[feature] oob oauth token support (#889)

* move helpful advice into oauth server

* rewrite HandleAuthorizeRequest to allow oob

internal/api/client/auth/auth.go

@@ -23,6 +23,7 @@ import (
 
 	"github.com/superseriousbusiness/gotosocial/internal/api"
 	"github.com/superseriousbusiness/gotosocial/internal/db"
+	"github.com/superseriousbusiness/gotosocial/internal/oauth"
 	"github.com/superseriousbusiness/gotosocial/internal/oidc"
 	"github.com/superseriousbusiness/gotosocial/internal/processing"
 	"github.com/superseriousbusiness/gotosocial/internal/router"
@@ -92,5 +93,7 @@ func (m *Module) Route(s router.Router) error {
 	s.AttachHandler(http.MethodPost, OauthAuthorizePath, m.AuthorizePOSTHandler)
 
 	s.AttachHandler(http.MethodGet, CallbackPath, m.CallbackGETHandler)
+
+	s.AttachHandler(http.MethodGet, oauth.OOBTokenPath, m.OobHandler)
 	return nil
 }

internal/api/client/auth/authorize.go

@@ -33,12 +33,9 @@ import (
 	"github.com/superseriousbusiness/gotosocial/internal/db"
 	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
 	"github.com/superseriousbusiness/gotosocial/internal/gtsmodel"
+	"github.com/superseriousbusiness/gotosocial/internal/oauth"
 )
 
-// helpfulAdvice is a handy hint to users;
-// particularly important during the login flow
-var helpfulAdvice = "If you arrived at this error during a login/oauth flow, please try clearing your session cookies and logging in again; if problems persist, make sure you're using the correct credentials"
-
 // AuthorizeGETHandler should be served as GET at https://example.org/oauth/authorize
 // The idea here is to present an oauth authorize page to the user, with a button
 // that they have to click to accept.
@@ -57,7 +54,7 @@ func (m *Module) AuthorizeGETHandler(c *gin.Context) {
 		form := &model.OAuthAuthorize{}
 		if err := c.ShouldBind(form); err != nil {
 			m.clearSession(s)
-			api.ErrorHandler(c, gtserror.NewErrorBadRequest(err, helpfulAdvice), m.processor.InstanceGet)
+			api.ErrorHandler(c, gtserror.NewErrorBadRequest(err, oauth.HelpfulAdvice), m.processor.InstanceGet)
 			return
 		}
 
@@ -76,7 +73,7 @@ func (m *Module) AuthorizeGETHandler(c *gin.Context) {
 	if !ok || clientID == "" {
 		m.clearSession(s)
 		err := fmt.Errorf("key %s was not found in session", sessionClientID)
-		api.ErrorHandler(c, gtserror.NewErrorBadRequest(err, helpfulAdvice), m.processor.InstanceGet)
+		api.ErrorHandler(c, gtserror.NewErrorBadRequest(err, oauth.HelpfulAdvice), m.processor.InstanceGet)
 		return
 	}
 
@@ -86,9 +83,9 @@ func (m *Module) AuthorizeGETHandler(c *gin.Context) {
 		safe := fmt.Sprintf("application for %s %s could not be retrieved", sessionClientID, clientID)
 		var errWithCode gtserror.WithCode
 		if err == db.ErrNoEntries {
-			errWithCode = gtserror.NewErrorBadRequest(err, safe, helpfulAdvice)
+			errWithCode = gtserror.NewErrorBadRequest(err, safe, oauth.HelpfulAdvice)
 		} else {
-			errWithCode = gtserror.NewErrorInternalError(err, safe, helpfulAdvice)
+			errWithCode = gtserror.NewErrorInternalError(err, safe, oauth.HelpfulAdvice)
 		}
 		api.ErrorHandler(c, errWithCode, m.processor.InstanceGet)
 		return
@@ -100,9 +97,9 @@ func (m *Module) AuthorizeGETHandler(c *gin.Context) {
 		safe := fmt.Sprintf("user with id %s could not be retrieved", userID)
 		var errWithCode gtserror.WithCode
 		if err == db.ErrNoEntries {
-			errWithCode = gtserror.NewErrorBadRequest(err, safe, helpfulAdvice)
+			errWithCode = gtserror.NewErrorBadRequest(err, safe, oauth.HelpfulAdvice)
 		} else {
-			errWithCode = gtserror.NewErrorInternalError(err, safe, helpfulAdvice)
+			errWithCode = gtserror.NewErrorInternalError(err, safe, oauth.HelpfulAdvice)
 		}
 		api.ErrorHandler(c, errWithCode, m.processor.InstanceGet)
 		return
@@ -114,9 +111,9 @@ func (m *Module) AuthorizeGETHandler(c *gin.Context) {
 		safe := fmt.Sprintf("account with id %s could not be retrieved", user.AccountID)
 		var errWithCode gtserror.WithCode
 		if err == db.ErrNoEntries {
-			errWithCode = gtserror.NewErrorBadRequest(err, safe, helpfulAdvice)
+			errWithCode = gtserror.NewErrorBadRequest(err, safe, oauth.HelpfulAdvice)
 		} else {
-			errWithCode = gtserror.NewErrorInternalError(err, safe, helpfulAdvice)
+			errWithCode = gtserror.NewErrorInternalError(err, safe, oauth.HelpfulAdvice)
 		}
 		api.ErrorHandler(c, errWithCode, m.processor.InstanceGet)
 		return
@@ -131,7 +128,7 @@ func (m *Module) AuthorizeGETHandler(c *gin.Context) {
 	if !ok || redirect == "" {
 		m.clearSession(s)
 		err := fmt.Errorf("key %s was not found in session", sessionRedirectURI)
-		api.ErrorHandler(c, gtserror.NewErrorBadRequest(err, helpfulAdvice), m.processor.InstanceGet)
+		api.ErrorHandler(c, gtserror.NewErrorBadRequest(err, oauth.HelpfulAdvice), m.processor.InstanceGet)
 		return
 	}
 
@@ -139,7 +136,7 @@ func (m *Module) AuthorizeGETHandler(c *gin.Context) {
 	if !ok || scope == "" {
 		m.clearSession(s)
 		err := fmt.Errorf("key %s was not found in session", sessionScope)
-		api.ErrorHandler(c, gtserror.NewErrorBadRequest(err, helpfulAdvice), m.processor.InstanceGet)
+		api.ErrorHandler(c, gtserror.NewErrorBadRequest(err, oauth.HelpfulAdvice), m.processor.InstanceGet)
 		return
 	}
 
@@ -208,7 +205,7 @@ func (m *Module) AuthorizePOSTHandler(c *gin.Context) {
 	}
 
 	if len(errs) != 0 {
-		errs = append(errs, helpfulAdvice)
+		errs = append(errs, oauth.HelpfulAdvice)
 		api.ErrorHandler(c, gtserror.NewErrorBadRequest(errors.New("one or more missing keys on session during AuthorizePOSTHandler"), errs...), m.processor.InstanceGet)
 		return
 	}
@@ -219,9 +216,9 @@ func (m *Module) AuthorizePOSTHandler(c *gin.Context) {
 		safe := fmt.Sprintf("user with id %s could not be retrieved", userID)
 		var errWithCode gtserror.WithCode
 		if err == db.ErrNoEntries {
-			errWithCode = gtserror.NewErrorBadRequest(err, safe, helpfulAdvice)
+			errWithCode = gtserror.NewErrorBadRequest(err, safe, oauth.HelpfulAdvice)
 		} else {
-			errWithCode = gtserror.NewErrorInternalError(err, safe, helpfulAdvice)
+			errWithCode = gtserror.NewErrorInternalError(err, safe, oauth.HelpfulAdvice)
 		}
 		api.ErrorHandler(c, errWithCode, m.processor.InstanceGet)
 		return
@@ -233,9 +230,9 @@ func (m *Module) AuthorizePOSTHandler(c *gin.Context) {
 		safe := fmt.Sprintf("account with id %s could not be retrieved", user.AccountID)
 		var errWithCode gtserror.WithCode
 		if err == db.ErrNoEntries {
-			errWithCode = gtserror.NewErrorBadRequest(err, safe, helpfulAdvice)
+			errWithCode = gtserror.NewErrorBadRequest(err, safe, oauth.HelpfulAdvice)
 		} else {
-			errWithCode = gtserror.NewErrorInternalError(err, safe, helpfulAdvice)
+			errWithCode = gtserror.NewErrorInternalError(err, safe, oauth.HelpfulAdvice)
 		}
 		api.ErrorHandler(c, errWithCode, m.processor.InstanceGet)
 		return
@@ -245,8 +242,10 @@ func (m *Module) AuthorizePOSTHandler(c *gin.Context) {
 		return
 	}
 
-	// we're done with the session now, so just clear it out
-	m.clearSession(s)
+	if redirectURI != oauth.OOBURI {
+		// we're done with the session now, so just clear it out
+		m.clearSession(s)
+	}
 
 	// we have to set the values on the request form
 	// so that they're picked up by the oauth server
@@ -263,8 +262,8 @@ func (m *Module) AuthorizePOSTHandler(c *gin.Context) {
 		c.Request.Form.Set("state", clientState)
 	}
 
-	if err := m.processor.OAuthHandleAuthorizeRequest(c.Writer, c.Request); err != nil {
-		api.ErrorHandler(c, gtserror.NewErrorBadRequest(err, err.Error(), helpfulAdvice), m.processor.InstanceGet)
+	if errWithCode := m.processor.OAuthHandleAuthorizeRequest(c.Writer, c.Request); errWithCode != nil {
+		api.ErrorHandler(c, errWithCode, m.processor.InstanceGet)
 	}
 }
 
@@ -273,22 +272,22 @@ func (m *Module) AuthorizePOSTHandler(c *gin.Context) {
 func saveAuthFormToSession(s sessions.Session, form *model.OAuthAuthorize) gtserror.WithCode {
 	if form == nil {
 		err := errors.New("OAuthAuthorize form was nil")
-		return gtserror.NewErrorBadRequest(err, err.Error(), helpfulAdvice)
+		return gtserror.NewErrorBadRequest(err, err.Error(), oauth.HelpfulAdvice)
 	}
 
 	if form.ResponseType == "" {
 		err := errors.New("field response_type was not set on OAuthAuthorize form")
-		return gtserror.NewErrorBadRequest(err, err.Error(), helpfulAdvice)
+		return gtserror.NewErrorBadRequest(err, err.Error(), oauth.HelpfulAdvice)
 	}
 
 	if form.ClientID == "" {
 		err := errors.New("field client_id was not set on OAuthAuthorize form")
-		return gtserror.NewErrorBadRequest(err, err.Error(), helpfulAdvice)
+		return gtserror.NewErrorBadRequest(err, err.Error(), oauth.HelpfulAdvice)
 	}
 
 	if form.RedirectURI == "" {
 		err := errors.New("field redirect_uri was not set on OAuthAuthorize form")
-		return gtserror.NewErrorBadRequest(err, err.Error(), helpfulAdvice)
+		return gtserror.NewErrorBadRequest(err, err.Error(), oauth.HelpfulAdvice)
 	}
 
 	// set default scope to read
@@ -307,7 +306,7 @@ func saveAuthFormToSession(s sessions.Session, form *model.OAuthAuthorize) gtser
 
 	if err := s.Save(); err != nil {
 		err := fmt.Errorf("error saving form values onto session: %s", err)
-		return gtserror.NewErrorInternalError(err, helpfulAdvice)
+		return gtserror.NewErrorInternalError(err, oauth.HelpfulAdvice)
 	}
 
 	return nil

internal/api/client/auth/callback.go

@@ -34,6 +34,7 @@ import (
 	"github.com/superseriousbusiness/gotosocial/internal/db"
 	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
 	"github.com/superseriousbusiness/gotosocial/internal/gtsmodel"
+	"github.com/superseriousbusiness/gotosocial/internal/oauth"
 	"github.com/superseriousbusiness/gotosocial/internal/oidc"
 	"github.com/superseriousbusiness/gotosocial/internal/validate"
 )
@@ -91,7 +92,7 @@ func (m *Module) CallbackGETHandler(c *gin.Context) {
 	if !ok || clientID == "" {
 		m.clearSession(s)
 		err := fmt.Errorf("key %s was not found in session", sessionClientID)
-		api.ErrorHandler(c, gtserror.NewErrorBadRequest(err, helpfulAdvice), m.processor.InstanceGet)
+		api.ErrorHandler(c, gtserror.NewErrorBadRequest(err, oauth.HelpfulAdvice), m.processor.InstanceGet)
 		return
 	}
 
@@ -101,9 +102,9 @@ func (m *Module) CallbackGETHandler(c *gin.Context) {
 		safe := fmt.Sprintf("application for %s %s could not be retrieved", sessionClientID, clientID)
 		var errWithCode gtserror.WithCode
 		if err == db.ErrNoEntries {
-			errWithCode = gtserror.NewErrorBadRequest(err, safe, helpfulAdvice)
+			errWithCode = gtserror.NewErrorBadRequest(err, safe, oauth.HelpfulAdvice)
 		} else {
-			errWithCode = gtserror.NewErrorInternalError(err, safe, helpfulAdvice)
+			errWithCode = gtserror.NewErrorInternalError(err, safe, oauth.HelpfulAdvice)
 		}
 		api.ErrorHandler(c, errWithCode, m.processor.InstanceGet)
 		return

internal/api/client/auth/oob.go

@@ -0,0 +1,111 @@
+/*
+   GoToSocial
+   Copyright (C) 2021-2022 GoToSocial Authors admin@gotosocial.org
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU Affero General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU Affero General Public License for more details.
+
+   You should have received a copy of the GNU Affero General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+package auth
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"net/http"
+
+	"github.com/gin-contrib/sessions"
+	"github.com/gin-gonic/gin"
+	"github.com/superseriousbusiness/gotosocial/internal/api"
+	"github.com/superseriousbusiness/gotosocial/internal/api/model"
+	"github.com/superseriousbusiness/gotosocial/internal/config"
+	"github.com/superseriousbusiness/gotosocial/internal/db"
+	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
+	"github.com/superseriousbusiness/gotosocial/internal/oauth"
+)
+
+func (m *Module) OobHandler(c *gin.Context) {
+	host := config.GetHost()
+	instance, errWithCode := m.processor.InstanceGet(c.Request.Context(), host)
+	if errWithCode != nil {
+		api.ErrorHandler(c, errWithCode, m.processor.InstanceGet)
+		return
+	}
+
+	instanceGet := func(ctx context.Context, domain string) (*model.Instance, gtserror.WithCode) { return instance, nil }
+
+	oobToken := c.Query("code")
+	if oobToken == "" {
+		err := errors.New("no 'code' query value provided in callback redirect")
+		api.ErrorHandler(c, gtserror.NewErrorBadRequest(err, err.Error(), oauth.HelpfulAdvice), instanceGet)
+		return
+	}
+
+	s := sessions.Default(c)
+
+	errs := []string{}
+
+	scope, ok := s.Get(sessionScope).(string)
+	if !ok {
+		errs = append(errs, fmt.Sprintf("key %s was not found in session", sessionScope))
+	}
+
+	userID, ok := s.Get(sessionUserID).(string)
+	if !ok {
+		errs = append(errs, fmt.Sprintf("key %s was not found in session", sessionUserID))
+	}
+
+	if len(errs) != 0 {
+		errs = append(errs, oauth.HelpfulAdvice)
+		api.ErrorHandler(c, gtserror.NewErrorBadRequest(errors.New("one or more missing keys on session during OobHandler"), errs...), m.processor.InstanceGet)
+		return
+	}
+
+	user, err := m.db.GetUserByID(c.Request.Context(), userID)
+	if err != nil {
+		m.clearSession(s)
+		safe := fmt.Sprintf("user with id %s could not be retrieved", userID)
+		var errWithCode gtserror.WithCode
+		if err == db.ErrNoEntries {
+			errWithCode = gtserror.NewErrorBadRequest(err, safe, oauth.HelpfulAdvice)
+		} else {
+			errWithCode = gtserror.NewErrorInternalError(err, safe, oauth.HelpfulAdvice)
+		}
+		api.ErrorHandler(c, errWithCode, instanceGet)
+		return
+	}
+
+	acct, err := m.db.GetAccountByID(c.Request.Context(), user.AccountID)
+	if err != nil {
+		m.clearSession(s)
+		safe := fmt.Sprintf("account with id %s could not be retrieved", user.AccountID)
+		var errWithCode gtserror.WithCode
+		if err == db.ErrNoEntries {
+			errWithCode = gtserror.NewErrorBadRequest(err, safe, oauth.HelpfulAdvice)
+		} else {
+			errWithCode = gtserror.NewErrorInternalError(err, safe, oauth.HelpfulAdvice)
+		}
+		api.ErrorHandler(c, errWithCode, instanceGet)
+		return
+	}
+
+	// we're done with the session now, so just clear it out
+	m.clearSession(s)
+
+	c.HTML(http.StatusOK, "oob.tmpl", gin.H{
+		"instance": instance,
+		"user":     acct.Username,
+		"oobToken": oobToken,
+		"scope":    scope,
+	})
+}

internal/api/client/auth/signin.go

@@ -29,6 +29,7 @@ import (
 	"github.com/superseriousbusiness/gotosocial/internal/api"
 	"github.com/superseriousbusiness/gotosocial/internal/config"
 	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
+	"github.com/superseriousbusiness/gotosocial/internal/oauth"
 	"golang.org/x/crypto/bcrypt"
 )
 
@@ -86,7 +87,7 @@ func (m *Module) SignInPOSTHandler(c *gin.Context) {
 	form := &login{}
 	if err := c.ShouldBind(form); err != nil {
 		m.clearSession(s)
-		api.ErrorHandler(c, gtserror.NewErrorBadRequest(err, helpfulAdvice), m.processor.InstanceGet)
+		api.ErrorHandler(c, gtserror.NewErrorBadRequest(err, oauth.HelpfulAdvice), m.processor.InstanceGet)
 		return
 	}
 
@@ -101,7 +102,7 @@ func (m *Module) SignInPOSTHandler(c *gin.Context) {
 	s.Set(sessionUserID, userid)
 	if err := s.Save(); err != nil {
 		err := fmt.Errorf("error saving user id onto session: %s", err)
-		api.ErrorHandler(c, gtserror.NewErrorInternalError(err, helpfulAdvice), m.processor.InstanceGet)
+		api.ErrorHandler(c, gtserror.NewErrorInternalError(err, oauth.HelpfulAdvice), m.processor.InstanceGet)
 	}
 
 	c.Redirect(http.StatusFound, OauthAuthorizePath)
@@ -140,5 +141,5 @@ func (m *Module) ValidatePassword(ctx context.Context, email string, password st
 // only a generic 'safe' error message to the user, to not give any info away.
 func incorrectPassword(err error) (string, gtserror.WithCode) {
 	safeErr := fmt.Errorf("password/email combination was incorrect")
-	return "", gtserror.NewErrorUnauthorized(err, safeErr.Error(), helpfulAdvice)
+	return "", gtserror.NewErrorUnauthorized(err, safeErr.Error(), oauth.HelpfulAdvice)
 }

internal/oauth/server.go

@@ -22,6 +22,7 @@ import (
 	"context"
 	"fmt"
 	"net/http"
+	"strings"
 
 	"github.com/superseriousbusiness/gotosocial/internal/db"
 	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
@@ -49,12 +50,19 @@ const (
 	// of a Client who has successfully passed Bearer token authorization.
 	// The interface returned from grabbing this key should be parsed as a *gtsmodel.Application
 	SessionAuthorizedApplication = "authorized_app"
+	// OOBURI is the out-of-band oauth token uri
+	OOBURI = "urn:ietf:wg:oauth:2.0:oob"
+	// OOBTokenPath is the path to redirect out-of-band token requests to.
+	OOBTokenPath = "/oob"
+	// HelpfulAdvice is a handy hint to users;
+	// particularly important during the login flow
+	HelpfulAdvice = "If you arrived at this error during a login/oauth flow, please try clearing your session cookies and logging in again; if problems persist, make sure you're using the correct credentials"
 )
 
 // Server wraps some oauth2 server functions in an interface, exposing only what is needed
 type Server interface {
 	HandleTokenRequest(r *http.Request) (map[string]interface{}, gtserror.WithCode)
-	HandleAuthorizeRequest(w http.ResponseWriter, r *http.Request) error
+	HandleAuthorizeRequest(w http.ResponseWriter, r *http.Request) gtserror.WithCode
 	ValidationBearerToken(r *http.Request) (oauth2.TokenInfo, error)
 	GenerateUserAccessToken(ctx context.Context, ti oauth2.TokenInfo, clientSecret string, userID string) (accessToken oauth2.TokenInfo, err error)
 	LoadAccessToken(ctx context.Context, access string) (accessToken oauth2.TokenInfo, err error)
@@ -123,13 +131,13 @@ func (s *s) HandleTokenRequest(r *http.Request) (map[string]interface{}, gtserro
 	gt, tgr, err := s.server.ValidationTokenRequest(r)
 	if err != nil {
 		help := fmt.Sprintf("could not validate token request: %s", err)
-		return nil, gtserror.NewErrorBadRequest(err, help)
+		return nil, gtserror.NewErrorBadRequest(err, help, HelpfulAdvice)
 	}
 
 	ti, err := s.server.GetAccessToken(ctx, gt, tgr)
 	if err != nil {
 		help := fmt.Sprintf("could not get access token: %s", err)
-		return nil, gtserror.NewErrorBadRequest(err, help)
+		return nil, gtserror.NewErrorBadRequest(err, help, HelpfulAdvice)
 	}
 
 	data := s.server.GetTokenData(ti)
@@ -145,7 +153,7 @@ func (s *s) HandleTokenRequest(r *http.Request) (map[string]interface{}, gtserro
 			}
 		default:
 			err := errors.New("expires_in was set on token response, but was not an int64")
-			return nil, gtserror.NewErrorInternalError(err)
+			return nil, gtserror.NewErrorInternalError(err, HelpfulAdvice)
 		}
 	}
 
@@ -155,9 +163,88 @@ func (s *s) HandleTokenRequest(r *http.Request) (map[string]interface{}, gtserro
 	return data, nil
 }
 
+func (s *s) errorOrRedirect(err error, w http.ResponseWriter, req *server.AuthorizeRequest) gtserror.WithCode {
+	if req == nil {
+		return gtserror.NewErrorUnauthorized(err, HelpfulAdvice)
+	}
+
+	data, _, _ := s.server.GetErrorData(err)
+	uri, err := s.server.GetRedirectURI(req, data)
+	if err != nil {
+		return gtserror.NewErrorInternalError(err, HelpfulAdvice)
+	}
+
+	w.Header().Set("Location", uri)
+	w.WriteHeader(http.StatusFound)
+	return nil
+}
+
 // HandleAuthorizeRequest wraps the oauth2 library's HandleAuthorizeRequest function
-func (s *s) HandleAuthorizeRequest(w http.ResponseWriter, r *http.Request) error {
-	return s.server.HandleAuthorizeRequest(w, r)
+func (s *s) HandleAuthorizeRequest(w http.ResponseWriter, r *http.Request) gtserror.WithCode {
+	ctx := r.Context()
+
+	req, err := s.server.ValidationAuthorizeRequest(r)
+	if err != nil {
+		return s.errorOrRedirect(err, w, req)
+	}
+
+	// user authorization
+	userID, err := s.server.UserAuthorizationHandler(w, r)
+	if err != nil {
+		return s.errorOrRedirect(err, w, req)
+	}
+	if userID == "" {
+		help := "userID was empty"
+		return gtserror.NewErrorUnauthorized(err, help, HelpfulAdvice)
+	}
+	req.UserID = userID
+
+	// specify the scope of authorization
+	if fn := s.server.AuthorizeScopeHandler; fn != nil {
+		scope, err := fn(w, r)
+		if err != nil {
+			return s.errorOrRedirect(err, w, req)
+		} else if scope != "" {
+			req.Scope = scope
+		}
+	}
+
+	// specify the expiration time of access token
+	if fn := s.server.AccessTokenExpHandler; fn != nil {
+		exp, err := fn(w, r)
+		if err != nil {
+			return s.errorOrRedirect(err, w, req)
+		}
+		req.AccessTokenExp = exp
+	}
+
+	ti, err := s.server.GetAuthorizeToken(ctx, req)
+	if err != nil {
+		return s.errorOrRedirect(err, w, req)
+	}
+
+	// If the redirect URI is empty, the default domain provided by the client is used.
+	if req.RedirectURI == "" {
+		client, err := s.server.Manager.GetClient(ctx, req.ClientID)
+		if err != nil {
+			return gtserror.NewErrorUnauthorized(err, HelpfulAdvice)
+		}
+		req.RedirectURI = client.GetDomain()
+	}
+
+	uri, err := s.server.GetRedirectURI(req, s.server.GetAuthorizeData(req.ResponseType, ti))
+	if err != nil {
+		return gtserror.NewErrorUnauthorized(err, HelpfulAdvice)
+	}
+
+	if strings.Contains(uri, OOBURI) {
+		w.Header().Set("Location", strings.ReplaceAll(uri, OOBURI, OOBTokenPath))
+	} else {
+		w.Header().Set("Location", uri)
+	}
+
+	w.WriteHeader(http.StatusFound)
+	return nil
 }
 
 // ValidationBearerToken wraps the oauth2 library's ValidationBearerToken function

internal/processing/oauth.go

@@ -24,7 +24,7 @@ import (
 	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
 )
 
-func (p *processor) OAuthHandleAuthorizeRequest(w http.ResponseWriter, r *http.Request) error {
+func (p *processor) OAuthHandleAuthorizeRequest(w http.ResponseWriter, r *http.Request) gtserror.WithCode {
 	// todo: some kind of metrics stuff here
 	return p.oauthServer.HandleAuthorizeRequest(w, r)
 }

internal/processing/processor.go

@@ -160,7 +160,7 @@ type Processor interface {
 	NotificationsClear(ctx context.Context, authed *oauth.Auth) gtserror.WithCode
 
 	OAuthHandleTokenRequest(r *http.Request) (map[string]interface{}, gtserror.WithCode)
-	OAuthHandleAuthorizeRequest(w http.ResponseWriter, r *http.Request) error
+	OAuthHandleAuthorizeRequest(w http.ResponseWriter, r *http.Request) gtserror.WithCode
 
 	// SearchGet performs a search with the given params, resolving/dereferencing remotely as desired
 	SearchGet(ctx context.Context, authed *oauth.Auth, searchQuery *apimodel.SearchQuery) (*apimodel.SearchResult, gtserror.WithCode)

web/template/oob.tmpl

@@ -0,0 +1,8 @@
+{{ template "header.tmpl" .}}
+    <main>
+        <h1>Hi {{ .user }}!</h1>
+        <p>Here's your out-of-band token with scope <em>{{.scope}}</em>:</p>
+        <p><code>{{ .oobToken }}</code><p>
+        <p>Use it wisely!</p>
+    </main>
+{{ template "footer.tmpl" .}}