Language: PHP
Label: safe
CWE: CWE-79 \u2014 Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
The product 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.

Code:
    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;
    }