Forward and Redirect 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 as input string to a request being initiated from a browser, it is important to validate and authorize the URL.

Programming 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 these above examples, the URL is being explicitly declared in the code. Suppose you want to use this URL as a query string to allow a more dynamic way of programming, such as building a URL list or saving these URL's in the database? We will explore these possibilities below.

Redirect Example
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://example.com/example.php?url=http://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.

Security Weakness
Applications frequently redirect users to other pages, or use internal forwards in a similar manner. Sometimes the target page is specified in unvalidated input, allowing attackers to choose the destination page or function they wish to perform.

Technical impacts
Such redirects may attempt to install malware or trick victims into disclosing passwords or other sensitive information. Unsafe forwards may allow access control bypass.

Consider the business value of retaining your users’ trust

 * What if they get owned by malware?


 * What if attackers can access internal only functions?


 * Am I Vulnerable To Unvalidated Redirects and Forwards?

The best way to find out if an application has any unvalidated redirects or forwards is to:

Review the code for all uses of redirect or forward (called a transfer in .NET). For each use, identify if the target URL is included in any parameter values. If so, verify the parameter(s) are validated to contain only an allowed destination, or element of a destination.

Also, spider the site to see if it generates any redirects (HTTP response codes 300-307, typically 302). Look at the parameters supplied prior to the redirect to see if they appear to be a target URL or a piece of such a URL. If so, change the URL target and observe whether the site redirects to the new target.

If code is unavailable, check all parameters to see if they look like part of a redirect or forward URL destination and test those that do.

How Do I Prevent 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.