XSS filter to protect from XSS attacks

Here are the excerpts from the chilling effect

Grossman, who founded his own research company, WhiteHat, claims XSS vulnerabilities can be found in 70 percent of websites. RSnake goes further. "I know Jeremiah says seven of 10. I'd say there's only one in 30 I come across where the XSS isn't totally obvious. I don't know of a company I couldn't break into [using XSS]."
If you apply Grossman's number to a recent Netcraft survey, which estimated that there are close to 100 million websites, you've got 70 million sites with XSS vulnerabilities. Repairing them one-off, two-off, 200,000-off is spitting in the proverbial ocean. Even if you've disclosed, you've done very little to reduce the overall risk of exploit. "Logistically, there's no way to disclose this stuff to all the interested parties," Grossman says. "I used to think it was my moral professional duty to report every vulnerability, but it would take up my whole day."


If we can put an XSS filter in the framework (for example, as a servlet filter in the Java EE) then it would make it a lot easier for the developers to not worry about all the places where they are accepting the input from the client browser. Instead they can just do a request.getParameter() and the servlet filter would do all the filtering for XSS characters with each and every request which comes in to the JSP pages or servlets.

I have created a xss filter which can be tested at http://www.attacklabs.com/xssfilter/. It filters only 8 characters but it protects from all the possible attack strings mentioned in the Rsnake’s cheat sheet. If you can find an xss attack which can bypass the filter, then please do submit it here or email it to me at anurag.agarwal@yahoo.com

Please bear in mind that this filter is not the right choice if you want your users to input HTML tags or urls.

Source Code

import java.io.*;
import java.net.*;

import javax.servlet.*;
import javax.servlet.http.*;

/**
*
* @author anurag agarwal
* @version 1.0
*/
public class XSSFilter extends HttpServlet {
private String[] filterChars = {"<", ">", "<", ">", "&#", "\"", "\\", "0x"};
private String[] replacementChars = {" ", " ", " ", " ", "#", "'", "/", "0 x"};

protected void processRequest(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html;charset=UTF-8");
PrintWriter out = response.getWriter();

String html = request.getParameter("html");
out.println("Here is the filtered output of the html you submitted.");

out.println(filterRequest(html));

out.close();
}

private String filterRequest(String param) {
String value = param;

if( param!=null) {

for(int i = 0; i < filterChars.length; i++) {
value = filterCharacters(filterChars[i], replacementChars[i], value);
}

}

return value;

}

private String filterCharacters(String originalChar, String newChar, String param)
{
StringBuffer sb = new StringBuffer(param);

for(int position = param.toLowerCase().indexOf(originalChar); position >= 0; ) {
sb.replace(position, position + originalChar.length(), newChar);
param = sb.toString();
position = param.toLowerCase().indexOf(originalChar);
}

return sb.toString();
}

protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}

protected void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}
}