function Standard::filterXss

Overrides EditorXssFilterInterface::filterXss

File

core/modules/editor/src/EditorXssFilter/Standard.php, line 18

Class

Standard

Defines the standard text editor XSS filter.

Namespace

Code

public static function filterXss($html, FilterFormatInterface $format, FilterFormatInterface $original_format = NULL) {
    // Apply XSS filtering, but blacklist the <script>, <style>, <link>, <embed>
    // and <object> tags.
    // The <script> and <style> tags are blacklisted because their contents
    // can be malicious (and therefore they are inherently unsafe), whereas for
    // all other tags, only their attributes can make them malicious. Since
    // \Drupal\Component\Utility\Xss::filter() protects against malicious
    // attributes, we take no blacklisting action.
    // The exceptions to the above rule are <link>, <embed> and <object>:
    // - <link> because the href attribute allows the attacker to import CSS
    //   using the HTTP(S) protocols which Xss::filter() considers safe by
    //   default. The imported remote CSS is applied to the main document, thus
    //   allowing for the same XSS attacks as a regular <style> tag.
    // - <embed> and <object> because these tags allow non-HTML applications or
    //   content to be embedded using the src or data attributes, respectively.
    //   This is safe in the case of HTML documents, but not in the case of
    //   Flash objects for example, that may access/modify the main document
    //   directly.
    // <iframe> is considered safe because it only allows HTML content to be
    // embedded, hence ensuring the same origin policy always applies.
    $dangerous_tags = [
        'script',
        'style',
        'link',
        'embed',
        'object',
    ];
    // Simply blacklisting these five dangerous tags would bring safety, but
    // also user frustration: what if a text format is configured to allow
    // <embed>, for example? Then we would strip that tag, even though it is
    // allowed, thereby causing data loss!
    // Therefore, we want to be smarter still. We want to take into account
    // which HTML tags are allowed and forbidden by the text format we're
    // filtering for, and if we're switching from another text format, we want
    // to take that format's allowed and forbidden tags into account as well.
    // In other words: we only expect markup allowed in both the original and
    // the new format to continue to exist.
    $format_restrictions = $format->getHtmlRestrictions();
    if ($original_format !== NULL) {
        $original_format_restrictions = $original_format->getHtmlRestrictions();
    }
    // Any tags that are explicitly blacklisted by the text format must be
    // appended to the list of default dangerous tags: if they're explicitly
    // forbidden, then we must respect that configuration.
    // When switching from another text format, we must use the union of
    // forbidden tags: if either text format is more restrictive, then the
    // safety expectations of *both* text formats apply.
    $forbidden_tags = self::getForbiddenTags($format_restrictions);
    if ($original_format !== NULL) {
        $forbidden_tags = array_merge($forbidden_tags, self::getForbiddenTags($original_format_restrictions));
    }
    // Any tags that are explicitly whitelisted by the text format must be
    // removed from the list of default dangerous tags: if they're explicitly
    // allowed, then we must respect that configuration.
    // When switching from another format, we must use the intersection of
    // allowed tags: if either format is more restrictive, then the safety
    // expectations of *both* formats apply.
    $allowed_tags = self::getAllowedTags($format_restrictions);
    if ($original_format !== NULL) {
        $allowed_tags = array_intersect($allowed_tags, self::getAllowedTags($original_format_restrictions));
    }
    // Don't blacklist dangerous tags that are explicitly allowed in both text
    // formats.
    $blacklisted_tags = array_diff($dangerous_tags, $allowed_tags);
    // Also blacklist tags that are explicitly forbidden in either text format.
    $blacklisted_tags = array_merge($blacklisted_tags, $forbidden_tags);
    $output = static::filter($html, $blacklisted_tags);
    // Since data-attributes can contain encoded HTML markup that could be
    // decoded and interpreted by editors, we need to apply XSS filtering to
    // their contents.
    return static::filterXssDataAttributes($output);
}