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# V51 OAuth 2.0 Protocol

This section describes and summarizes the [best current security practices](https://datatracker.ietf.org/doc/html/draft-ietf-oauth-security-topics#name-best-practices) for OAuth 2.0 as derived from its [RFC 6750](https://www.rfc-editor.org/info/rfc6750) and [6749](https://www.rfc-editor.org/info/rfc6749) for every OAuth implementor. OAuth became the standard for API protection and the basis for federated login using OpenID Connect. OpenID Connect 1.0 is a simple identity layer on top of the OAuth 2.0 protocol. It enables clients to verify the identity of the end-user based on the authentication performed by an authorization server, as well as to obtain basic profile information about the end-user in an interoperable and REST-like manner.
This section describes and summarizes the [best current security practices](https://datatracker.ietf.org/doc/html/draft-ietf-oauth-security-topics#name-best-practices) for OAuth 2.0 as derived from the relevant RFCs, [6750](https://www.rfc-editor.org/info/rfc6750) and [6749](https://www.rfc-editor.org/info/rfc6749). OAuth has become the standard for API authorization and the basis for federated login using OpenID Connect. OpenID Connect 1.0 is a simple identity layer on top of the OAuth 2.0 protocol. It enables clients to verify the identity of the end-user based on the authentication performed by an authorization server, as well as to obtain basic profile information about the end-user in an interoperable and REST-like manner.

There are various different personas in the OAuth process, described in more detail in the terminology section below. The requirements in this chapter are structured based on those personas as requirements for one persona may not be relevant for a different persona.

## Terminology

* Access tokens - provides an abstraction, replacing different authorization constructs (e.g., username and password, assertion) for a single token understood by the resource server. This abstraction enables issuing access tokens valid for a short time period, as well as removing the resource server's need to understand a wide range of authentication schemes.
* Refresh tokens - are credentials used to obtain access tokens. These 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).
* Client - generally refers to 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 (AS) - refers to the server issuing access tokens to the client after successfully authenticating the resource owner and obtaining authorization.
* Resource Owner (RO) - refers to 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.
* Resource Server (RS) - refers to the server hosting the protected resources, capable of accepting and responding to protected resource requests using access tokens.
* Access tokens - These provide an abstraction for a session mechanism, replacing different authorization constructs (e.g., username and password, assertion) for a single token understood by the resource server. This abstraction enables issuing access tokens valid for a short time period, as well as removing the resource server's need to understand a wide range of authentication schemes.
* Refresh tokens - These are credentials used to obtain access tokens without a full reauthentication. These 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).
* Client - This refers to 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 (AS) - This refers to the server issuing access tokens to the client after successfully authenticating the resource owner and obtaining authorization.
* Resource Owner (RO) - This refers to 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.
* Resource Server (RS) - This refers to the server hosting the protected resources, capable of accepting and responding to protected resource requests using access tokens.

## OAuth 2.0 Basics

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| # | Description | L1 | L2 | L3 |
| :---: | :--- | :---: | :---: | :---: |
| **51.1.1** | [ADDED] Verify that the Authorization Server supports CSRF protection either using the mechanism provided by PKCE, the "nonce" parameter or the "state" parameter. | ✓ | ✓ | ✓ |
| **51.1.2** | [ADDED] Verify that the replay of authorization codes into the authorization response is prevented either by using the PKCE flow or alternatively the OpenID Connect "nonce" parameter and the respective Claim in the ID Token. The PKCE challenge or OpenID Connect "nonce" must be transaction-specific and securely bound to the client and the user agent in which the transaction was started. | ✓ | ✓ | ✓ |
| **51.1.3** | [ADDED] Verify that Authorization Servers are mitigating PKCE Downgrade Attacks by ensuring a token request containing a "code_verifier" parameter is accepted only if a "code_challenge" parameter was present in the authorization request. | ✓ | ✓ | ✓ |
| **51.1.2** | [ADDED] Verify that the replay of authorization codes into the authorization response is prevented either by using the PKCE flow or alternatively the OpenID Connect "nonce" parameter and the respective claim in the ID Token. The PKCE challenge or OpenID Connect "nonce" must be transaction-specific and securely bound to the client and the user agent in which the transaction was started. | ✓ | ✓ | ✓ |
| **51.1.3** | [ADDED] Verify that the Authorization Server is mitigating PKCE Downgrade Attacks by ensuring a token request containing a "code_verifier" parameter is accepted only if a "code_challenge" parameter was present in the authorization request. | ✓ | ✓ | ✓ |
| **51.1.4** | [ADDED] Verify that refresh tokens are sender-constrained or use refresh token rotation. | ✓ | ✓ | ✓ |
| **51.1.5** | [ADDED] Verify that the Authorization Server publishes the element "code_challenge_methods_supported" in their Authorization Server metadata containing the supported PKCE challenge methods. | ✓ | ✓ | ✓ |

## V51.2 OAuth Client

| # | Description | L1 | L2 | L3 |
| :---: | :--- | :---: | :---: | :---: |
| **51.2.1** | [ADDED] Verify that when an OAuth Client can interact with more than one Authorization Server, Clients should verify that the issuer "iss" parameter value is what it expected from the authorization response to prevent against mix-up attacks. [reference link for Mix-up attacks: https://datatracker.ietf.org/doc/html/draft-ietf-oauth-security-topics-18#mix_up] In the absence of "iss" parameter, Clients may instead use distinct redirect URIs to identify authorization endpoints and token endpoints. | ✓ | ✓ | ✓ |
| **51.2.2** | [ADDED] Verify that the Client is using the PKCE flow or alternatively the OpenID Connect "nonce" parameter and the respective Claim in the ID Token. The PKCE challenge or OpenID Connect "nonce" must be transaction-specific and securely bound to the client and the user agent in which the transaction was started. | ✓ | ✓ | ✓ |
| **51.2.1** | [ADDED] Verify that if the Client can interact with more than one Authorization Server, it verifies that the issuer "iss" parameter value is what it expected from the authorization response to prevent against mix-up attacks. In the absence of an "iss" parameter, the Client may instead use distinct redirect URIs to identify authorization endpoints and token endpoints. | ✓ | ✓ | ✓ |
| **51.2.2** | [ADDED] Verify that the Client is using the PKCE flow or alternatively the OpenID Connect "nonce" parameter and the respective claim in the ID Token. The PKCE challenge or OpenID Connect "nonce" must be transaction-specific and securely bound to the client and the user agent in which the transaction was started. | ✓ | ✓ | ✓ |
| **51.2.3** | [ADDED] Verify that if a Client sends a valid PKCE "code_challenge" parameter in the authorization request, the Authorization Server enforces the correct usage of "code_verifier" at the token endpoint. | ✓ | ✓ | ✓ |
| **51.2.4** | [ADDED] Verify that Clients are utilizing the parameters "scope" and "resource", respectively to determine the Resource Server they want to access. | ✓ | ✓ | ✓ |
| **51.2.5** | [ADDED] Verify that Clients are utilizing the parameters "scope" and "authorization_details" to determine the requested/related resources and/or actions the access token are restricted to. | ✓ | ✓ | ✓ |
| **51.2.4** | [ADDED] Verify that the Client uses the "scope" or "resource" parameter to determine the Resource Server that it wants to access. | ✓ | ✓ | ✓ |
| **51.2.5** | [ADDED] Verify that the Client uses the "scope" and "authorization_details" parameters to determine the requested or related resources or the actions that the access token is restricted to. | ✓ | ✓ | ✓ |

* Reference link for Mix-up attacks: https://datatracker.ietf.org/doc/html/draft-ietf-oauth-security-topics-18#mix_up

## V51.3 Resource Server

| # | Description | L1 | L2 | L3 |
| :---: | :--- | :---: | :---: | :---: |
| **51.3.1** | [ADDED] Verify that Resource Servers are using mechanisms for sender-constraining access tokens to prevent token replay, such as Mutual TLS for OAuth 2.0 or OAuth Demonstration of Proof of Possession (DPoP). | ✓ | ✓ | ✓ |
| **51.3.2** | [ADDED] Verify that access tokens are restricted to certain Resource Servers (audience restriction), preferably to a single Resource Server. Every Resource Server is obliged to verify, for every request, whether the access token sent with that request was meant to be used for that particular Resource Server. If not, the Resource Server must refuse to serve the respective request. | ✓ | ✓ | ✓ |
| **51.3.3** | [ADDED] Verify that access tokens are restricted to certain resources and actions on Resource Servers or resources. Every Resource Server is obliged to verify, for every request, whether the access token sent with that request was meant to be used for that particular action on the particular resource. If not, the Resource Server must refuse to serve the respective request. | ✓ | ✓ | ✓ |
| **51.3.1** | [ADDED] Verify that the Resource Server is using a mechanisms for sender-constraining access tokens to prevent token replay, such as Mutual TLS for OAuth 2.0 or OAuth Demonstration of Proof of Possession (DPoP). | ✓ | ✓ | ✓ |
| **51.3.2** | [ADDED] Verify that access tokens are restricted to certain Resource Servers (audience restriction), preferably to a single Resource Server and that the Resource Server verifies this for every request. | ✓ | ✓ | ✓ |
| **51.3.3** | [ADDED] Verify that access tokens are restricted to certain resources and actions on Resource Servers or resources and that the Resource Server verifies this for every request. | ✓ | ✓ | ✓ |

## V51.4 Resource Owner

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