; and 2) enabling the authorization server to transmit the access token directly to the client, rather than exposing it to the user-agent (e.g., browser). The authorization code grant model supports both access tokens and refresh tokens, and is the preferred model for authorizing an external client to access FHIR resources.

2.1.2 If the requesting client is a registered partner organization with whom the FHIR resource-holder has an agreement to share resources, then the Client Credentials Grants model (section 4.4 of RFC6749) may be used. In such case, the use of signed JSON Web Tokens for transmitting the authorization request and authentication information, as described in RFC7523 is recommended.

2.1.3 To discourage online guessing of authorization codes, authorization servers should limit the number of times, within an established time period, a client may submit an invalid code to exchange for an access token.

2.2 End-User Authorization

2.2.1 Whether the authorization server needs to ask for user authorization before authorizing access to a resource is determined by the security policy of the organization holding the resource. When the authorization server requests user authorization, the end user should be provided information important in making this decision, such as the name of the client, the resources for which access is requested, the scope of access requested, and the period of time for which the access may be authorized. Authorization servers may provide end-users with the ability to accept or reject individual scopes of access, resulting in an approval that carries fewer scopes than the client requested.

2.3 Refresh Tokens

2.3.1 The use of refresh tokens eliminates the need for the authorization server to issue an access token with a long lifetime, thus reducing the risk of undesired access and use. Instead of issuing a single, long-term access token, issuing a long-term refresh token along with a short-term access token is recommended. However, because a refresh token provides extended access to a resource, they may be issued only through the use of the Authorization Code Grant model. It is the authorization server’s responsibility to ensure that refresh tokens cannot be generated, modified, or guessed by an interloper, and to assure that refresh tokens are protected in transit and during storage.

2.3.2 RFC6749 requires that the authorization server maintain the binding between a refresh token and the client to whom it was issued, and that the authorization server verify that binding when the refresh token is presented. Because only confidential clients are capable of authenticating their own identity, it is recommended that refresh tokens be issued only to confidential clients. If the authorization server’s policy allows issuance of refresh tokens to public clients, then the authorization server should use some other means of protecting against misuse, such as issuing a new refresh token with every access-token refresh response and retaining the used refresh token. If a refresh token is compromised and subsequently used by both the attacker and the legitimate client, one of them will present an invalidated refresh token, which will inform the authorization server of the breach.

2.3.3 The SMART on FHIR Scopes and Launch Contexts define two ways of asking for refresh tokens: requesting a refresh token using the online_access scope results in a refresh token that remains valid only while the end-user remains online; whereas requesting a refresh token using the offline_access scope results in a refresh token that remains valid even after the end-user is no longer online. In order to enforce online_access, the authorization server needs to implement some means of determining whether the user is still online. For example, the server may implement user sessions with automatic timeouts and automated session extension whenever the user shows signs of activity. Because of the increased risk presented by long-term, offline_access, the default for refresh tokens should be online_access. In addition, some means should be provided for a user to revoke offline_access (such as a permissions management web page or API).

2.3.4 If a user device known to have held a refresh token for an app is stolen, an authorization server should revoke access by refusing to refresh when a refresh token for that user is presented.

2.4 Token Lifetimes

2.4.1 Authorization servers should issue access tokens with lifetimes as short as is practical and reasonable, based on risk. The lifetimes of access tokens should be shorter than those for refresh tokens (e.g., 1 hour vs. 1 year). The lifetimes for access tokens issued to confidential clients may be longer than those issued to public clients.

2.4 Cross-Site Request Forgery (CSRF)

2.4.1 Cross-site request forgery (CSRF) is a category of attacks that trick the victim into submitting a malicious request. In a CSRF attack, a malicious application (or web site) runs within the same browser as an active session to which the end-user has authenticated, and this application tricks the end-user into submitting unauthorized HTTP requests to the site with which the victim has the active, authenticated session. This request allows the attacker to exploit the victim’s authorizations to perform actions on the target site. For example, a CSRF attack against a client’s redirection URI might cause the client to mistakenly obtain an access token using an attacker-supplied authorization code. RFC6749 requires that clients implement CSRF protection for its redirection URI. To fulfill this requirement, the SMART on FHIR Authorization Guide requires that each app generate an unpredictable (at least 128 bits of entropy) state parameter for each user session, and that the app validate the state value for any request sent to its redirect URL; include the state parameter with all requests sent to the authorization server; and validate the state value included in access tokens it receives. In addition, the authorization server should validate state parameters it receives from clients.

2.5 Access Token Phishing by Counterfeit Resource Servers

2.5.1 To prevent leakage of a genuine bearer token to a counterfeit resource server, the SMART on FHIR Authorization Guide requires that authorization requests include an aud parameter whose value is the URL of the FHIR resource server from which the app wishes to retrieve FHIR data. Authorization servers must validate that the aud parameter is the URL of a known and trusted resource server prior to returning an authorization code to the requester.

2.5.2 RFC6750 describes this threat more broadly as “token redirect” – when “an attacker uses a token generated for consumption by one resource server to gain access to a different resource server that mistakenly believes the token to be for it.” To deal with token redirect, it is important for the authorization server to identify the intended recipient (or recipients) of the access token, typically a single RS (or a list of RSs), in the token. This may be done through use of the aud parameter or by some other means devised by the authorization server, in coordination with its RSs. Then, upon receipt of an access token, the RS needs to check to assure that the access token it has received is intended to be used by that RS.

3.0 Best Practices for FHIR Resource Servers

4.0 Best Practices for End Users

4.1 Token Protection

4.1.1 Sometimes apps obtain tokens that enable them to access EHR and other sensitive information. While most tokens are effective for only a limited period of time, other tokens remain on a device for a longer period of time. To avoid misuse of the access privileges these tokens represent, it is important for users to lock device screens, shut down browsers, or power down devices when not in use.

4.2 Cross-Site Request Forgery (CSRF) Protection

4.2.1 For convenience, users often keep multiple web sites and apps open on their browser device simultaneously. Some sites and apps may not be as friendly as others and may try to perform actions and access data in unwanted ways. They may accomplish these devious actions through an attack called cross-site request forgery (CSRF). To help protect health information against CSRF attacks, after using an app to access sensitive health data, users should log off the app (or shut down the browser) before visiting another site, and clear the browser’s cookies at the end of each browser session.

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