OWASP Top 10 – A10 Unvalidated Redirects and Forwards

Description

If a user is redirected or forwarded to a page defined by an unverified entry, the target URL can be manipulated by an attacker.
This attack is used to redirect a user to a malicious website through a website with a trusted domain name (phishing) or to access an unauthorized internal page.

Examples

My bank provides a very interesting page “My favorites” to easily access all available services and manage my own favorites. Thanks to this page, I can retrieve all my favourites from any computer, everywhere in the world.
For example, when I want to consult my emails, I click on “My mailbox” I have added in that page. The URL https://www.mybank.com/redirect.jsp?url=www.mymailbox.com is called then I am redirected to http://www.mymailbox.com/.

Last time I’ve checked my mailbox, I have received an email from my bank to confirm my login and passcode for security reasons. I click on the provided link, enter my login and passcode and that’s it!
The problem is the link I have clicked on was https://www.mybank.com/redirect.jsp?url=www.trustme.com and I was silently redirected to http://www.trustme.com/ on a page with the look-and-feel of my bank’s website.
A few days later, I received a call from my bank. Someone has used my personal credentials to access my accounts and sent my money abroad.

Mitigations

It is very simple to mitigate this risk: never redirect or forward to unverified URLs or URLs modifiable with user entry.

An unverified URL is an URL entered by the user and not verified/trusted by the server.
A modifiable URL is a URL with parts (domain name, path, parameters…) that can be modified with unverified/untrusted values entered by user.
In both cases, the URL can be a full URL (for redirections) or a path to an internal resource (for forwards).

If you really need to redirect a user to another page (external or internal) registered by that user, the page URL must not be in a request parameter. Use an indirect reference instead. Refer to one of my previous posts about Insecure Direct Object References for details on generating an indirect object reference.
E.g. use https://www.mybank.com/redirect.jsp?urlId=abc321 instead of https://www.mybank.com/redirect.jsp?url=www.mymailbox.com, where urlId is an indirect reference to the URL http://www.mymailbox.com added in user’s favorites.
Besides, you can also check the URL is well-formed and can be trusted before storing it in your database. Some commercial antivirus/firewall products and online services provide blacklist of suspicious websites that can’t be trusted.

If the user can choose a landing page after some actions (e.g. its default homepage after login), instead of giving him a blank field to enter what he wants, propose a list of acceptable landing pages. This way, the user won’t be tempted to enter something like /secure/mgmt/users.xhtml to access user management without authorization! Obviously, if you have applied what I have said in my post about Missing Function Level Access Control, this attempt should not work 😉
And again, remember you must avoid direct references. Don’t give the real path to the internal resource in the list!

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