Record OAuth client type at registration (#21316)

The OAuth spec [defines two types of
client](https://datatracker.ietf.org/doc/html/rfc6749#section-2.1),
confidential and public. Previously Gitea assumed all clients to be
confidential.

> OAuth defines two client types, based on their ability to authenticate
securely with the authorization server (i.e., ability to
>   maintain the confidentiality of their client credentials):
>
>   confidential
> Clients capable of maintaining the confidentiality of their
credentials (e.g., client implemented on a secure server with
> restricted access to the client credentials), or capable of secure
client authentication using other means.
>
>   **public
> Clients incapable of maintaining the confidentiality of their
credentials (e.g., clients executing on the device used by the resource
owner, such as an installed native application or a web browser-based
application), and incapable of secure client authentication via any
other means.**
>
> The client type designation is based on the authorization server's
definition of secure authentication and its acceptable exposure levels
of client credentials. The authorization server SHOULD NOT make
assumptions about the client type.

 https://datatracker.ietf.org/doc/html/rfc8252#section-8.4

> Authorization servers MUST record the client type in the client
registration details in order to identify and process requests
accordingly.

Require PKCE for public clients:
https://datatracker.ietf.org/doc/html/rfc8252#section-8.1

> Authorization servers SHOULD reject authorization requests from native
apps that don't use PKCE by returning an error message

Fixes #21299

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

3 files changed, 32 insertions, 15 deletions

diff --git a/routers/api/v1/user/app.go b/routers/api/v1/user/app.go
index a94db79239..14f1592591 100644
--- a/routers/api/v1/user/app.go
+++ b/routers/api/v1/user/app.go
@@ -213,9 +213,10 @@ func CreateOauth2Application(ctx *context.APIContext) {
 	data := web.GetForm(ctx).(*api.CreateOAuth2ApplicationOptions)
 
 	app, err := auth_model.CreateOAuth2Application(ctx, auth_model.CreateOAuth2ApplicationOptions{
-		Name:         data.Name,
-		UserID:       ctx.Doer.ID,
-		RedirectURIs: data.RedirectURIs,
+		Name:               data.Name,
+		UserID:             ctx.Doer.ID,
+		RedirectURIs:       data.RedirectURIs,
+		ConfidentialClient: data.ConfidentialClient,
 	})
 	if err != nil {
 		ctx.Error(http.StatusBadRequest, "", "error creating oauth2 application")
@@ -363,10 +364,11 @@ func UpdateOauth2Application(ctx *context.APIContext) {
 	data := web.GetForm(ctx).(*api.CreateOAuth2ApplicationOptions)
 
 	app, err := auth_model.UpdateOAuth2Application(auth_model.UpdateOAuth2ApplicationOptions{
-		Name:         data.Name,
-		UserID:       ctx.Doer.ID,
-		ID:           appID,
-		RedirectURIs: data.RedirectURIs,
+		Name:               data.Name,
+		UserID:             ctx.Doer.ID,
+		ID:                 appID,
+		RedirectURIs:       data.RedirectURIs,
+		ConfidentialClient: data.ConfidentialClient,
 	})
 	if err != nil {
 		if auth_model.IsErrOauthClientIDInvalid(err) || auth_model.IsErrOAuthApplicationNotFound(err) {
diff --git a/routers/web/auth/oauth.go b/routers/web/auth/oauth.go
index c98385c8f6..9c929d990e 100644
--- a/routers/web/auth/oauth.go
+++ b/routers/web/auth/oauth.go
@@ -438,8 +438,21 @@ func AuthorizeOAuth(ctx *context.Context) {
 			log.Error("Unable to save changes to the session: %v", err)
 		}
 	case "":
-		break
+		// "Authorization servers SHOULD reject authorization requests from native apps that don't use PKCE by returning an error message"
+		// https://datatracker.ietf.org/doc/html/rfc8252#section-8.1
+		if !app.ConfidentialClient {
+			// "the authorization endpoint MUST return the authorization error response with the "error" value set to "invalid_request""
+			// https://datatracker.ietf.org/doc/html/rfc7636#section-4.4.1
+			handleAuthorizeError(ctx, AuthorizeError{
+				ErrorCode:        ErrorCodeInvalidRequest,
+				ErrorDescription: "PKCE is required for public clients",
+				State:            form.State,
+			}, form.RedirectURI)
+			return
+		}
 	default:
+		// "If the server supporting PKCE does not support the requested transformation, the authorization endpoint MUST return the authorization error response with "error" value set to "invalid_request"."
+		// https://www.rfc-editor.org/rfc/rfc7636#section-4.4.1
 		handleAuthorizeError(ctx, AuthorizeError{
 			ErrorCode:        ErrorCodeInvalidRequest,
 			ErrorDescription: "unsupported code challenge method",
diff --git a/routers/web/user/setting/oauth2_common.go b/routers/web/user/setting/oauth2_common.go
index f02f6ab041..49ee5c7c2f 100644
--- a/routers/web/user/setting/oauth2_common.go
+++ b/routers/web/user/setting/oauth2_common.go
@@ -39,9 +39,10 @@ func (oa *OAuth2CommonHandlers) AddApp(ctx *context.Context) {
 
 	// TODO validate redirect URI
 	app, err := auth.CreateOAuth2Application(ctx, auth.CreateOAuth2ApplicationOptions{
-		Name:         form.Name,
-		RedirectURIs: []string{form.RedirectURI},
-		UserID:       oa.OwnerID,
+		Name:               form.Name,
+		RedirectURIs:       []string{form.RedirectURI},
+		UserID:             oa.OwnerID,
+		ConfidentialClient: form.ConfidentialClient,
 	})
 	if err != nil {
 		ctx.ServerError("CreateOAuth2Application", err)
@@ -90,10 +91,11 @@ func (oa *OAuth2CommonHandlers) EditSave(ctx *context.Context) {
 	// TODO validate redirect URI
 	var err error
 	if ctx.Data["App"], err = auth.UpdateOAuth2Application(auth.UpdateOAuth2ApplicationOptions{
-		ID:           ctx.ParamsInt64("id"),
-		Name:         form.Name,
-		RedirectURIs: []string{form.RedirectURI},
-		UserID:       oa.OwnerID,
+		ID:                 ctx.ParamsInt64("id"),
+		Name:               form.Name,
+		RedirectURIs:       []string{form.RedirectURI},
+		UserID:             oa.OwnerID,
+		ConfidentialClient: form.ConfidentialClient,
 	}); err != nil {
 		ctx.ServerError("UpdateOAuth2Application", err)
 		return