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 the pie-chart below, created by the Web Hacking Incident Database for 2011 (WHID) clearly shows that whilst many different attack methods exist, SQL injection and XSS are the most popular. To add to this, many other attack methods, such as Information Disclosures, Content Spoofing and Stolen Credentials could all be side-effects of an XSS attack.
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.
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“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).
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