/Drupal 8

public static function UrlHelper::stripDangerousProtocols

public static UrlHelper::stripDangerousProtocols($uri)

Strips dangerous protocols (for example, 'javascript:') from a URI.

This function must be called for all URIs within user-entered input prior to being output to an HTML attribute value. It is often called as part of \Drupal\Component\Utility\UrlHelper::filterBadProtocol() or \Drupal\Component\Utility\Xss::filter(), but those functions return an HTML-encoded string, so this function can be called independently when the output needs to be a plain-text string for passing to functions that will call Html::escape() separately. The exact behavior depends on the value:

  • If the value is a well-formed (per RFC 3986) relative URL or absolute URL that does not use a dangerous protocol (like "javascript:"), then the URL remains unchanged. This includes all URLs generated via Url::toString() and UrlGeneratorTrait::url().
  • If the value is a well-formed absolute URL with a dangerous protocol, the protocol is stripped. This process is repeated on the remaining URL until it is stripped down to a safe protocol.
  • If the value is not a well-formed URL, the same sanitization behavior as for well-formed URLs will be invoked, which strips most substrings that precede a ":". The result can be used in URL attributes such as "href" or "src" (only after calling Html::escape() separately), but this may not produce valid HTML (for example, malformed URLs within "href" attributes fail HTML validation). This can be avoided by using Url::fromUri($possibly_not_a_url)->toString(), which either throws an exception or returns a well-formed URL.


string $uri: A plain-text URI that might contain dangerous protocols.

Return value

string A plain-text URI stripped of dangerous protocols. As with all plain-text strings, this return value must not be output to an HTML page without being sanitized first. However, it can be passed to functions expecting plain-text strings.

See also






core/lib/Drupal/Component/Utility/UrlHelper.php, line 341


Helper class URL based methods.




public static function stripDangerousProtocols($uri) {
  $allowed_protocols = array_flip(static::$allowedProtocols);

  // Iteratively remove any invalid protocol found.
  do {
    $before = $uri;
    $colonpos = strpos($uri, ':');
    if ($colonpos > 0) {
      // We found a colon, possibly a protocol. Verify.
      $protocol = substr($uri, 0, $colonpos);
      // If a colon is preceded by a slash, question mark or hash, it cannot
      // possibly be part of the URL scheme. This must be a relative URL, which
      // inherits the (safe) protocol of the base document.
      if (preg_match('![/?#]!', $protocol)) {
      // Check if this is a disallowed protocol. Per RFC2616, section 3.2.3
      // (URI Comparison) scheme comparison must be case-insensitive.
      if (!isset($allowed_protocols[strtolower($protocol)])) {
        $uri = substr($uri, $colonpos + 1);
  } while ($before != $uri);

  return $uri;

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