comments (not for humans)

The last item on the OWASP Top 10 is A10 - Unvalidated Redirects and Forwards.

First - here is the risk rating from OWASP:

Threat Agents Attack Vectors Security Weakness Technical Impacts Business Impacts
______ Exploitability
Consider anyone who can trick your users into submitting a request to your website. Any website or other HTML feed that your users use could do this. Attacker links to unvalidated redirect and tricks victims into clicking it. Victims are more likely to click on it, since the link is to a valid site. Attacker targets unsafe forward to bypass security checks. Applications frequently redirect users to other pages, or use internal forwards in a similar manner. Sometimes the target page is specified in an unvalidated parameter, allowing attackers to choose the destination page. 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?

What's the problem?

The problem is that javascript sets document.location or does some other kind of redirect to a URL under the attackers control. The attacker can use this to launch attacks towards the user, who is more likely to click the link, because the hostname is to a trusted site.

Server-side redirects

A vulnerable server-side redirect will typically be a simple http handler that blindly redirects to a URL given as a parameter: As you can imagine it's rather simple to make this redirect to a phishing site, or even a javascript: url.

Redirects in javascript

How do we do redirects in javascript? There is a decent list at the DOM XSS Wiki. Assignment of untrusted data to any of these can lead to trouble:

  • location
  • location.href
  • location.pathname
  • location.protocol
  • location.hostname
Also there is:
  • location.assign
  • location.replace

A real life example

I highly recommend you read A Twitter DomXss, a wrong fix and something more from Minded Security. It starts with the following code:

  var a=location.href.split("#!")[1];
and shows how Twitter tried to address the problem - in multiple rounds.

Another example

Consider an application that does the following code where we try to limit our redirects to redirects on the same server:

var relativePath = document.location.hash.substring(1);
var re = new RegExp("^/[a-z0-9/.]+$");
if (re.test(relativePath)) {
  document.location = relativePath;
At first we may think this is ok. We will allow things like /blog/test.html. However we forgot about scheme less URLs like // If the URL starts with //, it inherits the scheme from the current URL. Broken.


  • Don't blindly redirect to URLs on the server side
  • Make sure you validate values assigned to location-related properties from client side javascript

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