fix(activitypub): only return public activities on request (#12382)

The endpoint returning individual activities was missing access control checks, since IDs are sequential, this is not ideal.

Fixes #12333

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/12382
Reviewed-by: Gusted <gusted@noreply.codeberg.org>

models/activities/action.go

@@ -812,3 +812,36 @@ func FixActionCreatedUnixString(ctx context.Context) (int64, error) {
 	}
 	return 0, nil
 }
+
+func (a *Action) IsActionPrivate(ctx context.Context) (bool, error) {
+	if a.IsPrivate {
+		return true, nil
+	}
+
+	a.loadRepo(ctx)
+	if a.Repo == nil {
+		return true, repo_model.ErrRepoNotExist{}
+	}
+
+	repo := a.Repo
+	err := repo.LoadOwner(ctx)
+	if err != nil {
+		return true, err
+	}
+
+	if repo.IsPrivate || repo.Owner.KeepActivityPrivate || repo.Owner.Visibility != structs.VisibleTypePublic {
+		return true, nil
+	}
+
+	a.LoadActUser(ctx)
+	if a.ActUser == nil {
+		return true, user_model.ErrUserNotExist{}
+	}
+
+	user := a.ActUser
+	if user.KeepActivityPrivate || user.Visibility != structs.VisibleTypePublic {
+		return true, nil
+	}
+
+	return false, nil
+}

models/activities/action_test.go

@@ -371,3 +371,28 @@ func TestGetIssueInfos(t *testing.T) {
 		assert.Equal(t, test.field3, issueInfos[2])
 	}
 }
+
+func TestIsPrivate(t *testing.T) {
+	defer unittest.OverrideFixtures("models/activities/fixtures/TestIsPrivate")()
+	require.NoError(t, unittest.PrepareTestDatabase())
+
+	tt := []struct {
+		activityID int64
+		private    bool
+	}{
+		{1, true},  // private repo
+		{3, false}, // public activities, public repo
+		{11, true}, // private activities
+	}
+
+	for _, test := range tt {
+		ctx := t.Context()
+		action, err := activities_model.GetActivityByID(ctx, test.activityID)
+		require.NoError(t, err)
+
+		private, err := action.IsActionPrivate(ctx)
+		require.NoError(t, err)
+
+		assert.Equal(t, test.private, private, "action ID: %d", test.activityID)
+	}
+}

models/activities/error.go

@@ -0,0 +1,14 @@
+// Copyright 2026 The Forgejo Authors. All rights reserved.
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package activities
+
+import "fmt"
+
+type ErrActivityPrivate struct {
+	id int64
+}
+
+func (err ErrActivityPrivate) Error() string {
+	return fmt.Sprintf("Activity with id %d is private", err.id)
+}

models/activities/fixtures/TestIsPrivate/action.yml

@@ -0,0 +1,7 @@
+- id: 11
+  user_id: 44
+  op_type: 1 # create repo
+  act_user_id: 44 # private user activities
+  repo_id: 60 # public
+  is_private: false
+  created_unix: 1680454039

models/activities/fixtures/TestIsPrivate/user.yml

@@ -0,0 +1,36 @@
+- id: 44
+  lower_name: user44
+  name: user44
+  full_name: user44
+  email: user44@example.com
+  keep_email_private: false
+  email_notifications_preference: enabled
+  passwd: ZogKvWdyEx:password
+  passwd_hash_algo: dummy
+  must_change_password: false
+  login_source: 0
+  login_name: user44
+  type: 0
+  salt: ZogKvWdyEx
+  max_repo_creation: -1
+  is_active: true
+  is_admin: false
+  is_restricted: false
+  allow_git_hook: false
+  allow_import_local: false
+  allow_create_organization: true
+  prohibit_login: false
+  avatar: ""
+  avatar_email: user44@example.com
+  use_custom_avatar: true
+  num_followers: 0
+  num_following: 0
+  num_stars: 0
+  num_repos: 0
+  num_teams: 0
+  num_members: 0
+  visibility: 0
+  repo_admin_change_team_access: false
+  theme: ""
+  keep_activity_private: true
+  created_unix: 1672578380

models/fixtures/action.yml

@@ -81,4 +81,4 @@
   act_user_id: 40
   repo_id: 60 # public
   is_private: false
-  created_unix: 1577404800 # end of heatmap
+  created_unix: 1577404800 # end of heatmap

routers/api/v1/activitypub/person.go

@@ -148,6 +148,17 @@ func getActivity(ctx *context.APIContext, id int64) (*forgefed.ForgeUserActivity
 		return nil, err
 	}
 
+	private, err := action.IsActionPrivate(ctx)
+	if err != nil {
+		ctx.Error(http.StatusInternalServerError, "action.IsActionPrivate", err.Error())
+		return nil, err
+	}
+
+	if private {
+		ctx.NotFound()
+		return nil, activities.ErrActivityPrivate{}
+	}
+
 	actions := activities.ActionList{action}
 	if err := actions.LoadAttributes(ctx); err != nil {
 		ctx.Error(http.StatusInternalServerError, "action.LoadAttributes", err.Error())

services/convert/activitypub_user_action.go

@@ -13,6 +13,7 @@ import (
 
 	activities_model "forgejo.org/models/activities"
 	issues_model "forgejo.org/models/issues"
+	"forgejo.org/models/repo"
 	fm "forgejo.org/modules/forgefed"
 	"forgejo.org/modules/json"
 	"forgejo.org/modules/markup"
@@ -20,6 +21,10 @@ import (
 )
 
 func ActionToForgeUserActivity(ctx context.Context, action *activities_model.Action) (fm.ForgeUserActivity, error) {
+	if action.Repo == nil {
+		return fm.ForgeUserActivity{}, repo.ErrRepoNotExist{}
+	}
+
 	render := func(format string, args ...any) string {
 		return fmt.Sprintf(`<a href="%s" rel="nofollow">%s</a> %s`, action.ActUser.HTMLURL(), action.GetActDisplayName(ctx), fmt.Sprintf(format, args...))
 	}

services/federation/user_activity.go

@@ -9,8 +9,8 @@ import (
 	activities_model "forgejo.org/models/activities"
 	"forgejo.org/models/forgefed"
 	"forgejo.org/models/user"
+	"forgejo.org/modules/log"
 	"forgejo.org/modules/setting"
-	"forgejo.org/modules/structs"
 	"forgejo.org/services/convert"
 
 	ap "github.com/go-ap/activitypub"
@@ -64,17 +64,16 @@ func NotifyActivityPubFollowers(ctx context.Context, actions []activities_model.
 		return nil
 	}
 	for _, act := range actions {
-		if act.Repo != nil {
-			if act.Repo.IsPrivate {
-				continue
-			}
-			if act.Repo.Owner.KeepActivityPrivate || act.Repo.Owner.Visibility != structs.VisibleTypePublic {
-				continue
-			}
-		}
-		if act.ActUser.KeepActivityPrivate || act.ActUser.Visibility != structs.VisibleTypePublic {
+		private, err := act.IsActionPrivate(ctx)
+		if err != nil {
+			log.Error("Failed to check if action is private: %s", err.Error())
 			continue
 		}
+
+		if private {
+			continue
+		}
+
 		if err := SendUserActivity(ctx, act.ActUser, &act); err != nil {
 			return err
 		}

tests/integration/api_activitypub_person_inbox_useractivity_test.go

@@ -23,6 +23,7 @@ import (
 	"forgejo.org/services/federation"
 	"forgejo.org/tests"
 
+	ap "github.com/go-ap/activitypub"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -159,15 +160,22 @@ func TestActivityPubPersonInboxNoteToDistant(t *testing.T) {
 		require.NoError(t, err)
 		assert.Equal(t, http.StatusOK, resp.StatusCode)
 
-		// distant request activity & activity note
-		localUser2ActivityNote := fmt.Sprintf("%v/activities/1", localUser2URL)
-		localUser2Activity := fmt.Sprintf("%v/activities/1/activity", localUser2URL)
+		// ID of create activity and note delivered to distant
+		activity, err := ap.UnmarshalJSON([]byte(mock.LastPost))
+		require.NoError(t, err)
 
-		resp, err = c.Get(localUser2ActivityNote)
+		createNote := activity.(*ap.Create)
+		localUser2ActivityNote, err := createNote.Object.GetID().URL()
+		require.NoError(t, err)
+
+		localUser2Activity, err := createNote.GetID().URL()
+		require.NoError(t, err)
+
+		resp, err = c.Get(localUser2ActivityNote.String())
 		require.NoError(t, err)
 		assert.Equal(t, http.StatusOK, resp.StatusCode)
 
-		resp, err = c.Get(localUser2Activity)
+		resp, err = c.Get(localUser2Activity.String())
 		require.NoError(t, err)
 		assert.Equal(t, http.StatusOK, resp.StatusCode)