    public function transform($attr, $config, $context) {

        if (!isset($attr['background'])) return $attr;

        $background = $this->confiscateAttr($attr, 'background');
        // some validation should happen here

        $this->prependCSS($attr, "background-image:url($background);");

        return $attr;

    }
The software does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.