feat: Auth - OAuth2 (Dovecot PassDB) (#3480)

Co-authored-by: Brennan Kinney <5098581+polarathene@users.noreply.github.com>

docs/content/config/advanced/auth-oauth2.md

@@ -0,0 +1,69 @@
+---
+title: 'Advanced | Basic OAuth2 Authentication'
+---
+
+## Introduction
+
+!!! warning "This is only a supplement to the existing account provisioners"
+
+    Accounts must still be managed via the configured [`ACCOUNT_PROVISIONER`][env::account-provisioner] (FILE or LDAP).
+
+    Reasoning for this can be found in [#3480][gh-pr::oauth2]. Future iterations on this feature may allow it to become a full account provisioner.
+
+[gh-pr::oauth2]: https://github.com/docker-mailserver/docker-mailserver/pull/3480
+[env::account-provisioner]: ../environment.md#account_provisioner
+
+The present OAuth2 support provides the capability for 3rd-party applications such as Roundcube to authenticate with DMS (dovecot) by using a token obtained from an OAuth2 provider, instead of passing passwords around.
+
+## Example (Authentik & Roundcube)
+
+This example assumes you have:
+
+- A working DMS server set up
+- An Authentik server set up ([documentation](https://goauthentik.io/docs/installation/))
+- A Roundcube server set up (either [docker](https://hub.docker.com/r/roundcube/roundcubemail/) or [bare metal](https://github.com/roundcube/roundcubemail/wiki/Installation))
+
+!!! example "Setup Instructions"
+
+    === "1. Docker Mailserver"
+        Edit the following values in `mailserver.env`:
+        ```env
+        # -----------------------------------------------
+        # --- OAUTH2 Section ----------------------------
+        # -----------------------------------------------
+
+        # empty => OAUTH2 authentication is disabled
+        # 1 => OAUTH2 authentication is enabled
+        ENABLE_OAUTH2=1
+
+        # Specify the user info endpoint URL of the oauth2 provider
+        OAUTH2_INTROSPECTION_URL=https://authentik.example.com/application/o/userinfo/
+        ```
+
+    === "2. Authentik"
+        1. Create a new OAuth2 provider
+        2. Note the client id and client secret
+        3. Set the allowed redirect url to the equivalent of `https://roundcube.example.com/index.php/login/oauth` for your RoundCube instance.
+
+    === "3. Roundcube"
+        Add the following to `oauth2.inc.php` ([documentation](https://github.com/roundcube/roundcubemail/wiki/Configuration)):
+
+        ```php
+        $config['oauth_provider'] = 'generic';
+        $config['oauth_provider_name'] = 'Authentik';
+        $config['oauth_client_id'] = '<insert client id here>';
+        $config['oauth_client_secret'] = '<insert client secret here>';
+        $config['oauth_auth_uri'] = 'https://authentik.example.com/application/o/authorize/';
+        $config['oauth_token_uri'] = 'https://authentik.example.com/application/o/token/';
+        $config['oauth_identity_uri'] = 'https://authentik.example.com/application/o/userinfo/';
+
+        // Optional: disable SSL certificate check on HTTP requests to OAuth server. For possible values, see:
+        // http://docs.guzzlephp.org/en/stable/request-options.html#verify
+        $config['oauth_verify_peer'] = false;
+
+        $config['oauth_scope'] = 'email openid profile';
+        $config['oauth_identity_fields'] = ['email'];
+
+        // Boolean: automatically redirect to OAuth login when opening Roundcube without a valid session
+        $config['oauth_login_redirect'] = false;
+        ```

docs/content/config/environment.md

@@ -54,7 +54,15 @@ The Group ID assigned to the static vmail group for `/var/mail` (_Mail storage m
 
 Configures the provisioning source of user accounts (including aliases) for user queries and authentication by services managed by DMS (_Postfix and Dovecot_).
 
-User provisioning via OIDC is planned for the future, see [this tracking issue](https://github.com/docker-mailserver/docker-mailserver/issues/2713).
+!!! tip "OAuth2 Support"
+
+    Presently DMS supports OAuth2 only as an supplementary authentication method. 
+
+    - A third-party service must provide a valid token for the user which Dovecot validates with the authentication service provider. To enable this feature reference the [OAuth2 configuration example guide][docs::auth::oauth2-config-guide].
+    - User accounts must be provisioned to receive mail via one of the supported `ACCOUNT_PROVISIONER` providers.
+    - User provisioning via OIDC is planned for the future, see [this tracking issue](https://github.com/docker-mailserver/docker-mailserver/issues/2713).
+
+[docs::auth::oauth2-config-guide]: ./advanced/auth-oauth2.md
 
 - **empty** => use FILE
 - LDAP => use LDAP authentication
@@ -716,10 +724,20 @@ Enable or disable `getmail`.
 
 - **5** => `getmail` The number of minutes for the interval. Min: 1; Max: 30; Default: 5.
 
+
+#### OAUTH2
+
+##### ENABLE_OAUTH2
+
+- **empty** => OAUTH2 authentication is disabled
+- 1 => OAUTH2 authentication is enabled
+
+##### OAUTH2_INTROSPECTION_URL
+
+- => Specify the user info endpoint URL of the oauth2 provider (_eg: `https://oauth2.example.com/userinfo/`_)
+
 #### LDAP
 
-
-
 ##### LDAP_START_TLS
 
 - **empty** => no