fapi-2-baseline June 2022
Fett Standards Track [Page]
Intended Status:
Standards Track
D. Fett

FAPI 2.0 Baseline Profile


The OpenID Foundation (OIDF) promotes, protects and nurtures the OpenID community and technologies. As a non-profit international standardizing body, it is comprised by over 160 participating entities (workgroup participant). The work of preparing implementer drafts and final international standards is carried out through OIDF workgroups in accordance with the OpenID Process. Participants interested in a subject for which a workgroup has been established have the right to be represented in that workgroup. International organizations, governmental and non-governmental, in liaison with OIDF, also take part in the work. OIDF collaborates closely with other standardizing bodies in the related fields.

Final drafts adopted by the Workgroup through consensus are circulated publicly for the public review for 60 days and for the OIDF members for voting. Publication as an OIDF Standard requires approval by at least 50% of the members casting a vote. There is a possibility that some of the elements of this document may be the subject to patent rights. OIDF shall not be held responsible for identifying any or all such patent rights.

The Financial-grade API (FAPI) 2.0 Baseline profile is an API security profile based on the OAuth 2.0 Authorization Framework [RFC6749].


Financial-grade API (FAPI) 2.0 is an API security profile based on the OAuth 2.0 Authorization Framework [RFC6749] and related specifications suitable for protecting APIs in high-value scenarios. While the security profile was initially developed with a focus on financial applications, it is designed to be universally applicable for protecting APIs exposing high-value and sensitive (personal and other) data, for example, in e-health and e-government applications.


This document is not an OIDF International Standard. It is distributed for review and comment. It is subject to change without notice and may not be referred to as an International Standard.

Recipients of this draft are invited to submit, with their comments, notification of any relevant patent rights of which they are aware and to provide supporting documentation.

Notational Conventions

The keywords "shall", "shall not", "should", "should not", "may", and "can" in this document are to be interpreted as described in ISO Directive Part 2 [ISODIR2]. These keywords are not used as dictionary terms such that any occurrence of them shall be interpreted as keywords and are not to be interpreted with their natural language meanings.

Table of Contents

1. Scope

This document specifies the requirements for confidential Clients to securely obtain OAuth tokens from Authorization Servers and securely use those tokens to access REST APIs at Resource Servers.

2. Terms and definitions

For the purpose of this document, the terms defined in [RFC6749], [RFC6750], [RFC7636], [OIDC] and ISO29100 apply.

3. Symbols and Abbreviated terms

API - Application Programming Interface

FAPI - Financial-grade API

HTTP - Hyper Text Transfer Protocol

REST - Representational State Transfer

TLS - Transport Layer Security

DNS - Domain Name System

DNSSEC - Domain Name System Security Extensions

CAA - Certificate Authority Authorization

URI - Uniform Resource Identifier

4. Baseline Profile

4.1. Introduction

OIDF FAPI is an API security profile based on the OAuth 2.0 Authorization Framework [RFC6749]. This Baseline Profile aims to reach the security goals laid out in the Attacker Model [attackermodel].

We are not currently aware of any mechanisms that would allow public clients to be secured to the same degree and hence their use is not within the scope of this specification.

4.2. Network Layer Protections

4.2.1. Requirements for all endpoints

TLS connections shall be protected against network attackers. To this end, clients, authorization servers, and resource servers:

  1. shall only offer TLS protected endpoints and shall establish connections to other servers using TLS. TLS connections shall be set up to use TLS version 1.2 or later.

  2. when using TLS 1.2, follow the recommendations for Secure Use of Transport Layer Security in [RFC7525].

  3. should use DNSSEC to protect against DNS spoofing attacks that can lead to the issuance of rogue domain-validated TLS certificates.

  4. shall perform a TLS server certificate check, as per [RFC6125].

NOTE: Even if an endpoint uses only organization validated (OV) or extended validation (EV) TLS certificates, rogue domain-validated certificates can be used to impersonate the endpoints and conduct man-in-the-middle attacks. CAA records [RFC8659] can help to mitigate this risk.

4.2.2. Requirements for endpoints not used by web browsers

  1. when using TLS 1.2, only the following 4 cipher suites shall be permitted:





  2. When using the TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 or TLS_DHE_RSA_WITH_AES_256_GCM_SHA384 cipher suites, key lengths of at least 2048 bits are required.

4.2.3. Requirements for endpoints user by web browsers

Endpoints for the use by web browsers

  1. shall use methods to ensure that connections cannot be downgraded using TLS Stripping attacks. A preloaded [preload] HTTP Strict Transport Security policy [RFC6797] can be used for this purpose. Some top-level domains, like .bank and .insurance, have set such a policy and therefore protect all second-level domains below them.

  2. when using TLS 1.2, shall only use cipher suites allowed in [RFC7525]

4.3. Profile

In the following, a profile of the following technologies is defined:

  • OAuth 2.0 Authorization Framework [RFC6749]

  • OAuth 2.0 Bearer Tokens [RFC6750]

  • Proof Key for Code Exchange by OAuth Public Clients (PKCE) [RFC7636]

  • OAuth 2.0 Mutual-TLS Client Authentication and Certificate-Bound Access Tokens (MTLS) [RFC8705]

  • OAuth 2.0 Demonstrating Proof-of-Possession at the Application Layer (DPoP) [I-D.ietf-oauth-dpop]

  • OAuth 2.0 Pushed Authorization Requests (PAR) [RFC9126]

  • OAuth 2.0 Authorization Server Metadata [RFC8414]

  • OAuth 2.0 Authorization Server Issuer Identification [RFC9207]

  • OpenID Connect Core 1.0 incorporating errata set 1 [OIDC]

4.3.1. Requirements for Authorization Servers General Requirements

Authorization servers

  1. shall distribute discovery metadata (such as the authorization endpoint) via the metadata document as specified in [OIDD] and [RFC8414]

  2. shall reject requests using the resource owner password credentials grant or the implicit grant described in [RFC6749] or the hybrid flow as described in [OIDC]

  3. shall support confidential clients as defined in [RFC6749]

  4. shall only issue sender-constrained access tokens using one of the following methods:

  5. shall authenticate clients using one of the following methods:

    • MTLS as specified in section 2 of [RFC8705]

    • private_key_jwt as specified in section 9 of [OIDC]

  6. shall not expose open redirectors (see section 4.10 of [I-D.ietf-oauth-security-topics])

  7. shall accept its issuer identifier value (as defined in [RFC8414]) in the aud claim received in client authentication assertions.

  8. shall not use refresh token rotation unless, in the case a response with a new refresh token is not received and stored by the client, retrying the request (with the previous refresh token) will succeed.

  9. if using DPoP, may use the server provided nonce mechanism (as defined in section 8 of [I-D.ietf-oauth-dpop]).

NOTE: In order to facilitate interoperability the authorization server should also accept its token endpoint URL or the URL of the endpoint at which the assertion was received in the aud claim received in client authentication assertions.

NOTE: Refresh token rotation is an optional feature defined in [RFC6749] section 6 where the Authorization Server issues a new refresh token to the client as part of the refresh_token grant. This specification discourages the use of this feature as it doesn't bring any security benefits for confidential clients, and can cause significant operational issues. However to allow for operational agility, Authorization Servers may implement it providing they meet the requirement in clause 20.

NOTE: Other grants as appropriate may be supported, for example the client credentials grant, the Client Initiated Backchannel Authentication grant, etc. Authorization Code Flow

For the Authorization Code flow, Authorization servers

  1. shall support the authorization code grant (response_type=code & grant_type=authorization_code) described in [RFC6749]

  2. shall support client-authenticated pushed authorization requests according to [RFC9126]

  3. shall reject authorization requests sent without [RFC9126]

  4. shall reject pushed authorization requests without client authentication

  5. shall require PKCE [RFC7636] with S256 as the code challenge method

  6. shall require the redirect_uri parameter in pushed authorization requests

  7. shall return an iss parameter in the authorization response according to [RFC9207]

  8. shall not transmit authorization responses over unencrypted network connections, and, to this end, shall not allow redirect URIs that use the "http" scheme except for native clients that use Loopback Interface Redirection as described in [RFC8252], Section 7.3,

  9. shall reject an authorization code (section 1.3.1 of [RFC6749]) if it has been previously used

  10. shall not use the HTTP 307 status code when redirecting a request that contains user credentials to avoid forwarding the credentials to a third party accidentally (see section 4.11 of [I-D.ietf-oauth-security-topics]);

  11. should use the HTTP 303 status code when redirecting the user agent using status codes;

  12. shall issue pushed authorization requests request_uri with expires_in values of between 5 and 600 seconds.

NOTE: If replay identification of the authorization code is not possible, it is desirable to set the validity period of the authorization code to one minute or a suitable short period of time. The validity period may act as a cache control indicator of when to clear the authorization code cache if one is used

NOTE: To enable an interoperable solution to consent management it is anticipated that future versions of this specification will reference the FAPI WG's Grant Management API. Returning Authenticated User's Identifier

If it is desired to provide the authenticated user's identifier to the client in the token response, the authorization server shall support OpenID Connect [OIDC].

4.3.2. Requirements for Clients General Requirements


  1. shall support sender-constrained access tokens using one of the following methods:

  2. shall support client authentication using one of the following methods:

    • MTLS as specified in section 2 of [RFC8705]

    • private_key_jwt as specified in section 9 of [OIDC]

  3. shall send access tokens in the HTTP header as in Section 2.1 of OAuth 2.0 Bearer Token Usage [RFC6750]

  4. shall not expose open redirectors (see section 4.10 of [I-D.ietf-oauth-security-topics])

  5. if using private_key_jwt, shall use the Authorization Server's issuer identifier value (as defined in [RFC8414]) in the aud claim sent in client authentication assertions. The issuer identifier value shall be sent as a string not as an item in an array.

  6. shall support refresh tokens and their rotation.

  7. if using MTLS client authentication or MTLS sender-constrained access tokens, shall support the mtls_endpoint_aliases metadata defined in [RFC8705]

  8. if using DPoP, shall support the server provided nonce mechanism (as defined in section 8 of [I-D.ietf-oauth-dpop]). Authorization Code Flow

For the Authorization Code flow, Clients

  1. shall use the authorization code grant described in [RFC6749]

  2. shall use pushed authorization requests according to [RFC9126]

  3. shall use PKCE [RFC7636] with S256 as the code challenge method

  4. shall check the iss parameter in the authorization response according to [RFC9207] to prevent Mix-Up attacks

4.3.3. Requirements for Resource Servers

The FAPI 2.0 endpoints are OAuth 2.0 protected resource endpoints that return protected information for the resource owner associated with the submitted access token.

Resource servers with the FAPI endpoints

  1. shall accept access tokens in the HTTP header as in Section 2.1 of OAuth 2.0 Bearer Token Usage [RFC6750]

  2. shall not accept access tokens in the query parameters stated in Section 2.3 of OAuth 2.0 Bearer Token Usage [RFC6750]

  3. shall verify the validity, integrity, expiration and revocation status of access tokens

  4. shall verify that the authorization represented by the access token is sufficient for the requested resource access and otherwise return errors as in section 3.1 of [RFC6750]

  5. shall support and verify sender-constrained access tokens using one of the following methods:

4.4. Cryptography and Secrets

  1. Authorization Servers, Clients, and Resource Servers when creating or processing JWTs shall

    1. adhere to [RFC8725]

    2. use PS256, ES256, or EdDSA (using the Ed25519 subtype) algorithms

    3. not use or accept the none algorithm

  2. RSA keys shall have a minimum length of 2048 bits.

  3. Elliptic curve keys shall have a minimum length of 160 bits.

  4. Credentials not intended for handling by end-users (e.g., access tokens, refresh tokens, authorization codes, etc.) shall be created with at least 128 bits of entropy such that an attacker correctly guessing the value is computationally infeasible. Cf. Section 10.10 of [RFC6749].

4.5. Differences to FAPI 1.0

Table 1
FAPI 1.0 Read/Write FAPI 2.0 Reasons
JAR, JARM PAR integrity protection and compatibility improvements for authorization requests; only code in response
- shall adhere to Security BCP
s_hash - state integrity is protected by PAR; protection provided by state is now provided by PKCE
pre-registered redirect URIs redirect URIs in PAR pre-registration is not required with client authentication and PAR
response types code id_token or code response type code improve security: no ID token in front-channel; not needed
ID Token as detached signature - ID token does not need to serve as a detached signature
potentially encrypted ID Tokens encryption not required ID Tokens only exchanged in back channel
nbf & exp claims in request object request_uri has lifetime under 300 seconds Prevents pre-generation of requests.
x-fapi-* headers - Removed pending further discussion
MTLS for sender-constrained access tokens MTLS or DPoP

5. Security considerations


6. Privacy considerations


7. Acknowledgements

We would like to thank Takahiko Kawasaki, Filip Skokan, Dave Tonge, Nat Sakimura, Stuart Low, Dima Postnikov, Torsten Lodderstedt, Joseph Heenan, Travis Spencer, Brian Campbell, Ralph Bragg and Lukasz Jaromin for their valuable feedback and contributions that helped to evolve this specification.

8. Normative References

Sakimura, N., Bradley, J., Jones, M., de Medeiros, B., and C. Mortimore, "OpenID Connect Core 1.0 incorporating errata set 1", , <http://openid.net/specs/openid-connect-core-1_0.html>.
Fett, D., "FAPI 2.0 Attacker Model", , <https://bitbucket.org/openid/fapi/src/master/FAPI_2_0_Attacker_Model.md>.
Fett, D., Campbell, B., Bradley, J., Lodderstedt, T., Jones, M., and D. Waite, "OAuth 2.0 Demonstrating Proof-of-Possession at the Application Layer (DPoP)", Work in Progress, Internet-Draft, draft-ietf-oauth-dpop-09, , <https://tools.ietf.org/html/draft-ietf-oauth-dpop-09>.
Meyer zu Selhausen, K. and D. Fett, "OAuth 2.0 Authorization Server Issuer Identification", RFC 9207, DOI 10.17487/RFC9207, , <https://www.rfc-editor.org/info/rfc9207>.
Hardt, D., Ed., "The OAuth 2.0 Authorization Framework", RFC 6749, DOI 10.17487/RFC6749, , <https://www.rfc-editor.org/info/rfc6749>.
Standardization, I. O. F., "ISO/IEC Directives Part 2 -", , <https://www.iso.org/sites/directives/current/part2/index.xhtml>.
Saint-Andre, P. and J. Hodges, "Representation and Verification of Domain-Based Application Service Identity within Internet Public Key Infrastructure Using X.509 (PKIX) Certificates in the Context of Transport Layer Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, , <https://www.rfc-editor.org/info/rfc6125>.
Hodges, J., Jackson, C., and A. Barth, "HTTP Strict Transport Security (HSTS)", RFC 6797, DOI 10.17487/RFC6797, , <https://www.rfc-editor.org/info/rfc6797>.
Sakimura, N., Ed., Bradley, J., and N. Agarwal, "Proof Key for Code Exchange by OAuth Public Clients", RFC 7636, DOI 10.17487/RFC7636, , <https://www.rfc-editor.org/info/rfc7636>.
Sheffer, Y., Holz, R., and P. Saint-Andre, "Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, , <https://www.rfc-editor.org/info/rfc7525>.
Jones, M. and D. Hardt, "The OAuth 2.0 Authorization Framework: Bearer Token Usage", RFC 6750, DOI 10.17487/RFC6750, , <https://www.rfc-editor.org/info/rfc6750>.
Sakimura, N., Bradley, J., Jones, M., and E. Jay, "OpenID Connect Discovery 1.0 incorporating errata set 1", , <https://openid.net/specs/openid-connect-discovery-1_0.html>.
Lodderstedt, T., Campbell, B., Sakimura, N., Tonge, D., and F. Skokan, "OAuth 2.0 Pushed Authorization Requests", RFC 9126, DOI 10.17487/RFC9126, , <https://www.rfc-editor.org/info/rfc9126>.
Denniss, W. and J. Bradley, "OAuth 2.0 for Native Apps", BCP 212, RFC 8252, DOI 10.17487/RFC8252, , <https://www.rfc-editor.org/info/rfc8252>.
Sheffer, Y., Hardt, D., and M. Jones, "JSON Web Token Best Current Practices", BCP 225, RFC 8725, DOI 10.17487/RFC8725, , <https://www.rfc-editor.org/info/rfc8725>.
Hallam-Baker, P., Stradling, R., and J. Hoffman-Andrews, "DNS Certification Authority Authorization (CAA) Resource Record", RFC 8659, DOI 10.17487/RFC8659, , <https://www.rfc-editor.org/info/rfc8659>.
Campbell, B., Bradley, J., Sakimura, N., and T. Lodderstedt, "OAuth 2.0 Mutual-TLS Client Authentication and Certificate-Bound Access Tokens", RFC 8705, DOI 10.17487/RFC8705, , <https://www.rfc-editor.org/info/rfc8705>.

9. Informative References

Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0 Authorization Server Metadata", RFC 8414, DOI 10.17487/RFC8414, , <https://www.rfc-editor.org/info/rfc8414>.
Anonymous, "HSTS Preload List Submission", , <https://hstspreload.org/>.
Lodderstedt, T., Bradley, J., Labunets, A., and D. Fett, "OAuth 2.0 Security Best Current Practice", Work in Progress, Internet-Draft, draft-ietf-oauth-security-topics-19, , <https://tools.ietf.org/html/draft-ietf-oauth-security-topics-19>.

Appendix A. Notices

Copyright (c) 2021 The OpenID Foundation.

The OpenID Foundation (OIDF) grants to any Contributor, developer, implementer, or other interested party a non-exclusive, royalty free, worldwide copyright license to reproduce, prepare derivative works from, distribute, perform and display, this Implementers Draft or Final Specification solely for the purposes of (i) developing specifications, and (ii) implementing Implementers Drafts and Final Specifications based on such documents, provided that attribution be made to the OIDF as the source of the material, but that such attribution does not indicate an endorsement by the OIDF.

The technology described in this specification was made available from contributions from various sources, including members of the OpenID Foundation and others. Although the OpenID Foundation has taken steps to help ensure that the technology is available for distribution, it takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in this specification or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any independent effort to identify any such rights. The OpenID Foundation and the contributors to this specification make no (and hereby expressly disclaim any) warranties (express, implied, or otherwise), including implied warranties of merchantability, non-infringement, fitness for a particular purpose, or title, related to this specification, and the entire risk as to implementing this specification is assumed by the implementer. The OpenID Intellectual Property Rights policy requires contributors to offer a patent promise not to assert certain patent claims against other contributors and against implementers. The OpenID Foundation invites any interested party to bring to its attention any copyrights, patents, patent applications, or other proprietary rights that may cover technology that may be required to practice this specification.

Author's Address

Daniel Fett