XSS - Cross Site Scripting

Cross site scripting (XSS) flaws are a relatively common issue in web application security. 
Cross-Site Scripting attacks are a type of injection problem, in which malicious scripts are injected into the otherwise benign and trusted web sites. Cross-site scripting (XSS) attacks occur when an attacker uses a web application to send malicious code, generally in the form of a browser side script, to a different end user. Flaws that allow these attacks to succeed are quite widespread and occur anywhere a web application uses input from a user in the output it generates without validating or encoding it.
An attacker can use XSS to send a malicious script to an unsuspecting user. The end user’s browser has no way to know that the script should not be trusted, and will execute the script. Because it thinks the script came from a trusted source, the malicious script can access any cookies, session tokens, or other sensitive information retained by your browser and used with that site. These scripts can even rewrite the content of the HTML page.

Cross-Site Scripting (XSS) attacks occur when:

• Data enters a Web application through an untrusted source, most frequently a web request.
• The data is included in dynamic content that is sent to a web user without being validated for malicious code.

Black Box testing and example

One way to test for XSS vulnerabilities is to verify whether an application or web server will respond to requests containing simple scripts with an HTTP response that could be executed by a browser. For example, Sambar Server (version 5.3) is a popular freeware web server with known XSS vulnerabilities. Sending the server a request such as the following generates a response from the server that will be executed by a web browser:

http://server/cgi-bin/testcgi.exe?<SCRIPT>alert(“Cookie”+document.cookie)</SCRIPT>

The script is executed by the browser because the application generates an error message containing the original script, and the browser interprets the response as an executable script originating from the server. All web servers and web applications are potentially vulnerable to this type of misuse, and preventing such attacks is extremely difficult.

Example 1:
Since JavaScript is case sensitive, some people attempt to filter XSS by converting all characters to upper case, rendering Cross Site Scripting utilizing inline JavaScript useless. If this is the case, you may want to use VBScript since it is not a case sensitive language.

JavaScript:
<script>alert(document.cookie);</script>
VBScript:
<script type="text/vbscript">alert(DOCUMENT.COOKIE)</script>

The purpose of the security test is to discover the vulnerabilities of the web application so that the developers can then remove these vulnerabilities from the application and make the web application and data safe from unauthorized actions.

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