Hackers are constantly experimenting with a wide repertoire of hacking techniques to compromise websites and web applications and make off with a treasure trove of sensitive data including credit card numbers, social security numbers and even medical records.

Cross Site Scripting (also known as XSS or CSS) is generally believed to be one of the most common application layer hacking techniques.

In general, cross-site scripting refers to that hacking technique that leverages vulnerabilities in the code of a web application to allow an attacker to send malicious content from an end-user and collect some type of data from the victim.

Today, websites rely heavily on complex web applications to deliver different output or content to a wide variety of users according to set preferences and specific needs. This arms organizations with the ability to provide better value to their customers and prospects. However, dynamic websites suffer from serious vulnerabilities rendering organizations helpless and prone to cross site scripting attacks on their data.

"A web page contains both text and HTML markup that is generated by the server and interpreted by the client browser. Web sites that generate only static pages are able to have full control over how the browser interprets these pages. Web sites that generate dynamic pages do not have complete control over how their outputs are interpreted by the client. The heart of the issue is that if mistrusted content can be introduced into a dynamic page, neither the web site nor the client has enough information to recognize that this has happened and take protective actions." (CERT Coordination Center).

Cross Site Scripting allows an attacker to embed malicious JavaScript, VBScript, ActiveX, HTML, or Flash into a vulnerable dynamic page to fool the user, executing the script on his machine in order to gather data. The use of XSS might compromise private information, manipulate or steal cookies, create requests that can be mistaken for those of a valid user, or execute malicious code on the end-user systems. The data is usually formatted as a hyperlink containing malicious content and which is distributed over any possible means on the internet.

As a hacking tool, the attacker can formulate and distribute a custom-crafted CSS URL just by using a browser to test the dynamic website response. The attacker also needs to know some HTML, JavaScript and a dynamic language, to produce a URL which is not too suspicious-looking, in order to attack a XSS vulnerable website.

Any web page which passes parameters to a database can be vulnerable to this hacking technique. Usually these are present in Login forms, Forgot Password forms, etc…

N.B. Often people refer to Cross Site Scripting as CSS or XSS, which is can be confused with Cascading Style Sheets (CSS).

Is your site vulnerable to Cross Site Scripting
Our experience leads us to conclude that the cross-site scripting vulnerability is one of the most highly widespread flaw on the Internet and will occur anywhere a web application uses input from a user in the output it generates without validating it. Our own research shows that over a third of the organizations applying for our free audit service are vulnerable to Cross Site Scripting. And the trend is upward.

Example of a Cross Site Scripting attack
As a simple example, imagine a search engine site which is open to an XSS attack. The query screen of the search engine is a simple single field form with a submit button. Whereas the results page, displays both the matched results and the text you are looking for.

Example:
Search Results for "XSS Vulnerability"

To be able to bookmark pages, search engines generally leave the entered variables in the URL address. In this case the URL would look like:

http://test.searchengine.com/search.php?q=XSS%20

Vulnerability

Next we try to send the following query to the search engine:

<script type="text/javascript"> alert('This is an XSS Vulnerability') </script>

By submitting the query to search.php, it is encoded and the resulting URL would be something like:

http://test.searchengine.com/search.php?q=%3Cscript%3

Ealert%28%91This%20is%20an%20XSS%20Vulnerability%92%2

9%3C%2Fscript%3E

Upon loading the results page, the test search engine would probably display no results for the search but it will display a JavaScript alert which was injected into the page by using the XSS vulnerability.