The OAuth 2.1 Authorization Framework

1.1. Roles

OAuth defines four roles:¶

"resource owner":

An entity capable of granting access to a protected resource. When the resource owner is a person, it is referred to as an end-user. This is sometimes abbreviated as "RO".¶

"resource server":

The server hosting the protected resources, capable of accepting and responding to protected resource requests using access tokens. The resource server is often accessible via an API. This is sometimes abbreviated as "RS".¶

"client":

An application making protected resource requests on behalf of the resource owner and with its authorization. The term "client" does not imply any particular implementation characteristics (e.g., whether the application executes on a server, a desktop, or other devices).¶

"authorization server":

The server issuing access tokens to the client after successfully authenticating the resource owner and obtaining authorization. This is sometimes abbreviated as "AS".¶

The interaction between the authorization server and resource server is beyond the scope of this specification, however several extension have been defined to provide an option for interoperability between resource servers and authorization servers. The authorization server may be the same server as the resource server or a separate entity. A single authorization server may issue access tokens accepted by multiple resource servers.¶

1.2. Protocol Flow

     +--------+                               +---------------+
     |        |--(1)- Authorization Request ->|   Resource    |
     |        |                               |     Owner     |
     |        |<-(2)-- Authorization Grant ---|               |
     |        |                               +---------------+
     |        |
     |        |                               +---------------+
     |        |--(3)-- Authorization Grant -->| Authorization |
     | Client |                               |     Server    |
     |        |<-(4)----- Access Token -------|               |
     |        |                               +---------------+
     |        |
     |        |                               +---------------+
     |        |--(5)----- Access Token ------>|    Resource   |
     |        |                               |     Server    |
     |        |<-(6)--- Protected Resource ---|               |
     +--------+                               +---------------+

The abstract OAuth 2.1 flow illustrated in Figure 1 describes the interaction between the four roles and includes the following steps:¶

1. The client requests authorization from the resource owner. The authorization request can be made directly to the resource owner (as shown), or preferably indirectly via the authorization server as an intermediary.¶
2. The client receives an authorization grant, which is a credential representing the resource owner's authorization, expressed using one of two authorization grant types defined in this specification or using an extension grant type. The authorization grant type depends on the method used by the client to request authorization and the types supported by the authorization server.¶
3. The client requests an access token by authenticating with the authorization server and presenting the authorization grant.¶
4. The authorization server authenticates the client and validates the authorization grant, and if valid, issues an access token.¶
5. The client requests the protected resource from the resource server and authenticates by presenting the access token.¶
6. The resource server validates the access token, and if valid, serves the request.¶

The preferred method for the client to obtain an authorization grant from the resource owner (depicted in steps (1) and (2)) is to use the authorization server as an intermediary, which is illustrated in Figure 3 in Section 4.1.¶

1.3. Authorization Grant

An authorization grant is a credential representing the resource owner's authorization (to access its protected resources) used by the client to obtain an access token. This specification defines two grant types - authorization code and client credentials - as well as an extensibility mechanism for defining additional types.¶

1.4. Access Token

Access tokens are credentials used to access protected resources. An access token is a string representing an authorization issued to the client. The string is considered opaque to the client, even if it has a structure. Depending on the authorization server, the access token string may be parseable by the resource server.¶

Access tokens represent specific scopes and durations of access, granted by the resource owner, and enforced by the resource server and authorization server.¶

The token may be used by the RS to retrieve the authorization information, or the token may self-contain the authorization information in a verifiable manner (i.e., a token string consisting of a signed data payload). One example of a token retrieval mechanism is Token Introspection [RFC7662], in which the RS calls an endpoint on the AS to validate the token presented by the client. One example of a structured token format is [I-D.ietf-oauth-access-token-jwt], a method of encoding access token data as a JSON Web Token [RFC7519].¶

Additional authentication credentials, which are beyond the scope of this specification, may be required in order for the client to use an access token. This is typically referred to as a sender-constrained access token, such as Mutual TLS Access Tokens [RFC8705].¶

The access token provides an abstraction layer, replacing different authorization constructs (e.g., username and password) with a single token understood by the resource server. This abstraction enables issuing access tokens more restrictive than the authorization grant used to obtain them, as well as removing the resource server's need to understand a wide range of authentication methods.¶

Access tokens can have different formats, structures, and methods of utilization (e.g., cryptographic properties) based on the resource server security requirements. Access token attributes and the methods used to access protected resources may be extended beyond what is described in this specification.¶

1.5. Refresh Token

Refresh tokens are credentials used to obtain access tokens. Refresh tokens are issued to the client by the authorization server and are used to obtain a new access token when the current access token becomes invalid or expires, or to obtain additional access tokens with identical or narrower scope (access tokens may have a shorter lifetime and fewer permissions than authorized by the resource owner). Issuing a refresh token is optional at the discretion of the authorization server, and may be issued based on properties of the client, properties of the request, policies within the authorization server, or any other criteria. If the authorization server issues a refresh token, it is included when issuing an access token (i.e., step (2) in Figure 2).¶

A refresh token is a string representing the authorization granted to the client by the resource owner. The string is considered opaque to the client. The refresh token may be an identifier used to retrieve the authorization information or may encode this information into the string itself. Unlike access tokens, refresh tokens are intended for use only with authorization servers and are never sent to resource servers.¶

+--------+                                           +---------------+
|        |--(1)------- Authorization Grant --------->|               |
|        |                                           |               |
|        |<-(2)----------- Access Token -------------|               |
|        |               & Refresh Token             |               |
|        |                                           |               |
|        |                            +----------+   |               |
|        |--(3)---- Access Token ---->|          |   |               |
|        |                            |          |   |               |
|        |<-(4)- Protected Resource --| Resource |   | Authorization |
| Client |                            |  Server  |   |     Server    |
|        |--(5)---- Access Token ---->|          |   |               |
|        |                            |          |   |               |
|        |<-(6)- Invalid Token Error -|          |   |               |
|        |                            +----------+   |               |
|        |                                           |               |
|        |--(7)----------- Refresh Token ----------->|               |
|        |                                           |               |
|        |<-(8)----------- Access Token -------------|               |
+--------+           & Optional Refresh Token        +---------------+

The flow illustrated in Figure 2 includes the following steps:¶

1. The client requests an access token by authenticating with the authorization server and presenting an authorization grant.¶
2. The authorization server authenticates the client and validates the authorization grant, and if valid, issues an access token and optionally a refresh token.¶
3. The client makes a protected resource request to the resource server by presenting the access token.¶
4. The resource server validates the access token, and if valid, serves the request.¶
5. Steps (3) and (4) repeat until the access token expires. If the client knows the access token expired, it skips to step (7); otherwise, it makes another protected resource request.¶
6. Since the access token is invalid, the resource server returns an invalid token error.¶
7. The client requests a new access token by presenting the refresh token and providing client authentication if it has been issued credentials. The client authentication requirements are based on the client type and on the authorization server policies.¶
8. The authorization server authenticates the client and validates the refresh token, and if valid, issues a new access token (and, optionally, a new refresh token).¶

1.6. TLS Version

Whenever Transport Layer Security (TLS) is used by this specification, the appropriate version (or versions) of TLS will vary over time, based on the widespread deployment and known security vulnerabilities. Refer to [BCP195] for up to date recommendations on transport layer security.¶

Implementations MAY also support additional transport-layer security mechanisms that meet their security requirements.¶

1.7. HTTP Redirections

This specification makes extensive use of HTTP redirections, in which the client or the authorization server directs the resource owner's user-agent to another destination. While the examples in this specification show the use of the HTTP 302 status code, any other method available via the user-agent to accomplish this redirection, with the exception of HTTP 307, is allowed and is considered to be an implementation detail. See Section 9.7.2 for details.¶

1.8. Interoperability

OAuth 2.1 provides a rich authorization framework with well-defined security properties.¶

This specification leaves a few required components partially or fully undefined (e.g., client registration, authorization server capabilities, endpoint discovery). Some of these behaviors are defined in optional extensions which implementations can choose to use, such as:¶

• [RFC8414]: Authorization Server Metadata, defining an endpoint clients can use to look up the information needed to interact with a particular OAuth server¶
• [RFC7591]: Dynamic Client Registration, providing a mechanism for programmatically registering clients with an authorization server¶
• [RFC7592]: Dynamic Client Management, providing a mechanism for updating dynamically registered client information¶
• [RFC7662]: Token Introspection, defining a mechanism for resource servers to obtain information about access tokens¶

Please refer to Appendix C for a list of current known extensions at the time of this publication.¶

1.9. Compatibility with OAuth 2.0

OAuth 2.1 is compatible with OAuth 2.0 with the extensions and restrictions from known best current practices applied. Specifically, features not specified in OAuth 2.0 core, such as PKCE, are required in OAuth 2.1. Additionally, some features available in OAuth 2.0, such as the Implicit or Resource Owner Credentials grant types, are not specified in OAuth 2.1. Furthermore, some behaviors allowed in OAuth 2.0 are restricted in OAuth 2.1, such as the strict string matching of redirect URIs required by OAuth 2.1.¶

See Section 12 for more details on the differences from OAuth 2.0.¶

1.10. Notational Conventions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

This specification uses the Augmented Backus-Naur Form (ABNF) notation of [RFC5234]. Additionally, the rule URI-reference is included from "Uniform Resource Identifier (URI): Generic Syntax" [RFC3986].¶

Certain security-related terms are to be understood in the sense defined in [RFC4949]. These terms include, but are not limited to, "attack", "authentication", "authorization", "certificate", "confidentiality", "credential", "encryption", "identity", "sign", "signature", "trust", "validate", and "verify".¶

The term "payload" is to be interpreted as described in Section 3.3 of [RFC7231].¶

Unless otherwise noted, all the protocol parameter names and values are case sensitive.¶

2.1. Client Types

OAuth 2.1 defines three client types based on their ability to authenticate securely with the authorization server as well as the authorization server's assurance of the client's identity.¶

"confidential":

Clients that have credentials and their identity has been confirmed by the AS are designated as "confidential clients"¶

"credentialed":

Clients that have credentials and their identity has been not been confirmed by the AS are designated as "credentialed clients"¶

"public":

Clients without credentials are called "public clients"¶

Any clients with credentials MUST take precautions to prevent leakage and abuse of their credentials.¶

Authorization servers SHOULD consider the level of confidence in a client's identity when deciding whether they allow such a client access to more critical functions, such as the Client Credentials grant type.¶

A single client_id MUST NOT be treated as more than one type of client.¶

This specification has been designed around the following client profiles:¶

"web application":

A web application is a confidential client running on a web server. Resource owners access the client via an HTML user interface rendered in a user-agent on the device used by the resource owner. The client credentials as well as any access token issued to the client are stored on the web server and are not exposed to or accessible by the resource owner.¶

"browser-based application":

A browser-based application is a public client in which the client code is downloaded from a web server and executes within a user-agent (e.g., web browser) on the device used by the resource owner. Protocol data and credentials are easily accessible (and often visible) to the resource owner. Since such applications reside within the user-agent, they can make seamless use of the user-agent capabilities when requesting authorization.¶

"native application":

A native application is a public client installed and executed on the device used by the resource owner. Protocol data and credentials are accessible to the resource owner. It is assumed that any client authentication credentials included in the application can be extracted. On the other hand, dynamically issued credentials such as access tokens or refresh tokens can receive an acceptable level of protection. At a minimum, these credentials are protected from hostile servers with which the application may interact. On some platforms, these credentials might be protected from other applications residing on the same device.¶

2.2. Client Identifier

The authorization server issues the registered client a client identifier - a unique string representing the registration information provided by the client. The client identifier is not a secret; it is exposed to the resource owner and MUST NOT be used alone for client authentication. The client identifier is unique to the authorization server.¶

The client identifier string size is left undefined by this specification. The client should avoid making assumptions about the identifier size. The authorization server SHOULD document the size of any identifier it issues.¶

Authorization servers SHOULD NOT allow clients to choose or influence their client_id value. See Section 9.6 for details.¶

2.3. Client Authentication

Confidential and credentialed clients establish a client authentication method with the authorization server suitable for the security requirements of the authorization server. The authorization server MAY accept any form of client authentication meeting its security requirements.¶

Confidential and credentialed clients are typically issued (or establish) a set of client credentials used for authenticating with the authorization server (e.g., password, public/private key pair).¶

Authorization servers SHOULD use client authentication if possible.¶

It is RECOMMENDED to use asymmetric (public-key based) methods for client authentication such as mTLS [RFC8705] or "private_key_jwt" [OpenID]. When asymmetric methods for client authentication are used, authorization servers do not need to store sensitive symmetric keys, making these methods more robust against a number of attacks.¶

The authorization server MAY establish a client authentication method with public clients, which converts them to credentialed clients. However, the authorization server MUST NOT rely on credentialed client authentication for the purpose of identifying the client.¶

The client MUST NOT use more than one authentication method in each request.¶

Authorization: Basic czZCaGRSa3F0Mzo3RmpmcDBaQnIxS3REUmJuZlZkbUl3

POST /token HTTP/1.1
Host: server.example.com
Content-Type: application/x-www-form-urlencoded

grant_type=refresh_token&refresh_token=tGzv3JOkF0XG5Qx2TlKWIA
&client_id=s6BhdRkqt3&client_secret=7Fjfp0ZBr1KtDRbnfVdmIw

2.4. Unregistered Clients

This specification does not exclude the use of unregistered clients. However, the use of such clients is beyond the scope of this specification and requires additional security analysis and review of its interoperability impact.¶

3.1. Authorization Endpoint

The authorization endpoint is used to interact with the resource owner and obtain an authorization grant. The authorization server MUST first verify the identity of the resource owner. The way in which the authorization server authenticates the resource owner (e.g., username and password login, session cookies) is beyond the scope of this specification.¶

The means through which the client obtains the location of the authorization endpoint are beyond the scope of this specification, but the location is typically provided in the service documentation, or in the authorization server's metadata document ([RFC8414]).¶

The endpoint URI MAY include an "application/x-www-form-urlencoded" formatted (per Appendix B) query component ([RFC3986] Section 3.4), which MUST be retained when adding additional query parameters. The endpoint URI MUST NOT include a fragment component.¶

Since requests to the authorization endpoint result in user authentication and the transmission of clear-text credentials (in the HTTP response), the authorization server MUST require the use of TLS as described in Section 1.6 when sending requests to the authorization endpoint.¶

The authorization server MUST support the use of the HTTP GET method [RFC7231] for the authorization endpoint and MAY support the use of the POST method as well.¶

The authorization server MUST ignore unrecognized request parameters.¶

Request and response parameters defined by this specification MUST NOT be included more than once. Parameters sent without a value MUST be treated as if they were omitted from the request.¶

3.2. Token Endpoint

The token endpoint is used by the client to obtain an access token using a grant such as those described in Section 4 and Section 6.¶

The means through which the client obtains the location of the token endpoint are beyond the scope of this specification, but the location is typically provided in the service documentation and configured during development of the client, or provided in the authorization server's metadata document ([RFC8414]) and fetched programmatically at runtime.¶

The endpoint URI MAY include an application/x-www-form-urlencoded formatted (per Appendix B) query component ([RFC3986] Section 3.4) and MUST NOT include a fragment component.¶

Since requests to the token endpoint result in the transmission of clear-text credentials (in the HTTP request and response), the authorization server MUST require the use of TLS as described in Section 1.6 when sending requests to the token endpoint.¶

The client MUST use the HTTP POST method when making access token requests.¶

The authorization server MUST ignore unrecognized request parameters.¶

Parameters sent without a value MUST be treated as if they were omitted from the request. Request and response parameters defined by this specification MUST NOT be included more than once.¶

3.3. Access Token Scope

The authorization and token endpoints allow the client to specify the scope of the access request using the scope request parameter. In turn, the authorization server uses the scope response parameter to inform the client of the scope of the access token issued.¶

The value of the scope parameter is expressed as a list of space- delimited, case-sensitive strings. The strings are defined by the authorization server. If the value contains multiple space-delimited strings, their order does not matter, and each string adds an additional access range to the requested scope.¶

    scope       = scope-token *( SP scope-token )
    scope-token = 1*( %x21 / %x23-5B / %x5D-7E )

The authorization server MAY fully or partially ignore the scope requested by the client, based on the authorization server policy or the resource owner's instructions. If the issued access token scope is different from the one requested by the client, the authorization server MUST include the scope response parameter to inform the client of the actual scope granted.¶

If the client omits the scope parameter when requesting authorization, the authorization server MUST either process the request using a pre-defined default value or fail the request indicating an invalid scope. The authorization server SHOULD document its scope requirements and default value (if defined).¶

4.1. Authorization Code Grant

The authorization code grant type is used to obtain both access tokens and refresh tokens.¶

Since this is a redirect-based flow, the client must be capable of initiating the flow with the resource owner's user-agent (typically a web browser) and capable of being redirected back to from the authorization server.¶

+----------+
| Resource |
|   Owner  |
|          |
+----------+
     ^
     |
    (2)
+----|-----+          Client Identifier      +---------------+
|         -+----(1)-- & Redirect URI    ---->|               |
|  User-   |                                 | Authorization |
|  Agent  -+----(2)-- User authenticates --->|     Server    |
|          |                                 |               |
|         -+----(3)-- Authorization Code ---<|               |
+-|----|---+                                 +---------------+
  |    |                                         ^      v
 (1)  (3)                                        |      |
  |    |                                         |      |
  ^    v                                         |      |
+---------+                                      |      |
|         |>---(4)-- Authorization Code ---------'      |
|  Client |          & Redirect URI                     |
|         |                                             |
|         |<---(5)----- Access Token -------------------'
+---------+       (w/ Optional Refresh Token)

Note: The lines illustrating steps (1), (2), and (3) are broken into
two parts as they pass through the user-agent.

The flow illustrated in Figure 3 includes the following steps:¶

(1) The client initiates the flow by directing the resource owner's user-agent to the authorization endpoint. The client includes its client identifier, code challenge (derived from a generated code verifier), optional requested scope, optional local state, and a redirect URI to which the authorization server will send the user-agent back once access is granted (or denied).¶

(2) The authorization server authenticates the resource owner (via the user-agent) and establishes whether the resource owner grants or denies the client's access request.¶

(3) Assuming the resource owner grants access, the authorization server redirects the user-agent back to the client using the redirect URI provided earlier (in the request or during client registration). The redirect URI includes an authorization code and any local state provided by the client earlier.¶

(4) The client requests an access token from the authorization server's token endpoint by including the authorization code received in the previous step, and including its code verifier. When making the request, the client authenticates with the authorization server if it can. The client includes the redirect URI used to obtain the authorization code for verification.¶

(5) The authorization server authenticates the client when possible, validates the authorization code, validates the code verifier, and ensures that the redirect URI received matches the URI used to redirect the client in step (3). If valid, the authorization server responds back with an access token and, optionally, a refresh token.¶

code-verifier = 43*128unreserved
unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~"
ALPHA = %x41-5A / %x61-7A
DIGIT = %x30-39

S256
  code_challenge = BASE64URL-ENCODE(SHA256(ASCII(code_verifier)))

plain
  code_challenge = code_verifier

code-challenge = 43*128unreserved
unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~"
ALPHA = %x41-5A / %x61-7A
DIGIT = %x30-39

GET /authorize?response_type=code&client_id=s6BhdRkqt3&state=xyz
    &redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb
    &code_challenge=6fdkQaPm51l13DSukcAH3Mdx7_ntecHYd1vi3n0hMZY
    &code_challenge_method=S256 HTTP/1.1
Host: server.example.com

HTTP/1.1 302 Found
Location: https://client.example.com/cb?code=SplxlOBeZQQYbYS6WxSbIA
          &state=xyz

HTTP/1.1 302 Found
Location: https://client.example.com/cb?error=access_denied&state=xyz

POST /token HTTP/1.1
Host: server.example.com
Authorization: Basic czZCaGRSa3F0MzpnWDFmQmF0M2JW
Content-Type: application/x-www-form-urlencoded

grant_type=authorization_code&code=SplxlOBeZQQYbYS6WxSbIA
&redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb
&code_verifier=3641a2d12d66101249cdf7a79c000c1f8c05d2aafcf14bf146497bed

HTTP/1.1 200 OK
Content-Type: application/json
Cache-Control: no-store
Pragma: no-cache

{
  "access_token": "2YotnFZFEjr1zCsicMWpAA",
  "token_type": "Bearer",
  "expires_in": 3600,
  "refresh_token": "tGzv3JOkF0XG5Qx2TlKWIA",
  "example_parameter": "example_value"
}

4.2. Client Credentials Grant

The client can request an access token using only its client credentials (or other supported means of authentication) when the client is requesting access to the protected resources under its control, or those of another resource owner that have been previously arranged with the authorization server (the method of which is beyond the scope of this specification).¶

The client credentials grant type MUST only be used by confidential or credentialed clients.¶

     +---------+                                  +---------------+
     |         |                                  |               |
     |         |>--(1)- Client Authentication --->| Authorization |
     | Client  |                                  |     Server    |
     |         |<--(2)---- Access Token ---------<|               |
     |         |                                  |               |
     +---------+                                  +---------------+

The flow illustrated in Figure 4 includes the following steps:¶

(1) The client authenticates with the authorization server and requests an access token from the token endpoint.¶

(2) The authorization server authenticates the client, and if valid, issues an access token.¶

POST /token HTTP/1.1
Host: server.example.com
Authorization: Basic czZCaGRSa3F0MzpnWDFmQmF0M2JW
Content-Type: application/x-www-form-urlencoded

grant_type=client_credentials

HTTP/1.1 200 OK
Content-Type: application/json
Cache-Control: no-store
Pragma: no-cache

{
  "access_token": "2YotnFZFEjr1zCsicMWpAA",
  "token_type": "Bearer",
  "expires_in": 3600,
  "example_parameter": "example_value"
}

4.3. Extension Grants

The client uses an extension grant type by specifying the grant type using an absolute URI (defined by the authorization server) as the value of the grant_type parameter of the token endpoint, and by adding any additional parameters necessary.¶

For example, to request an access token using the Device Authorization Grant as defined by [RFC8628] after the user has authorized the client on a separate device, the client makes the following HTTP request using TLS (with extra line breaks for display purposes only):¶

  POST /token HTTP/1.1
  Host: server.example.com
  Content-Type: application/x-www-form-urlencoded

  grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Adevice_code
  &device_code=GmRhmhcxhwEzkoEqiMEg_DnyEysNkuNhszIySk9eS
  &client_id=C409020731

If the access token request is valid and authorized, the authorization server issues an access token and optional refresh token as described in Section 5.1. If the request failed client authentication or is invalid, the authorization server returns an error response as described in Section 5.2.¶

5.1. Successful Response

The authorization server issues an access token and optional refresh token by creating an HTTP response body using the application/json media type as defined by [RFC8259] with the following parameters and an HTTP 200 (OK) status code:¶

"access_token":

REQUIRED. The access token issued by the authorization server.¶

"token_type":

REQUIRED. The type of the access token issued as described in Section 7.1. Value is case insensitive.¶

"expires_in":

RECOMMENDED. The lifetime in seconds of the access token. For example, the value 3600 denotes that the access token will expire in one hour from the time the response was generated. If omitted, the authorization server SHOULD provide the expiration time via other means or document the default value.¶

"refresh_token":

OPTIONAL. The refresh token, which can be used to obtain new access tokens using the same authorization grant as described in Section 6.¶

"scope":

OPTIONAL, if identical to the scope requested by the client; otherwise, REQUIRED. The scope of the access token as described by Section 3.3.¶

The parameters are serialized into a JavaScript Object Notation (JSON) structure by adding each parameter at the highest structure level. Parameter names and string values are included as JSON strings. Numerical values are included as JSON numbers. The order of parameters does not matter and can vary.¶

The authorization server MUST include the HTTP Cache-Control response header field [RFC7234] with a value of no-store in any response containing tokens, credentials, or other sensitive information, as well as the Pragma response header field [RFC7234] with a value of no-cache.¶

For example:¶

HTTP/1.1 200 OK
Content-Type: application/json
Cache-Control: no-store
Pragma: no-cache

{
  "access_token":"2YotnFZFEjr1zCsicMWpAA",
  "token_type":"Bearer",
  "expires_in":3600,
  "refresh_token":"tGzv3JOkF0XG5Qx2TlKWIA",
  "example_parameter":"example_value"
}

The client MUST ignore unrecognized value names in the response. The sizes of tokens and other values received from the authorization server are left undefined. The client should avoid making assumptions about value sizes. The authorization server SHOULD document the size of any value it issues.¶

5.2. Error Response

The authorization server responds with an HTTP 400 (Bad Request) status code (unless specified otherwise) and includes the following parameters with the response:¶

"error":

REQUIRED. A single ASCII [USASCII] error code from the following:¶

"invalid_request":

The request is missing a required parameter, includes an unsupported parameter value (other than grant type), repeats a parameter, includes multiple credentials, utilizes more than one mechanism for authenticating the client, contains a code_verifier although no code_challenge was sent in the authorization request, or is otherwise malformed.¶

"invalid_client":

Client authentication failed (e.g., unknown client, no client authentication included, or unsupported authentication method). The authorization server MAY return an HTTP 401 (Unauthorized) status code to indicate which HTTP authentication schemes are supported. If the client attempted to authenticate via the Authorization request header field, the authorization server MUST respond with an HTTP 401 (Unauthorized) status code and include the WWW-Authenticate response header field matching the authentication scheme used by the client.¶

"invalid_grant":

The provided authorization grant (e.g., authorization code, resource owner credentials) or refresh token is invalid, expired, revoked, does not match the redirect URI used in the authorization request, or was issued to another client.¶

"unauthorized_client":

The authenticated client is not authorized to use this authorization grant type.¶

"unsupported_grant_type":

The authorization grant type is not supported by the authorization server.¶

"invalid_scope":

The requested scope is invalid, unknown, malformed, or exceeds the scope granted by the resource owner.¶

Values for the error parameter MUST NOT include characters outside the set %x20-21 / %x23-5B / %x5D-7E.¶

"error_description":

OPTIONAL. Human-readable ASCII [USASCII] text providing additional information, used to assist the client developer in understanding the error that occurred. Values for the error_description parameter MUST NOT include characters outside the set %x20-21 / %x23-5B / %x5D-7E.¶

"error_uri":

OPTIONAL. A URI identifying a human-readable web page with information about the error, used to provide the client developer with additional information about the error. Values for the error_uri parameter MUST conform to the URI-reference syntax and thus MUST NOT include characters outside the set %x21 / %x23-5B / %x5D-7E.¶

The parameters are included in the payload of the HTTP response using the application/json media type as defined by [RFC7159]. The parameters are serialized into a JSON structure by adding each parameter at the highest structure level. Parameter names and string values are included as JSON strings. Numerical values are included as JSON numbers. The order of parameters does not matter and can vary.¶

For example:¶

HTTP/1.1 400 Bad Request
Content-Type: application/json
Cache-Control: no-store
Pragma: no-cache

{
 "error":"invalid_request"
}

6.1. Refresh Token Request

To use a refresh token to obtain a new access token, the client makes a request to the token endpoint by adding the following parameters using the application/x-www-form-urlencoded format (per Appendix B) with a character encoding of UTF-8 in the HTTP request payload:¶

"grant_type":

REQUIRED. Value MUST be set to refresh_token.¶

"refresh_token":

REQUIRED. The refresh token issued to the client.¶

"scope":

OPTIONAL. The scope of the access request as described by Section 3.3. The requested scope MUST NOT include any scope not originally granted by the resource owner, and if omitted is treated as equal to the scope originally granted by the resource owner.¶

Because refresh tokens are typically long-lasting credentials used to request additional access tokens, the refresh token is bound to the client to which it was issued. Confidential or credentialed clients MUST authenticate with the authorization server as described in Section 3.2.1.¶

For example, the client makes the following HTTP request using transport-layer security (with extra line breaks for display purposes only):¶

POST /token HTTP/1.1
Host: server.example.com
Authorization: Basic czZCaGRSa3F0MzpnWDFmQmF0M2JW
Content-Type: application/x-www-form-urlencoded

grant_type=refresh_token&refresh_token=tGzv3JOkF0XG5Qx2TlKWIA

The authorization server MUST:¶

• require client authentication for confidential or credentialed clients¶
• authenticate the client if client authentication is included and ensure that the refresh token was issued to the authenticated client, and¶
• validate the refresh token.¶

Authorization servers SHOULD utilize one of these methods to detect refresh token replay by malicious actors for public clients:¶

• Sender-constrained refresh tokens: the authorization server cryptographically binds the refresh token to a certain client instance by utilizing [I-D.ietf-oauth-token-binding], [RFC8705], [I-D.ietf-oauth-dpop], or another suitable method.¶
• Refresh token rotation: the authorization server issues a new refresh token with every access token refresh response. The previous refresh token is invalidated but information about the relationship is retained by the authorization server. 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. The authorization server cannot determine which party submitted the invalid refresh token, but it will revoke the active refresh token. This stops the attack at the cost of forcing the legitimate client to obtain a fresh authorization grant.¶

Implementation note: the grant to which a refresh token belongs may be encoded into the refresh token itself. This can enable an authorization server to efficiently determine the grant to which a refresh token belongs, and by extension, all refresh tokens that need to be revoked. Authorization servers MUST ensure the integrity of the refresh token value in this case, for example, using signatures.¶

6.2. Refresh Token Response

If valid and authorized, the authorization server issues an access token as described in Section 5.1. If the request failed verification or is invalid, the authorization server returns an error response as described in Section 5.2.¶

The authorization server MAY issue a new refresh token, in which case the client MUST discard the old refresh token and replace it with the new refresh token. The authorization server MAY revoke the old refresh token after issuing a new refresh token to the client. If a new refresh token is issued, the refresh token scope MUST be identical to that of the refresh token included by the client in the request.¶

Authorization servers MAY revoke refresh tokens automatically in case of a security event, such as:¶

• password change¶
• logout at the authorization server¶

Refresh tokens SHOULD expire if the client has been inactive for some time, i.e., the refresh token has not been used to obtain new access tokens for some time. The expiration time is at the discretion of the authorization server. It might be a global value or determined based on the client policy or the grant associated with the refresh token (and its sensitivity).¶

7.1. Access Token Types

The access token type provides the client with the information required to successfully utilize the access token to make a protected resource request (along with type-specific attributes). The client MUST NOT use an access token if it does not understand the token type.¶

For example, the Bearer token type defined in this specification is utilized by simply including the access token string in the request:¶

GET /resource/1 HTTP/1.1
Host: example.com
Authorization: Bearer mF_9.B5f-4.1JqM

The above example is provided for illustration purposes only.¶

Each access token type definition specifies the additional attributes (if any) sent to the client together with the access_token response parameter. It also defines the HTTP authentication method used to include the access token when making a protected resource request.¶

7.2. Bearer Tokens

A Bearer Token is a security token with the property that any party in possession of the token (a "bearer") can use the token in any way that any other party in possession of it can. Using a bearer token does not require a bearer to prove possession of cryptographic key material (proof-of-possession).¶

Bearer tokens may be extended to include proof-of-possession techniques by other specifications.¶

 GET /resource HTTP/1.1
 Host: server.example.com
 Authorization: Bearer mF_9.B5f-4.1JqM

b64token    = 1*( ALPHA / DIGIT /
                 "-" / "." / "_" / "~" / "+" / "/" ) *"="
credentials = "Bearer" 1*SP b64token

POST /resource HTTP/1.1
Host: server.example.com
Content-Type: application/x-www-form-urlencoded

access_token=mF_9.B5f-4.1JqM

scope="openid profile email"
scope="urn:example:channel=HBO&urn:example:rating=G,PG-13"

HTTP/1.1 401 Unauthorized
WWW-Authenticate: Bearer realm="example"

HTTP/1.1 401 Unauthorized
WWW-Authenticate: Bearer realm="example",
                  error="invalid_token",
                  error_description="The access token expired"

HTTP/1.1 401 Unauthorized
WWW-Authenticate: Bearer realm="example"

7.3. Error Response

If a resource access request fails, the resource server SHOULD inform the client of the error. The method by which the resource server does this is determined by the particular token type, such as the description of Bearer tokens in Section 7.2.3.¶

7.4. Access Token Security Considerations

8.1. Defining Access Token Types

Access token types can be defined in one of two ways: registered in the Access Token Types registry (following the procedures in Section 11.1 of [RFC6749]), or by using a unique absolute URI as its name.¶

Types utilizing a URI name SHOULD be limited to vendor-specific implementations that are not commonly applicable, and are specific to the implementation details of the resource server where they are used.¶

All other types MUST be registered. Type names MUST conform to the type-name ABNF. If the type definition includes a new HTTP authentication scheme, the type name SHOULD be identical to the HTTP authentication scheme name (as defined by [RFC2617]). The token type example is reserved for use in examples.¶

type-name  = 1*name-char
name-char  = "-" / "." / "_" / DIGIT / ALPHA

8.2. Defining New Endpoint Parameters

New request or response parameters for use with the authorization endpoint or the token endpoint are defined and registered in the OAuth Parameters registry following the procedure in Section 11.2 of [RFC6749].¶

Parameter names MUST conform to the param-name ABNF, and parameter values syntax MUST be well-defined (e.g., using ABNF, or a reference to the syntax of an existing parameter).¶

param-name  = 1*name-char
name-char   = "-" / "." / "_" / DIGIT / ALPHA

Unregistered vendor-specific parameter extensions that are not commonly applicable and that are specific to the implementation details of the authorization server where they are used SHOULD utilize a vendor-specific prefix that is not likely to conflict with other registered values (e.g., begin with 'companyname_').¶

8.3. Defining New Authorization Grant Types

New authorization grant types can be defined by assigning them a unique absolute URI for use with the grant_type parameter. If the extension grant type requires additional token endpoint parameters, they MUST be registered in the OAuth Parameters registry as described by Section 11.2 of [RFC6749].¶

8.4. Defining New Authorization Endpoint Response Types

New response types for use with the authorization endpoint are defined and registered in the Authorization Endpoint Response Types registry following the procedure in Section 11.3 of [RFC6749]. Response type names MUST conform to the response-type ABNF.¶

response-type  = response-name *( SP response-name )
response-name  = 1*response-char
response-char  = "_" / DIGIT / ALPHA

If a response type contains one or more space characters (%x20), it is compared as a space-delimited list of values in which the order of values does not matter. Only one order of values can be registered, which covers all other arrangements of the same set of values.¶

For example, an extension can define and register the code other_token response type. Once registered, the same combination cannot be registered as other_token code, but both values can be used to denote the same response type.¶

8.5. Defining Additional Error Codes

In cases where protocol extensions (i.e., access token types, extension parameters, or extension grant types) require additional error codes to be used with the authorization code grant error response (Section 4.1.2.1), the token error response (Section 5.2), or the resource access error response (Section 7.3), such error codes MAY be defined.¶

Extension error codes MUST be registered (following the procedures in Section 11.4 of [RFC6749]) if the extension they are used in conjunction with is a registered access token type, a registered endpoint parameter, or an extension grant type. Error codes used with unregistered extensions MAY be registered.¶

Error codes MUST conform to the error ABNF and SHOULD be prefixed by an identifying name when possible. For example, an error identifying an invalid value set to the extension parameter example SHOULD be named example_invalid.¶

error      = 1*error-char
error-char = %x20-21 / %x23-5B / %x5D-7E

9.1. Client Authentication

Authorization servers SHOULD use client authentication if possible.¶

It is RECOMMENDED to use asymmetric (public-key based) methods for client authentication such as mTLS [RFC8705] or private_key_jwt [OpenID]. When asymmetric methods for client authentication are used, authorization servers do not need to store sensitive symmetric keys, making these methods more robust against a number of attacks.¶

Authorization server MUST only rely on client authentication if the process of issuance/registration and distribution of the underlying credentials ensures their confidentiality.¶

When client authentication is not possible, the authorization server SHOULD employ other means to validate the client's identity - for example, by requiring the registration of the client redirect URI or enlisting the resource owner to confirm identity. A valid redirect URI is not sufficient to verify the client's identity when asking for resource owner authorization but can be used to prevent delivering credentials to a counterfeit client after obtaining resource owner authorization.¶

The authorization server must consider the security implications of interacting with unauthenticated clients and take measures to limit the potential exposure of other credentials (e.g., refresh tokens) issued to such clients.¶

The privileges an authorization server associates with a certain client identity MUST depend on the assessment of the overall process for client identification and client credential lifecycle management. For example, authentication of a dynamically registered client just ensures the authorization server it is talking to the same client again. In contrast, if there is a web application whose developer's identity was verified, who signed a contract and is issued a client secret that is only used in a secure backend service, the authorization server might allow this client to access more sensible services or to use the client credential grant type.¶

9.2. Registration of Native App Clients

Except when using a mechanism like Dynamic Client Registration [RFC7591] to provision per-instance secrets, native apps are classified as public clients, as defined in Section 2.1; they MUST be registered with the authorization server as such. Authorization servers MUST record the client type in the client registration details in order to identify and process requests accordingly.¶

Authorization servers MUST require clients to register their complete redirect URI (including the path component) and reject authorization requests that specify a redirect URI that doesn't exactly match the one that was registered; the exception is loopback redirects, where an exact match is required except for the port URI component.¶

For private-use URI scheme-based redirects, authorization servers SHOULD enforce the requirement in Section 10.3.1 that clients use schemes that are reverse domain name based. At a minimum, any private-use URI scheme that doesn't contain a period character (.) SHOULD be rejected.¶

In addition to the collision-resistant properties, requiring a URI scheme based on a domain name that is under the control of the app can help to prove ownership in the event of a dispute where two apps claim the same private-use URI scheme (where one app is acting maliciously). For example, if two apps claimed com.example.app, the owner of example.com could petition the app store operator to remove the counterfeit app. Such a petition is harder to prove if a generic URI scheme was used.¶

Authorization servers MAY request the inclusion of other platform- specific information, such as the app package or bundle name, or other information that may be useful for verifying the calling app's identity on operating systems that support such functions.¶

9.3. Client Impersonation

A malicious client can impersonate another client and obtain access to protected resources if the impersonated client fails to, or is unable to, keep its client credentials confidential.¶

The authorization server MUST authenticate the client whenever possible. If the authorization server cannot authenticate the client due to the client's nature, the authorization server MUST require the registration of any redirect URI used for receiving authorization responses and SHOULD utilize other means to protect resource owners from such potentially malicious clients. For example, the authorization server can engage the resource owner to assist in identifying the client and its origin.¶

The authorization server SHOULD enforce explicit resource owner authentication and provide the resource owner with information about the client and the requested authorization scope and lifetime. It is up to the resource owner to review the information in the context of the current client and to authorize or deny the request.¶

The authorization server SHOULD NOT process repeated authorization requests automatically (without active resource owner interaction) without authenticating the client or relying on other measures to ensure that the repeated request comes from the original client and not an impersonator.¶

9.4. Access Tokens

Access token credentials (as well as any confidential access token attributes) MUST be kept confidential in transit and storage, and only shared among the authorization server, the resource servers the access token is valid for, and the client to whom the access token is issued. Access token credentials MUST only be transmitted using TLS as described in Section 1.6 with server authentication as defined by [RFC2818].¶

The authorization server MUST ensure that access tokens cannot be generated, modified, or guessed to produce valid access tokens by unauthorized parties.¶

9.5. Refresh Tokens

Authorization servers MAY issue refresh tokens to clients.¶

Refresh tokens MUST be kept confidential in transit and storage, and shared only among the authorization server and the client to whom the refresh tokens were issued. The authorization server MUST maintain the binding between a refresh token and the client to whom it was issued. Refresh tokens MUST only be transmitted using TLS as described in Section 1.6 with server authentication as defined by [RFC2818].¶

The authorization server MUST verify the binding between the refresh token and client identity whenever the client identity can be authenticated. When client authentication is not possible, the authorization server SHOULD issue sender-constrained refresh tokens or use refresh token rotation as described in (#refreshing-an-access-token).¶

The authorization server MUST ensure that refresh tokens cannot be generated, modified, or guessed to produce valid refresh tokens by unauthorized parties.¶

9.6. Client Impersonating Resource Owner

Resource servers may make access control decisions based on the identity of the resource owner as communicated in the sub claim returned by the authorization server in a token introspection response [RFC7662] or other mechanisms. If a client is able to choose its own client_id during registration with the authorization server, then there is a risk that it can register with the same sub value as a privileged user. A subsequent access token obtained under the client credentials grant may be mistaken for an access token authorized by the privileged user if the resource server does not perform additional checks.¶

Authorization servers SHOULD NOT allow clients to influence their client_id or sub value or any other claim if that can cause confusion with a genuine resource owner. Where this cannot be avoided, authorization servers MUST provide other means for the resource server to distinguish between access tokens authorized by a resource owner from access tokens authorized by the client itself.¶

9.7. Protecting Redirect-Based Flows

When comparing client redirect URIs against pre-registered URIs, authorization servers MUST utilize exact string matching. This measure contributes to the prevention of leakage of authorization codes and access tokens (see (#insufficient_uri_validation)). It can also help to detect mix-up attacks (see (#mix_up)).¶

Clients MUST NOT expose URLs that forward the user's browser to arbitrary URIs obtained from a query parameter ("open redirector"). Open redirectors can enable exfiltration of authorization codes and access tokens, see (#open_redirector_on_client).¶

Clients MUST prevent Cross-Site Request Forgery (CSRF). In this context, CSRF refers to requests to the redirection endpoint that do not originate at the authorization server, but a malicious third party (see Section 4.4.1.8. of [RFC6819] for details). Clients that have ensured that the authorization server supports the code_challenge parameter MAY rely the CSRF protection provided by that mechanism. In OpenID Connect flows, the nonce parameter provides CSRF protection. Otherwise, one-time use CSRF tokens carried in the state parameter that are securely bound to the user agent MUST be used for CSRF protection (see (#csrf_countermeasures)).¶

In order to prevent mix-up attacks (see (#mix_up)), clients MUST only process redirect responses of the authorization server they sent the respective request to and from the same user agent this authorization request was initiated with. Clients MUST store the authorization server they sent an authorization request to and bind this information to the user agent and check that the authorization request was received from the correct authorization server. Clients MUST ensure that the subsequent access token request, if applicable, is sent to the same authorization server. Clients SHOULD use distinct redirect URIs for each authorization server as a means to identify the authorization server a particular response came from.¶

An AS that redirects a request potentially containing user credentials MUST avoid forwarding these user credentials accidentally (see Section 9.7.2 for details).¶

9.8. Authorization Codes

The transmission of authorization codes MUST be made over a secure channel, and the client MUST require the use of TLS with its redirect URI if the URI identifies a network resource. Since authorization codes are transmitted via user-agent redirections, they could potentially be disclosed through user-agent history and HTTP referrer headers.¶

Authorization codes MUST be short lived and single-use. If the authorization server observes multiple attempts to exchange an authorization code for an access token, the authorization server SHOULD attempt to revoke all refresh and access tokens already granted based on the compromised authorization code.¶

If the client can be authenticated, the authorization servers MUST authenticate the client and ensure that the authorization code was issued to the same client.¶

Clients MUST prevent injection (replay) of authorization codes into the authorization response by attackers. To this end, using code_challenge and code_verifier is REQUIRED for clients and authorization servers MUST enforce their use, unless both of the following criteria are met:¶

• The client is a confidential client.¶
• In the specific deployment and the specific request, there is reasonable assurance for authorization server that the client implements the OpenID Connect nonce mechanism properly.¶

In this case, using and enforcing code_challenge and code_verifier is still RECOMMENDED.¶

The code_challenge or OpenID Connect nonce value MUST be transaction-specific and securely bound to the client and the user agent in which the transaction was started. If a transaction leads to an error, fresh values for code_challenge or nonce MUST be chosen.¶

Historic note: Although PKCE [RFC7636] was originally designed as a mechanism to protect native apps, this advice applies to all kinds of OAuth clients, including web applications and other confidential clients.¶

Clients SHOULD use code challenge methods that do not expose the code_verifier in the authorization request. Otherwise, attackers that can read the authorization request (cf. Attacker A4 in (#secmodel)) can break the security provided by this mechanism. Currently, S256 is the only such method.¶

When an authorization code arrives at the token endpoint, the authorization server MUST do the following check:¶

1. If there was a code_challenge in the authorization request for which this code was issued, there must be a code_verifier in the token request, and it MUST be verified according to the steps in Section 4.1.3. (This is no change from the current behavior in [RFC7636].)¶
2. If there was no code_challenge in the authorization request, any request to the token endpoint containing a code_verifier MUST be rejected.¶

Authorization servers MUST support the code_challenge and code_verifier parameters.¶

Authorization servers MUST provide a way to detect their support for the code_challenge mechanism. To this end, they MUST either (a) publish the element code_challenge_methods_supported in their AS metadata ([RFC8414]) containing the supported code_challenge_methods (which can be used by the client to detect support) or (b) provide a deployment-specific way to ensure or determine support by the AS.¶

9.9. Request Confidentiality

Access tokens, refresh tokens, authorization codes, and client credentials MUST NOT be transmitted in the clear.¶

The state and scope parameters SHOULD NOT include sensitive client or resource owner information in plain text, as they can be transmitted over insecure channels or stored insecurely.¶

9.10. Ensuring Endpoint Authenticity

In order to prevent man-in-the-middle attacks, the authorization server MUST require the use of TLS with server authentication as defined by [RFC2818] for any request sent to the authorization and token endpoints. The client MUST validate the authorization server's TLS certificate as defined by [RFC6125] and in accordance with its requirements for server identity authentication.¶

9.11. Credentials-Guessing Attacks

The authorization server MUST prevent attackers from guessing access tokens, authorization codes, refresh tokens, resource owner passwords, and client credentials.¶

The probability of an attacker guessing generated tokens (and other credentials not intended for handling by end-users) MUST be less than or equal to 2^(-128) and SHOULD be less than or equal to 2^(-160).¶

The authorization server MUST utilize other means to protect credentials intended for end-user usage.¶

9.12. Phishing Attacks

Wide deployment of this and similar protocols may cause end-users to become inured to the practice of being redirected to websites where they are asked to enter their passwords. If end-users are not careful to verify the authenticity of these websites before entering their credentials, it will be possible for attackers to exploit this practice to steal resource owners' passwords.¶

Service providers should attempt to educate end-users about the risks phishing attacks pose and should provide mechanisms that make it easy for end-users to confirm the authenticity of their sites. Client developers should consider the security implications of how they interact with the user-agent (e.g., external, embedded), and the ability of the end-user to verify the authenticity of the authorization server.¶

To reduce the risk of phishing attacks, the authorization servers MUST require the use of TLS on every endpoint used for end-user interaction.¶

9.13. Fake External User-Agents in Native Apps

The native app that is initiating the authorization request has a large degree of control over the user interface and can potentially present a fake external user-agent, that is, an embedded user-agent made to appear as an external user-agent.¶

When all good actors are using external user-agents, the advantage is that it is possible for security experts to detect bad actors, as anyone faking an external user-agent is provably bad. On the other hand, if good and bad actors alike are using embedded user-agents, bad actors don't need to fake anything, making them harder to detect. Once a malicious app is detected, it may be possible to use this knowledge to blacklist the app's signature in malware scanning software, take removal action (in the case of apps distributed by app stores) and other steps to reduce the impact and spread of the malicious app.¶

Authorization servers can also directly protect against fake external user-agents by requiring an authentication factor only available to true external user-agents.¶

Users who are particularly concerned about their security when using in-app browser tabs may also take the additional step of opening the request in the full browser from the in-app browser tab and complete the authorization there, as most implementations of the in-app browser tab pattern offer such functionality.¶

9.14. Malicious External User-Agents in Native Apps

If a malicious app is able to configure itself as the default handler for https scheme URIs in the operating system, it will be able to intercept authorization requests that use the default browser and abuse this position of trust for malicious ends such as phishing the user.¶

This attack is not confined to OAuth; a malicious app configured in this way would present a general and ongoing risk to the user beyond OAuth usage by native apps. Many operating systems mitigate this issue by requiring an explicit user action to change the default handler for http and https scheme URIs.¶

9.15. Cross-Site Request Forgery

An attacker might attempt to inject a request to the redirect URI of the legitimate client on the victim's device, e.g., to cause the client to access resources under the attacker's control. This is a variant of an attack known as Cross-Site Request Forgery (CSRF).¶

The traditional countermeasure are CSRF tokens that are bound to the user agent and passed in the state parameter to the authorization server as described in [RFC6819]. The same protection is provided by the code_verifier parameter or the OpenID Connect nonce value.¶

When using code_verifier instead of state or nonce for CSRF protection, it is important to note that:¶

• Clients MUST ensure that the AS supports the code_challenge_method intended to be used by the client. If an authorization server does not support the requested method, state or nonce MUST be used for CSRF protection instead.¶
• If state is used for carrying application state, and integrity of its contents is a concern, clients MUST protect state against tampering and swapping. This can be achieved by binding the contents of state to the browser session and/or signed/encrypted state values [I-D.bradley-oauth-jwt-encoded-state].¶

AS therefore MUST provide a way to detect their supported code challenge methods either via AS metadata according to [RFC8414] or provide a deployment-specific way to ensure or determine support.¶

9.16. Clickjacking

As described in Section 4.4.1.9 of [RFC6819], the authorization request is susceptible to clickjacking. An attacker can use this vector to obtain the user's authentication credentials, change the scope of access granted to the client, and potentially access the user's resources.¶

Authorization servers MUST prevent clickjacking attacks. Multiple countermeasures are described in [RFC6819], including the use of the X-Frame-Options HTTP response header field and frame-busting JavaScript. In addition to those, authorization servers SHOULD also use Content Security Policy (CSP) level 2 [CSP-2] or greater.¶

To be effective, CSP must be used on the authorization endpoint and, if applicable, other endpoints used to authenticate the user and authorize the client (e.g., the device authorization endpoint, login pages, error pages, etc.). This prevents framing by unauthorized origins in user agents that support CSP. The client MAY permit being framed by some other origin than the one used in its redirection endpoint. For this reason, authorization servers SHOULD allow administrators to configure allowed origins for particular clients and/or for clients to register these dynamically.¶

Using CSP allows authorization servers to specify multiple origins in a single response header field and to constrain these using flexible patterns (see [CSP-2] for details). Level 2 of this standard provides a robust mechanism for protecting against clickjacking by using policies that restrict the origin of frames (using frame-ancestors) together with those that restrict the sources of scripts allowed to execute on an HTML page (by using script-src). A non-normative example of such a policy is shown in the following listing:¶

HTTP/1.1 200 OK Content-Security-Policy: frame-ancestors https://ext.example.org:8000 Content-Security-Policy: script-src 'self' X-Frame-Options: ALLOW-FROM https://ext.example.org:8000 ... ¶

Because some user agents do not support [CSP-2], this technique SHOULD be combined with others, including those described in [RFC6819], unless such legacy user agents are explicitly unsupported by the authorization server. Even in such cases, additional countermeasures SHOULD still be employed.¶

9.17. Code Injection and Input Validation

A code injection attack occurs when an input or otherwise external variable is used by an application unsanitized and causes modification to the application logic. This may allow an attacker to gain access to the application device or its data, cause denial of service, or introduce a wide range of malicious side-effects.¶

The authorization server and client MUST sanitize (and validate when possible) any value received - in particular, the value of the state and redirect_uri parameters.¶

9.18. Open Redirectors

The following attacks can occur when an AS or client has an open redirector. An open redirector is an endpoint that forwards a user's browser to an arbitrary URI obtained from a query parameter.¶

9.19. Authorization Server Mix-Up Mitigation in Native Apps

(TODO: merge this with the regular mix-up section when it is brought in)¶

To protect against a compromised or malicious authorization server attacking another authorization server used by the same app, it is REQUIRED that a unique redirect URI is used for each authorization server used by the app (for example, by varying the path component), and that authorization responses are rejected if the redirect URI they were received on doesn't match the redirect URI in an outgoing authorization request.¶

The native app MUST store the redirect URI used in the authorization request with the authorization session data (i.e., along with state and other related data) and MUST verify that the URI on which the authorization response was received exactly matches it.¶

The requirement of Section 9.2, specifically that authorization servers reject requests with URIs that don't match what was registered, is also required to prevent such attacks.¶

9.20. Embedded User Agents in Native Apps

Embedded user-agents are a technically possible method for authorizing native apps. These embedded user-agents are unsafe for use by third parties to the authorization server by definition, as the app that hosts the embedded user-agent can access the user's full authentication credential, not just the OAuth authorization grant that was intended for the app.¶

In typical web-view-based implementations of embedded user-agents, the host application can record every keystroke entered in the login form to capture usernames and passwords, automatically submit forms to bypass user consent, and copy session cookies and use them to perform authenticated actions as the user.¶

Even when used by trusted apps belonging to the same party as the authorization server, embedded user-agents violate the principle of least privilege by having access to more powerful credentials than they need, potentially increasing the attack surface.¶

Encouraging users to enter credentials in an embedded user-agent without the usual address bar and visible certificate validation features that browsers have makes it impossible for the user to know if they are signing in to the legitimate site; even when they are, it trains them that it's OK to enter credentials without validating the site first.¶

Aside from the security concerns, embedded user-agents do not share the authentication state with other apps or the browser, requiring the user to log in for every authorization request, which is often considered an inferior user experience.¶

9.21. Other Recommendations

Authorization servers SHOULD NOT allow clients to influence their client_id or sub value or any other claim if that can cause confusion with a genuine resource owner (see (#client_impersonating)).¶

10.1. Using Inter-App URI Communication for OAuth in Native Apps

Just as URIs are used for OAuth on the web to initiate the authorization request and return the authorization response to the requesting website, URIs can be used by native apps to initiate the authorization request in the device's browser and return the response to the requesting native app.¶

By adopting the same methods used on the web for OAuth, benefits seen in the web context like the usability of a single sign-on session and the security of a separate authentication context are likewise gained in the native app context. Reusing the same approach also reduces the implementation complexity and increases interoperability by relying on standards-based web flows that are not specific to a particular platform.¶

Native apps MUST use an external user-agent to perform OAuth authorization requests. This is achieved by opening the authorization request in the browser (detailed in Section 10.2) and using a redirect URI that will return the authorization response back to the native app (defined in Section 10.3).¶

10.2. Initiating the Authorization Request from a Native App

Native apps needing user authorization create an authorization request URI with the authorization code grant type per Section 4.1 using a redirect URI capable of being received by the native app.¶

The function of the redirect URI for a native app authorization request is similar to that of a web-based authorization request. Rather than returning the authorization response to the OAuth client's server, the redirect URI used by a native app returns the response to the app. Several options for a redirect URI that will return the authorization response to the native app in different platforms are documented in Section 10.3. Any redirect URI that allows the app to receive the URI and inspect its parameters is viable.¶

After constructing the authorization request URI, the app uses platform-specific APIs to open the URI in an external user-agent. Typically, the external user-agent used is the default browser, that is, the application configured for handling http and https scheme URIs on the system; however, different browser selection criteria and other categories of external user-agents MAY be used.¶

This best practice focuses on the browser as the RECOMMENDED external user-agent for native apps. An external user-agent designed specifically for user authorization and capable of processing authorization requests and responses like a browser MAY also be used. Other external user-agents, such as a native app provided by the authorization server may meet the criteria set out in this best practice, including using the same redirect URI properties, but their use is out of scope for this specification.¶

Some platforms support a browser feature known as "in-app browser tabs", where an app can present a tab of the browser within the app context without switching apps, but still retain key benefits of the browser such as a shared authentication state and security context. On platforms where they are supported, it is RECOMMENDED, for usability reasons, that apps use in-app browser tabs for the authorization request.¶

10.3. Receiving the Authorization Response in a Native App

There are several redirect URI options available to native apps for receiving the authorization response from the browser, the availability and user experience of which varies by platform.¶

To fully support native apps, authorization servers MUST offer at least the three redirect URI options described in the following subsections to native apps. Native apps MAY use whichever redirect option suits their needs best, taking into account platform-specific implementation details.¶

com.example.app:/oauth2redirect/example-provider

https://app.example.com/oauth2redirect/example-provider

http://127.0.0.1:51004/oauth2redirect/example-provider

http://[::1]:61023/oauth2redirect/example-provider

A.1. "client_id" Syntax

The client_id element is defined in Section 2.3.1:¶

client-id     = *VSCHAR

A.2. "client_secret" Syntax

The client_secret element is defined in Section 2.3.1:¶

client-secret = *VSCHAR

A.3. "response_type" Syntax

The response_type element is defined in Section 3.1.1 and Section 8.4:¶

response-type = response-name *( SP response-name )
response-name = 1*response-char
response-char = "_" / DIGIT / ALPHA

A.4. "scope" Syntax

The scope element is defined in Section 3.3:¶

 scope       = scope-token *( SP scope-token )
 scope-token = 1*NQCHAR

A.5. "state" Syntax

The state element is defined in Section 4.1.1, Section 4.1.2, and Section 4.1.2.1:¶

 state      = 1*VSCHAR

A.6. "redirect_uri" Syntax

The redirect_uri element is defined in Section 4.1.1, and Section 4.1.3:¶

 redirect-uri      = URI-reference

A.7. "error" Syntax

The error element is defined in Sections Section 4.1.2.1, Section 5.2, 7.2, and 8.5:¶

 error             = 1*NQSCHAR

A.8. "error_description" Syntax

The error_description element is defined in Sections Section 4.1.2.1, Section 5.2, and Section 7.3:¶

 error-description = 1*NQSCHAR

A.9. "error_uri" Syntax

The error_uri element is defined in Sections Section 4.1.2.1, Section 5.2, and 7.2:¶

 error-uri         = URI-reference

A.10. "grant_type" Syntax

The grant_type element is defined in Sections Section 4.1.3, Section 4.2.3, Section 4.2.2, Section 4.3, and Section 6:¶

 grant-type = grant-name / URI-reference
 grant-name = 1*name-char
 name-char  = "-" / "." / "_" / DIGIT / ALPHA

A.11. "code" Syntax

The code element is defined in Section 4.1.3:¶

 code       = 1*VSCHAR

A.12. "access_token" Syntax

The access_token element is defined in Section 4.2.3 and Section 5.1:¶

 access-token = 1*VSCHAR

A.13. "token_type" Syntax

The token_type element is defined in Section 5.1, and Section 8.1:¶

 token-type = type-name / URI-reference
 type-name  = 1*name-char
 name-char  = "-" / "." / "_" / DIGIT / ALPHA

A.14. "expires_in" Syntax

The expires_in element is defined in Section 5.1:¶

 expires-in = 1*DIGIT

A.15. "refresh_token" Syntax

The refresh_token element is defined in Section 5.1 and Section 6:¶

 refresh-token = 1*VSCHAR

A.16. Endpoint Parameter Syntax

The syntax for new endpoint parameters is defined in Section 8.2:¶

 param-name = 1*name-char
 name-char  = "-" / "." / "_" / DIGIT / ALPHA

A.17. "code_verifier" Syntax

ABNF for code_verifier is as follows.¶

code-verifier = 43*128unreserved
unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~"
ALPHA = %x41-5A / %x61-7A
DIGIT = %x30-39

A.18. "code_challenge" Syntax

ABNF for code_challenge is as follows.¶

code-challenge = 43*128unreserved
unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~"
ALPHA = %x41-5A / %x61-7A
DIGIT = %x30-39