diff --git a/lib/pleroma/web/controller_helper.ex b/lib/pleroma/web/controller_helper.ex
index 8f36329ee..5915ea40e 100644
--- a/lib/pleroma/web/controller_helper.ex
+++ b/lib/pleroma/web/controller_helper.ex
@@ -6,6 +6,7 @@ defmodule Pleroma.Web.ControllerHelper do
   use Pleroma.Web, :controller
 
   def oauth_scopes(params, default) do
+    # Note: `scopes` is used by Mastodon — supporting it but sticking to OAuth's standard `scope` wherever we control it
     Pleroma.Web.OAuth.parse_scopes(params["scope"] || params["scopes"], default)
   end
 
diff --git a/lib/pleroma/web/mastodon_api/mastodon_api_controller.ex b/lib/pleroma/web/mastodon_api/mastodon_api_controller.ex
index 0e77ec907..5d51e913d 100644
--- a/lib/pleroma/web/mastodon_api/mastodon_api_controller.ex
+++ b/lib/pleroma/web/mastodon_api/mastodon_api_controller.ex
@@ -1156,7 +1156,7 @@ defmodule Pleroma.Web.MastodonAPI.MastodonAPIController do
           response_type: "code",
           client_id: app.client_id,
           redirect_uri: ".",
-          scope: app.scopes
+          scope: Enum.join(app.scopes, " ")
         )
 
       conn
diff --git a/lib/pleroma/web/oauth/oauth_controller.ex b/lib/pleroma/web/oauth/oauth_controller.ex
index d8d3ea5b4..fe2c958c9 100644
--- a/lib/pleroma/web/oauth/oauth_controller.ex
+++ b/lib/pleroma/web/oauth/oauth_controller.ex
@@ -17,20 +17,15 @@ defmodule Pleroma.Web.OAuth.OAuthController do
   action_fallback(Pleroma.Web.OAuth.FallbackController)
 
   def authorize(conn, params) do
-    params_scopes = oauth_scopes(params, nil)
-
-    scopes =
-      if params_scopes do
-        params_scopes
-      else
-        app = Repo.get_by(App, client_id: params["client_id"])
-        app && app.scopes
-      end
+    app = Repo.get_by(App, client_id: params["client_id"])
+    available_scopes = (app && app.scopes) || []
+    scopes = oauth_scopes(params, nil) || available_scopes
 
     render(conn, "show.html", %{
       response_type: params["response_type"],
       client_id: params["client_id"],
-      scopes: scopes || [],
+      available_scopes: available_scopes,
+      scopes: scopes,
       redirect_uri: params["redirect_uri"],
       state: params["state"]
     })
@@ -47,12 +42,13 @@ defmodule Pleroma.Web.OAuth.OAuthController do
       }) do
     with %User{} = user <- User.get_by_nickname_or_email(name),
          true <- Pbkdf2.checkpw(password, user.password_hash),
-         {:auth_active, true} <- {:auth_active, User.auth_active?(user)},
          %App{} = app <- Repo.get_by(App, client_id: client_id),
          true <- redirect_uri in String.split(app.redirect_uris),
          scopes <- oauth_scopes(auth_params, []),
-         [] <- scopes -- app.scopes,
-         true <- Enum.any?(scopes),
+         {:unsupported_scopes, []} <- {:unsupported_scopes, scopes -- app.scopes},
+         # Note: `scope` param is intentionally not optional in this context
+         {:missing_scopes, false} <- {:missing_scopes, scopes == []},
+         {:auth_active, true} <- {:auth_active, User.auth_active?(user)},
          {:ok, auth} <- Authorization.create_authorization(app, user, scopes) do
       # Special case: Local MastodonFE.
       redirect_uri =
@@ -85,20 +81,20 @@ defmodule Pleroma.Web.OAuth.OAuthController do
           redirect(conn, external: url)
       end
     else
-      res ->
-        msg =
-          if res == {:auth_active, false},
-            do: "Account confirmation pending",
-            else: "Invalid Username/Password/Permissions"
-
-        app = Repo.get_by(App, client_id: client_id)
-        available_scopes = (app && app.scopes) || oauth_scopes(auth_params, [])
-        scope_param = Enum.join(available_scopes, " ")
-
+      {scopes_issue, _} when scopes_issue in [:unsupported_scopes, :missing_scopes] ->
         conn
-        |> put_flash(:error, msg)
+        |> put_flash(:error, "Permissions not specified.")
         |> put_status(:unauthorized)
-        |> authorize(Map.merge(auth_params, %{"scope" => scope_param}))
+        |> authorize(auth_params)
+
+      {:auth_active, false} ->
+        conn
+        |> put_flash(:error, "Account confirmation pending.")
+        |> put_status(:forbidden)
+        |> authorize(auth_params)
+
+      error ->
+        error
     end
   end
 
diff --git a/lib/pleroma/web/templates/o_auth/o_auth/show.html.eex b/lib/pleroma/web/templates/o_auth/o_auth/show.html.eex
index 41b58aca0..6e88efe11 100644
--- a/lib/pleroma/web/templates/o_auth/o_auth/show.html.eex
+++ b/lib/pleroma/web/templates/o_auth/o_auth/show.html.eex
@@ -13,9 +13,10 @@
 
 <%= label f, :scope, "Permissions" %>
 <br>
-<%= for scope <- @scopes do %>
-  <%= checkbox f, :"scopes_#{scope}", value: scope, checked_value: scope, unchecked_value: "", name: "authorization[scopes][]" %>
-  <%= label f, :"scopes_#{scope}", String.capitalize(scope) %>
+<%= for scope <- @available_scopes do %>
+  <%# Note: using hidden input with `unchecked_value` in order to distinguish user's empty selection from `scope` param being omitted %>
+  <%= checkbox f, :"scope_#{scope}", value: scope in @scopes && scope, checked_value: scope, unchecked_value: "", name: "authorization[scope][]" %>
+  <%= label f, :"scope_#{scope}", String.capitalize(scope) %>
   <br>
 <% end %>
 
diff --git a/test/support/factory.ex b/test/support/factory.ex
index eaa6f0ce2..fa5d60bfc 100644
--- a/test/support/factory.ex
+++ b/test/support/factory.ex
@@ -214,7 +214,7 @@ defmodule Pleroma.Factory do
     %Pleroma.Web.OAuth.App{
       client_name: "Some client",
       redirect_uris: "https://example.com/callback",
-      scopes: ["read"],
+      scopes: ["read", "write", "follow"],
       website: "https://example.com",
       client_id: "aaabbb==",
       client_secret: "aaa;/&bbb"