Unvalidated Redirects and Forwards Cheat Sheet

= Introduction =

The Forward and Redirect Cheat Sheet is a technical guide that explains how to prevent unvalidated forwards and redirects. Forward and redirect attacks are possible when a web application accepts untrusted input that could cause the web application to redirect the request to a URL contained within the untrusted input. 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.

When using an URL from untrusted input that could lead to a redirect or forward, it is important to validate and authorize the URL.

Safe URL redirects
When we want to redirect a user automatically to another page (without an action of the visitor such as clicking on a hyperlink) you might implement a code such as the following:

PHP 

ASP.NET Response.Redirect("~/folder/Login.aspx")

In the examples above, the URL is being explicitly declared in the code and cannot be manipulated by an attacker.

Dangerous URL Redirects
An application request is sent which contains a url as input, for example an evil url site called 'malicious.example.com'. If this request includes a url as input that is not validated by the server, the browser can be redirected to a malicious url to perform any number of undesirable actions.

Example 1: The following PHP code obtains a URL from the query string and then redirects the user to that URL. $redirect_url = $_GET['url']; header("Location: " . $redirect_url); A similar example of C# .NET Vulnerable Code: string url = request.QueryString["url"]; Response.Redirect(url);

The above code is vulnerable to an attack if no validation or extra method controls are applied to verify the certainty of the url. This vulnerability could be used as part of a phishing scam by redirecting users to a malicious site. If no validation is applied, a malicious user could create a hyperlink to redirect your users to an unvalidated malicious website, for example:

http&#58;//example.com/example.php?url=http&#58;//malicious.example.com

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

Example 2

ASP.NET MVC 1 & 2 websites are particularly vulnerable to open redirection attacks. In order to avoid this vulnerability, you need to apply MVC 3.

The code for the LogOn action in an ASP.NET MVC 2 application is shown below. After a successful login, the controller returns a redirect to the returnUrl. You can see that no validation is being performed against the returnUrl parameter.

Listing 1 – ASP.NET MVC 2 LogOn action in AccountController.cs

[HttpPost] public ActionResult LogOn(LogOnModel model, string returnUrl) {   if (ModelState.IsValid) {       if (MembershipService.ValidateUser(model.UserName, model.Password)) {           FormsService.SignIn(model.UserName, model.RememberMe); if (!String.IsNullOrEmpty(returnUrl)) {               return Redirect(returnUrl); }           else {               return RedirectToAction("Index", "Home"); }       }        else {           ModelState.AddModelError("", "The user name or password provided is incorrect."); }   }    // If we got this far, something failed, redisplay form return View(model); }

Forward Example
When applications allow user input to forward requests between different parts of the site, the application must check that the user is authorized to access the url, perform the functions it provides, and it is an appropriate url request. If the application fails to perform these checks, an attacker crafted URL may pass the application’s access control check and then forward the attacker to an administrative function that is not normally permitted.

http://www.example.com/function.jsp?fwd=admin.jsp

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); }     }    }

Attack Vectors and Risk
When considering the risk, think about the implications of anyone who can trick your users into clicking on a malicious link. This can be used to direct users to malicious sites when they think they are actually going to your site or another legitimate site or one of its pages. Any website or other HTML feed that your users use could do this.

Attacker changes the url to an unvalidated redirect and the unsuspecting victim clicks on it because they are unaware of the deception.

Attacker targets unsafe forward to bypass security checks to perform actions that they are not authorized to perform.

Preventing Unvalidated Redirects and Forwards
Safe use of redirects and forwards can be done in a number of ways:


 * Simply avoid using redirects and forwards.


 * If used, don’t allow the url as user input for the destination. This can usually be done. In this case, you should have a method to validate URL.

The following example uses a behind code (C-Sharp) to validate the URL after the user has click a certain submit button:

Example validation in .NET: 

protected btRedirect_Click( object sender, EventArgs e ) {  if ( this.IsValid ) Response.Redirect( ... ); }


 * If user input can’t be avoided, ensure that the supplied *value* is valid, appropriate for the application, and *authorized* for the user.
 * It is recommended that any such destination input be mapped to a value, rather than the actual URL or portion of the URL, and that server side code translate this value to the target URL.
 * Sanitize input by creating a list of trusted URL's.
 * Force all redirects to first go through a page notifying users that they are going off of your site, and have them click a link to confirm.