URL Redirection to Untrusted Site ('Open Redirect')

Description

An http parameter may contain a URL value and could cause the web application to redirect the request to the specified URL. By modifying the URL value to a malicious site, an attacker may successfully launch a phishing scam and steal user credentials. Because the server name in the modified link is identical to the original site, phishing attempts have a more trustworthy appearance. Whether this issue poses a vulnerability will be subject to the intended behavior of the application. For example, a search engine might intentionally provide redirects to arbitrary URLs.

Background

Phishing is a general term for deceptive attempts to coerce private information from users that will be used for identity theft.

Demonstrations

The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

The following code obtains a URL from the query string and then redirects the user to that URL.

$redirect_url = $_GET['url'];
header("Location: " . $redirect_url);

The problem with the above code is that an attacker could use this page as part of a phishing scam by redirecting users to a malicious site. For example, assume the above code is in the file example.php. An attacker could supply a user with the following link:

http://example.com/example.php?url=http://malicious.example.com

The user sees the link pointing to the original trusted site (example.com) and does not realize the redirection that could take place.

Example Two

The following code is a Java servlet that will receive a GET request with a url parameter in the request to redirect the browser to the address specified in the url parameter. The servlet will retrieve the url parameter value from the request and send a response to redirect the browser to the url address.

public class RedirectServlet extends HttpServlet {


  protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    String query = request.getQueryString();
    if (query.contains("url")) {
      String url = request.getParameter("url");
      response.sendRedirect(url);
    }
  }

}

The problem with this Java servlet code is that an attacker could use the RedirectServlet as part of an e-mail phishing scam to redirect users to a malicious site. An attacker could send an HTML formatted e-mail directing the user to log into their account by including in the e-mail the following link:

<a href="http://bank.example.com/redirect?url=http://attacker.example.net">Click here to log in</a>

The user may assume that the link is safe since the URL starts with their trusted bank, bank.example.com. However, the user will then be redirected to the attacker's web site (attacker.example.net) which the attacker may have made to appear very similar to bank.example.com. The user may then unwittingly enter credentials into the attacker's web page and compromise their bank account. A Java servlet should never redirect a user to a URL without verifying that the redirect address is a trusted site.

See Also