Difference between revisions of "Forward and Redirect Cheat Sheet"

From OWASP
Jump to: navigation, search
 
(27 intermediate revisions by 2 users not shown)
Line 1: Line 1:
= Introduction =
+
Moved to the [[Unvalidated Redirects and Forwards Cheat Sheet]].
 
 
When using an URL as input to a request being initiated from a browser, it
 
is important to validate the URL, it is essential to determine that the redirect
 
or forward is appropriate based on the request and the user is authorized to access the target. Allowing a
 
URL as input without validation before redirecting the browser , could
 
result in sending the user to a malicious link without their knowledge.
 
Forwarding a user to a page without first determining if it is appropriate
 
and they are authorized could result in them being allowed to perform actions
 
that are not acceptable.
 
 
 
== Redirect Example ==
 
 
 
An application request is sent that contains a url as input,
 
malicious.example.com, from the example below. 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);
 
 
 
 
 
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:
 
 
 
(Attack)
 
 
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 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. Note that upon 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.
 
 
 
== Business impacts ==
 
 
 
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.
 
 
 
*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.
 
 
 
Applications can use ESAPI to override the
 
*sendRedirect()*<ref><http://owasp-esapi-java.googlecode.com/svn/trunk_doc/latest/org/owasp/esapi/filters/SecurityWrapperResponse.html></ref> method
 
to make sure all redirect destinations are safe.
 
 
 
Avoiding such flaws is extremely important as they are a favorite target of
 
phishers trying to gain the user’s trust to exploit.
 
**
 
== References ==
 
 
 
 
 
• OWASP Article on Open Redirects
 
https://www.owasp.org/index.php/Open_redirect
 
 
 
• ESAPI SecurityWrapperResponse.sendRedirect() method
 
 
 
== External ==
 
 
 
• CWE Entry 601 on Open Redirects
 
http://cwe.mitre.org/data/definitions/601.html
 
 
 
• WASC Article on URL Redirector Abuse
 
http://projects.webappsec.org/w/page/13246981/URL%20Redirector%20Abuse
 
 
 
• Google blog article on the dangers of open redirects
 
http://googlewebmastercentral.blogspot.com/2009/01/open-redirect-urls-is-your-site-being.html
 

Latest revision as of 09:48, 13 March 2013

Moved to the Unvalidated Redirects and Forwards Cheat Sheet.