Disallow certain templates

Author: binwiederhier

server/errors.go

@@ -89,7 +89,7 @@ var (
 	errHTTPBadRequestSinceInvalid                    = &errHTTP{40008, http.StatusBadRequest, "invalid since parameter", "https://ntfy.sh/docs/subscribe/api/#fetch-cached-messages", nil}
 	errHTTPBadRequestTopicInvalid                    = &errHTTP{40009, http.StatusBadRequest, "invalid request: topic invalid", "", nil}
 	errHTTPBadRequestTopicDisallowed                 = &errHTTP{40010, http.StatusBadRequest, "invalid request: topic name is not allowed", "", nil}
-	errHTTPBadRequestMessageNotUTF8                  = &errHTTP{40011, http.StatusBadRequest, "invalid message: message must be UTF-8 encoded", "", nil}
+	errHTTPBadRequestMessageNotUTF8                  = &errHTTP{40011, http.StatusBadRequest, "invalid request: message must be UTF-8 encoded", "", nil}
 	errHTTPBadRequestAttachmentURLInvalid            = &errHTTP{40013, http.StatusBadRequest, "invalid request: attachment URL is invalid", "https://ntfy.sh/docs/publish/#attachments", nil}
 	errHTTPBadRequestAttachmentsDisallowed           = &errHTTP{40014, http.StatusBadRequest, "invalid request: attachments not allowed", "https://ntfy.sh/docs/config/#attachments", nil}
 	errHTTPBadRequestAttachmentsExpiryBeforeDelivery = &errHTTP{40015, http.StatusBadRequest, "invalid request: attachment expiry before delayed delivery date", "https://ntfy.sh/docs/publish/#scheduled-delivery", nil}
@@ -113,14 +113,15 @@ var (
 	errHTTPBadRequestPhoneNumberNotVerified          = &errHTTP{40034, http.StatusBadRequest, "invalid request: phone number not verified, or no matching verified numbers found", "https://ntfy.sh/docs/publish/#phone-calls", nil}
 	errHTTPBadRequestAnonymousCallsNotAllowed        = &errHTTP{40035, http.StatusBadRequest, "invalid request: anonymous phone calls are not allowed", "https://ntfy.sh/docs/publish/#phone-calls", nil}
 	errHTTPBadRequestPhoneNumberVerifyChannelInvalid = &errHTTP{40036, http.StatusBadRequest, "invalid request: verification channel must be 'sms' or 'call'", "https://ntfy.sh/docs/publish/#phone-calls", nil}
-	errHTTPBadRequestDelayNoCall                     = &errHTTP{40037, http.StatusBadRequest, "delayed call notifications are not supported", "", nil}
+	errHTTPBadRequestDelayNoCall                     = &errHTTP{40037, http.StatusBadRequest, "invalid request: delayed call notifications are not supported", "", nil}
 	errHTTPBadRequestWebPushSubscriptionInvalid      = &errHTTP{40038, http.StatusBadRequest, "invalid request: web push payload malformed", "", nil}
 	errHTTPBadRequestWebPushEndpointUnknown          = &errHTTP{40039, http.StatusBadRequest, "invalid request: web push endpoint unknown", "", nil}
 	errHTTPBadRequestWebPushTopicCountTooHigh        = &errHTTP{40040, http.StatusBadRequest, "invalid request: too many web push topic subscriptions", "", nil}
-	errHTTPBadRequestTemplatedMessageTooLarge        = &errHTTP{40041, http.StatusBadRequest, "invalid request: message or title is too large after replacing template", "", nil}
-	errHTTPBadRequestTemplatedMessageNotJSON         = &errHTTP{40042, http.StatusBadRequest, "invalid request: message body must be JSON if templating is enabled", "", nil}
-	errHTTPBadRequestTemplateInvalid                 = &errHTTP{40043, http.StatusBadRequest, "invalid request: could not parse template", "", nil}
-	errHTTPBadRequestTemplateExecutionFailed         = &errHTTP{40044, http.StatusBadRequest, "invalid request: template execution failed", "", nil}
+	errHTTPBadRequestTemplateMessageTooLarge         = &errHTTP{40041, http.StatusBadRequest, "invalid request: message or title is too large after replacing template", "https://ntfy.sh/docs/publish/#message-templating", nil}
+	errHTTPBadRequestTemplateMessageNotJSON          = &errHTTP{40042, http.StatusBadRequest, "invalid request: message body must be JSON if templating is enabled", "https://ntfy.sh/docs/publish/#message-templating", nil}
+	errHTTPBadRequestTemplateInvalid                 = &errHTTP{40043, http.StatusBadRequest, "invalid request: could not parse template", "https://ntfy.sh/docs/publish/#message-templating", nil}
+	errHTTPBadRequestTemplateDisallowedFunctionCalls = &errHTTP{40044, http.StatusBadRequest, "invalid request: template contains disallowed function calls, e.g. template, call, or define", "https://ntfy.sh/docs/publish/#message-templating", nil}
+	errHTTPBadRequestTemplateExecuteFailed           = &errHTTP{40045, http.StatusBadRequest, "invalid request: template execution failed", "https://ntfy.sh/docs/publish/#message-templating", nil}
 	errHTTPNotFound                                  = &errHTTP{40401, http.StatusNotFound, "page not found", "", nil}
 	errHTTPUnauthorized                              = &errHTTP{40101, http.StatusUnauthorized, "unauthorized", "https://ntfy.sh/docs/publish/#authentication", nil}
 	errHTTPForbidden                                 = &errHTTP{40301, http.StatusForbidden, "forbidden", "https://ntfy.sh/docs/publish/#authentication", nil}

server/server.go

@@ -136,6 +136,12 @@ const (
 	templateMaxExecutionTime = 100 * time.Millisecond
 )
 
+var (
+	// templateDisallowedRegex tests a template for disallowed expressions. While not really dangerous, they
+	// are not useful, and seem potentially troublesome.
+	templateDisallowedRegex = regexp.MustCompile(`(?m)\{\{-?\s*(call|template|define)\b`)
+)
+
 // WebSocket constants
 const (
 	wsWriteWait  = 2 * time.Second
@@ -1108,23 +1114,26 @@ func (s *Server) handleBodyAsTemplatedTextMessage(m *message, body *util.PeekedR
 		return err
 	}
 	if len(m.Message) > s.config.MessageSizeLimit {
-		return errHTTPBadRequestTemplatedMessageTooLarge
+		return errHTTPBadRequestTemplateMessageTooLarge
 	}
 	return nil
 }
 
 func replaceTemplate(tpl string, source string) (string, error) {
+	if templateDisallowedRegex.MatchString(tpl) {
+		return "", errHTTPBadRequestTemplateDisallowedFunctionCalls
+	}
 	var data any
 	if err := json.Unmarshal([]byte(source), &data); err != nil {
-		return "", errHTTPBadRequestTemplatedMessageNotJSON
+		return "", errHTTPBadRequestTemplateMessageNotJSON
 	}
 	t, err := template.New("").Parse(tpl)
 	if err != nil {
 		return "", errHTTPBadRequestTemplateInvalid
 	}
 	var buf bytes.Buffer
 	if err := t.Execute(util.NewTimeoutWriter(&buf, templateMaxExecutionTime), data); err != nil {
-		return "", errHTTPBadRequestTemplateExecutionFailed
+		return "", errHTTPBadRequestTemplateExecuteFailed
 	}
 	return buf.String(), nil
 }