Limit the number of users on an instance

custom/conf/app.example.ini

@@ -896,6 +896,11 @@ LEVEL = Info
 ;USER_DELETE_WITH_COMMENTS_MAX_TIME = 0
 ;; Valid site url schemes for user profiles
 ;VALID_SITE_URL_SCHEMES=http,https
+;; The maximum number of users allowed to exist before creation is disabled.
+;; This is not a security prevention measure.
+;; Note, Admins will also be unable to create new users via the dashboard when the limit has been reached
+;; `-1` means no limit
+;MAX_USER_CREATE_LIMIT=-1
 
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

docs/content/administration/config-cheat-sheet.en-us.md

@@ -686,6 +686,7 @@ And the following unique queues:
   The user's email will be replaced with a concatenation of the user name in lower case, "@" and NO_REPLY_ADDRESS.
 - `USER_DELETE_WITH_COMMENTS_MAX_TIME`: **0** Minimum amount of time a user must exist before comments are kept when the user is deleted.
 - `VALID_SITE_URL_SCHEMES`: **http, https**: Valid site url schemes for user profiles
+- `MAX_USER_CREATE_LIMIT`: **-1**: The maximum number of users allowed to exist before creation is disabled. Note: This also prevents admins from creating new users. `-1` means no limit.
 
 ### Service - Explore (`service.explore`)
 

models/user/user.go

@@ -600,6 +600,11 @@ func createUser(ctx context.Context, u *User, createdByAdmin bool, overwriteDefa
 		return err
 	}
 
+	// Check the number of users already in the system, and if there are too many forbid any new ones.
+	if HitCreationLimit(ctx) {
+		return fmt.Errorf("The system has exceeded the user creation limit")
+	}
+
 	// set system defaults
 	u.KeepEmailPrivate = setting.Service.DefaultKeepEmailPrivate
 	u.Visibility = setting.Service.DefaultUserVisibilityMode
@@ -729,6 +734,15 @@ func createUser(ctx context.Context, u *User, createdByAdmin bool, overwriteDefa
 	return committer.Commit()
 }
 
+func HitCreationLimit(ctx context.Context) bool {
+	// don't bother calling DB if limit not set
+	if setting.Service.MaxUserCreationLimit < 0 ||
+		int64(setting.Service.MaxUserCreationLimit) < CountUsers(ctx, &CountUserFilter{}) {
+		return false
+	}
+	return true
+}
+
 // IsLastAdminUser check whether user is the last admin
 func IsLastAdminUser(ctx context.Context, user *User) bool {
 	if user.IsAdmin && CountUsers(ctx, &CountUserFilter{IsAdmin: optional.Some(true)}) <= 1 {

modules/setting/service.go

@@ -81,6 +81,7 @@ var Service = struct {
 	DefaultOrgMemberVisible                 bool
 	UserDeleteWithCommentsMaxTime           time.Duration
 	ValidSiteURLSchemes                     []string
+	MaxUserCreationLimit                    int
 
 	// OpenID settings
 	EnableOpenIDSignIn bool
@@ -234,6 +235,8 @@ func loadServiceFrom(rootCfg ConfigProvider) {
 	}
 	Service.ValidSiteURLSchemes = schemes
 
+	Service.MaxUserCreationLimit = sec.Key("MAX_USER_CREATE_LIMIT").MustInt(-1)
+
 	mustMapSetting(rootCfg, "service.explore", &Service.Explore)
 
 	loadOpenIDSetting(rootCfg)

routers/web/auth/auth.go

@@ -717,6 +717,13 @@ func ActivatePost(ctx *context.Context) {
 		return
 	}
 
+	// Check the number of users already in the system, and if there are too many forbid any new ones.
+	if user_model.HitCreationLimit(ctx) {
+		ctx.Flash.Error("The system has exceeded the user creation limit", true)
+		renderActivationChangeEmail(ctx)
+		return
+	}
+
 	if code == "" {
 		newEmail := strings.TrimSpace(ctx.FormString("change_email"))
 		if ctx.Doer != nil && newEmail != "" && !strings.EqualFold(ctx.Doer.Email, newEmail) {