Project #3 FAQ

This is the FAQ for CSE 127 Project 3.


General Questions

Is magic_quotes_gpc enabled on the Web server?

No, it’s not. magic_quotes_gpc is deprecated. Zoobar performs SQL sanitization on database-bound input using $db->quote(), which is a wrapper around the SQLite-specific sanitization function. Note that this sanitization, like magic_quotes_gpc, doesn’t do anything to angle brackets (<>). For that, you want htmlspecialchars.

Is register_globals enabled on the web server?

No, it's off.

Why do I get an error when I try to visit login.php with my web browser?

Actually, login.php isn’t intended to be viewed directly. It’s just a library of functions that get called if the user tries to view one of the other pages and isn’t logged in yet.

Is http://zoobar.org/ different from http://www.zoobar.org/?

Yes. www.zoobar.org may not work inside your Boxes 2/X VM, and in any case is a separate domain from zoobar.org for purposes of the Same-Origin Principle. Each domain will have its own cookies and they can’t talk to each other. You should use zoobar.org in your project. (If one of your attacks isn’t working, check to make sure you haven’t added the “www” by mistake.)

Um … Same-Origin Principle? Do I need to know what that is?

You should be familiar with the Same-Origin Principle before attempting this project. The Same-Origin Principle is the most important idea behind Web application security; ignoring it will cause you to run into lots of browser security exceptions.

Resources

Where can I find more information about JavaScript?

You can find a lot of references on Google. There’s a decent one at W3Schools. Pay particular attention the DOM examples.

My JavaScript isn’t working, and I don’t know why. What can I do?

Two extremely useful tools for debugging in Mozilla Firefox are the JavaScript console and the DOM Inspector. Both can be accessed from the Tools menu. The JavaScript console lets you see which exceptions are being thrown and why. The DOM Inspector lets you peek at the structure of the page and the properties and methods of each node it contains. (If the DOM Inspector isn’t installed, make sure it’s selected when you install Mozilla Firefox.) You might also want to try Firebug.

What do I need to know about CSS?

You only need to know enough to make your attacks disappear. You should know what basic syntax like <style>.warning{display:none}</style> means, and you should feel free to use stealthy attributes like style="display: none; visibility: hidden; height: 0; width: 0; position: absolute" in the HTML of your attacks. Beware that frames and images may behave strangely with display: none, so you might want to use visibility: hidden instead.

How can I see cookies and form data that the browser sends?

Try the LiveHTTPHeaders browser extension.

Where can I find more information about PHP?

The definitive resource on PHP is php.net. You can find some introductory tutorials there.

Hints on the Attacks

Am I allowed to load scripts or images from other domains?

No, your attacks should not load data from domains other than zoobar.org and zoomail.org. We are enforcing this policy so that the submission deadline is a hard one.

Are we allowed to include additional files?

Please limit yourself to the files requested. One file per attack, the README file, and the ID file.

Attack A.

How are the graders going to test our URL?

We will put it into the browser’s address bar and click the "Go" button.

The example attack doesn’t seem to do anything. What’s wrong?

You need to be logged in to zoobar.org before the attack will work. When you click the link, you should get a browser alert with the contents of document.cookie.

Why would someone want to steal document.cookie?

The cookie is the user’s authentication credential. If you steal someone else’s cookie, it is easy to hijack that user’s session (although we won’t ask you to do so in this project).

How am I supposed to email document.cookie? I didn’t know browsers could send email.

You have to convince the browser to send a GET request to the CS155 email script, which will cause an email to be sent by the zoomail.org server inside the Boxes 2/X VM to the user account.

How do I convince the browser to send a GET request to an URL of my choosing?

The email script inside the Boxes 2/X VM provides some example code. You allocate a JavaScript Image object, and set its src attribute to be the URL you want the browser to retrieve. The browser immediately tries to fetch the content of this image, even though the URL isn’t actually pointing at an image and the image hasn’t been inserted into the page anywhere. Pretty sneaky.

How to I put code into a URL? I want to have newlines and stuff.

It helps if you URL-encode it.

Should I include the javascript:void part in my code?

If you do, your attack might still work, but it’s not necessary if you’re already within a <script> tag. Just copy the part starting with (new Image()).src=...

There are two people in our group. What should we put in the netid parameter?

Put in the Leland username of a group member. You can put in both if you want, but it’s not necessary. As long as it’s clear to the grader that the attack is from your group, you’ll be fine.

What’s up with this random query parameter?

If you try your attack more than once, the browser might think that it already has the image in its cache, and so it wouldn’t send a second GET request. Adding a random number to the end of the URL ensures that the browser will think the new URL is different, and won’t use the cache. The email script ignores this parameter, so it’s only useful for bypassing the cache.

Why are the characters reflected back different from the ones in the URL?

Your query parameter is URL decoded by the server before being reflected back at the user. You’ll need to made sure that your attack code is URL encoded. For example, use + instead of space and %2b instead of +. Here is a URL encoding reference and a handy conversion tool.

Are there any restrictions on the length of my solution URL?

No. Internet Explorer has a limit of 2083 characters or so for URLs, but the limit is much longer in Firefox, which is what we are using for grading. The grader’s solution was 297 characters, and shorter solutions are possible.

Since a sending an email is a side effect of the request, wouldn’t it make more sense for the browser’s request to be a POST instead of a GET?

Yes, but that would prevent the Image trick from working. Real-world attackers are rarely bothered by such semantic distinctions.

What should the email look like if the attack worked?

The parts that might be different are italicized.

From: Apache <apache@theory.stanford.edu>
To: grader@zoobar.org, youremail@stanford.edu
Subject: Message from group 'yourlelandusername'

Payload:

ZoobarLogin=YToyOntpOjA7czo4OiJhbnRpZ3JhdiI7aToxO3M6MzI6
ImZlOGIzMTM3YjgyNTFlMDQ2YTIzMzRkNTgxOWM2YWZiIjt9=

My attack is working. What should I do to make it invisible to the user?

The text box should its usual size and in the normal place. No warning text or characters that are normally part of the page should be visible. From the point of view of the visitor, it should appear as if they just went to users.php and didn’t put in a username yet (with the possible exception of the address bar, which can be whatever you want). It’s ok if the page briefly looks weird before correcting itself. Don’t worry about the fact that the header at the top of the page changes with each page view — that’s normal site behavior.

That sounds hard.

There are actually several quick and easy ways to do it, so try to think outside the box on this one. If you can’t figure it out, try moving on to the other attacks and come back to this one when you’re done.

Attack B.

Is this a cross site scripting attack?

No, this is a cross site request forgery attack. You are exploiting the fact that zoobar.org uses only a cookie to authenticate requests, even ones with side effects.

Can I use the vulnerability in user.php from Attack A?

No. Since this not a cross site scripting attack, you do not need to use the vulnerability in user.php. All you have to do convince the user’s browser to post malicious form data to transfer.php.

How do I convince the browser to send a malicious POST request to transfer.php?

Put together a form in your HTML document, with http://zoobar.org/transfer.php as the action attribute.

How can I get the form to be submitted with no user interaction?

You can call the "Send" button’s click method. Or, you can use JavaScript to call the form’s submit method. Keep in mind that the site is looking for a parameter named "submission" in the form data, so if it’s not there, the transfer won’t happen.

What <input> fields should the form contain?

Use the browser’s view source function on transfer.php and you’ll get a pretty good idea.

How can I submit a form to zoobar.org without causing the browser’s address bar to change to zoobar.org?

Create a hidden <iframe> and make sure the form’s target attribute matches the frame’s name attribute.

Uh oh, iframes. Will the grader have third-party cookie blocking enabled?

No, third party cookie blocking will not be enabled. It’s probably off by default on your browser, but if your attack isn’t working because the login cookie isn’t sent, you might want to check to make sure. Tools → Options → Privacy → Cookies → Uncheck “for the originating site only.”

How do I make the iframe hidden?

There are lots of ways to do it, but the easiest is probably <iframe style="visibility: hidden" ...>.

How do I redirect the browser to the CSE 127 home page?

Change the document.location property. Note that it is okay (and required, in fact) for the browser’s address bar to change to http://cseweb.ucsd.edu/classes/fa11/cse127-a/ once your attack is complete.

How do ensure that the redirect doesn’t happen until after the form data has been posted?

You can trigger the redirect from the frame’s onload handler. Depending on how your code is written, this onload handler may get called twice — once when the page initially loads and once when the form is submitted. If this is the case, you’ll have to make sure that you change document.location on the second time only.

Attack C.

Does PHP's SQLite driver support multiple SQL statements in a single query, separated by a semicolon?

No, it doesn't appear to. Your solution will somehow have to work without relying on injecting extra statements after those performed by the Zoobar site.

Attack D

Is this a phishing attack?

No.

What can we assume the grader will do when logging in?

The grader will type a username, then click the password field, type a password, then move the mouse over the login button and click it. The grader will then wait for the login to complete.

Why is the site using JavaScript to focus the username field?

It’s a convenience for the visitor, so they don’t have to select the username field manually when they first come to the page. You may it useful as well.

How do I get the browser to call my injected code?

Because your code is sanitized with htmlspecialchars, you won’t be able to inject a simple <script> tag like you did in Attack A. Trick the browser into running your code another way.

Does it have something to do with the GBK character set vulnerability discussed in class?

No, that’s not it.

I think I figured it out, but when I call alert to test my attack, nothing happens.

It turns out that calling alert would lead to an infinite loop of dialog boxes, so Firefox is trying to be helpful by preventing it. Try using something like document.loginform.login_username.value=42 to test whether your attack is working.

How am I supposed to invoke the email script from my script without using any characters that will get escaped?

There are numerous static methods of String that you might find usable. Also, don’t forget about escape, unescape, and eval.

Encoding by hand is incredibly tedious. Help?

You can save yourself some headaches if you write out your attack as a string in your attack page and then encode it programmatically, using those static String methods.

What do I need to do to make my attack invisible to the user?

You’ll have to get a handle on the relevant DOM nodes and make the extraneous text disappear, either by deleting the text or setting the style.display property to "none".

How can I get a handle on the warning message? It doesn’t have an id attribute.

It does have other distinguishing characteristics. You may find getElementsByTagName useful. Depending on how your attack works, you may not see a warning message anyway.

I’m sending the email when the form is submitted, but it isn’t working. What’s wrong?

There’s a race condition here where the form may be getting submitted before the email image is downloaded. Once the form submit starts, the thread that’s downloading the image is killed. So, to ensure that your attack always works, you should delay the form submission a little bit. You can use addEventListener with an event handler such as function(evt) { evt.preventDefault(); ... }. In this way, you can prevent the form submission until you’re ready to trigger it yourself.

Ok, I prevented the submission. How long should I wait?

If you’re lazy, you can use setTimeout with a reasonable number of milliseconds. But the precise way of doing it is to use addEventListener to wait for the "error" event to fire on the Image object you created. That event indicates that the email script’s server has started to respond with a non-image file, meaning that it successfully processed the email send request.

What should I do after the email is sent?

You can trigger the submission manually using the login button’s click() method. Use removeEventListener() to avoid infinite loops.

Is it okay if the attack doesn’t work when the HTML file is clicked on the desktop, but it does work when the URL of the HTML file is put into the browser address bar directly?

That is fine. We will test your attack by putting in the URL of your HTML file in the browser’s address bar.

What should the email look like if the attack worked?

The parts that might be different are italicized.

From: Apache <apache@theory.stanford.edu>
To: grader@zoobar.org, youremail@stanford.edu
Subject: Message from group 'yourlelandusername'

Payload:

grader,topsecret

Firefox keeps asking if I want to save the password. Is there any way I can turn this off?

You can probably turn it off by setting autocomplete="no" in each of your form fields, but it shouldn’t present much of an issue for grading. The grader will have already hit "Never for this site" before testing your attack, so the dialog won’t come up.

Attack E.

How does the site sanitize profiles?

It uses strip_tags() to restrict the tags that can be used, and it uses a regular expression to replace certain dangerous words like "onmouseover" with a space character. Note that strip_tags() will remove all tags that are more than 1024 characters. If your solution occurs inside a tag, you will have to make sure it fits inside this limit, or it won’t render to the user.

What tags does zoobar.org allow in profiles?

<a> <br> <b> <h1> <h2> <h3> <h4> <i> <img> <li> <ol> <p> <strong> <table> <tr> <td> <th> <u> <ul> <em> <span>

What strings does zoobar.org not allow in profiles?

javascript: eval setTimeout setInterval window target onAbort onBlur onChange onClick onDblClick onDragDrop onError onFocus onKeyDown onKeyPress onKeyUp onLoad onMouseDown onMouseMove onMouseOut onMouseOver onMouseUp onMove onReset onResize onSelect onSubmit onUnload
Keep in mind that this is an example of what not to do. Blacklisting keywords is a recipe for disaster and will annoy, but not limit, a determined attacker.

How do I transfer the zoobar?

Create an <iframe> pointing to transfer.php, set the appropriate form fields inside it, and post the form. Alternatively, you can create a form on the current page with transfer.php as its target, and then post it with the target pointing at an <iframe>. Another option is to use XMLHttpRequest, since you’re actually making a same-site request this time. Pick whichever approach you prefer.

How do I create an <iframe>?

You can use the DOM methods document.createElement and document.body.appendChild.

How do I get a handle on form fields inside the <iframe>?

It differs by browser, and only works when the frame’s domain matches the parent page (that’s the Same Origin Principle). Here’s the Firefox way of doing it: iframe.contentDocument.forms[0].zoobars.value = 1;

Are there any alternatives to target, which is blacklisted?

You can use string concatenation to express "target" without actually saying it. The following are equivalent in JavaScript: x.target, x["target"], x["tar"+"get"].

How do I replace the profile?

Use the same technique you just used on transfer.php, but point at index.php instead.

Is there an easy way to get a copy of the current profile?

You can use document.getElementById('profile').innerHTML, but it may mangle quotes in your profile, so be sure to check that the replicated profile is still functional. Also, note that only display:inline tags can be nested inside a <p>.

Thanks

This is the Project 2 FAQ from Stanford’s CS 155, Computer and Network Security. See here for the original version. Thanks to Dan Boneh, John Mitchell, Collin Jackson, and the 155 TAs.


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