Web Application Exploits and Defenses (Part 3)

A Codelab by Bruce Leban, Mugdha Bendre, and Parisa Tabriz

Table of Contents

Client-State Manipulation

When a user interacts with a web application, they do it indirectly through a browser. When the user clicks a button or submits a form, the browser sends a request back to the web server. Because the browser runs on a machine that can be controlled by an attacker, the application must not trust any data sent by the browser.

It might seem that not trusting any user data would make it impossible to write a web application but that's not the case. If the user submits a form that says they wish to purchase an item, it's OK to trust that data. But if the submitted form also includes the price of the item, that's something that cannot be trusted.

Elevation of Privilege

Convert your account to an administrator account.

Hint 1

Hint 2

Exploit and Fixes

Cookie Manipulation

Because the HTTP protocol is stateless, there's no way a web server can automatically know that two requests are from the same user. For this reason, cookies were invented. When a web site includes a cookie (an arbitrary string) in a HTTP response, the browser automatically sends the cookie back to the browser on the next request. Web sites can use the cookie to save session state. Gruyere uses cookies to remember the identity of the logged in user. Since the cookie is stored on the client side, it's vulnerable to manipulation. Gruyere protects the cookies from manipulation by adding a hash to it. Notwithstanding the fact that this hash isn't very good protection, you don't need to break the hash to execute an attack.

Get Gruyere to issue you a cookie for someone else's account.

Hint 1

Hint 2

Exploit and Fix

Cross-Site Request Forgery (XSRF)

The previous section said "If the user submits a form that says they wish to purchase an item, it's OK to trust that data." That's true as long as it really was the user that submitted the form. If your site is vulnerable to XSS, then the attacker can fake any request as if it came from the user. But even if you've protected against XSS, there's another attack that you need to protect against: cross-site request forgery.

When a browser makes requests to a site, it always sends along any cookies it has for that site, regardless of where the request comes from. Additionally, web servers generally cannot distinguish between a request initiated by a deliberate user action (e.g., user clicking on "Submit" button) versus a request made by the browser without user action (e.g., request for an embedded image in a page). Therefore, if a site receives a request to perform some action (like deleting a mail, changing contact address), it cannot know whether this action was knowingly initiated by the user — even if the request contains authentication cookies. An attacker can use this fact to fool the server into performing actions the user did not intend to perform.

More details

XSRF Challenge

The goal here is to find a way to perform an account changing action on behalf of a logged in Gruyere user without their knowledge. Assume you can get them to visit a web page under your control.

Find a way to get someone to delete one of their Gruyere snippets.


Exploit and Fix

Cross Site Script Inclusion (XSSI)

Browsers prevent pages of one domain from reading pages in other domains. But they do not prevent pages of a domain from referencing resources in other domains. In particular, they allow images to be rendered from other domains and scripts to be executed from other domains. An included script doesn't have its own security context. It runs in the security context of the page that included it. For example, if www.evil.example.com includes a script hosted on www.google.com then that script runs in the evil context not in the google context. So any user data in that script will "leak."

XSSI Challenge

Find a way to read someone else's private snippet using XSSI.

That is, create a page on another web site and put something in that page that can read your private snippet. (You don't need to post it to a web site: you can just create a .html in your home directory and double click on it to open in a browser.)

Hint 1

Hint 2

Exploit and Fix

Continue >>

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The code portions of this codelab are licensed under the Creative Commons Attribution-No Derivative Works 3.0 United States license <https://creativecommons.org/licenses/by-nd/3.0/us>. Brief excerpts of the code may be used for educational or instructional purposes provided this notice is kept intact. Except as otherwise noted the remainder of this codelab is licensed under the Creative Commons Attribution 3.0 United States license <https://creativecommons.org/licenses/by/3.0/us>.