GarciaPL Notes - Java and Others

We all know what Cross Site Scripting (XSS) [1] means. In short the idea is that input parameters in our application should be checked for containing characters with special meaning in HTML for instance <, >, /. Those signs should be escaped by application into &amp;, &lt;, &gt;, &#x27;, &quot;, &#x2F before they will be processed by backend.

First of all you need implement Filter [7][2] which will intercept incoming requests to backend for further processing.

package pl.garciapl.xss;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import javax.servlet.*;
import java.io.IOException;

public class RequestFilter implements Filter {

    private final static Logger logger = LoggerFactory.getLogger(RequestFilter.class);

    public void init(FilterConfig fConfig) throws ServletException {
        logger.debug("RequestFilter initialized");
    }

    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
        chain.doFilter(new XSSRequestWrapper((HttpServletRequest) request), response);
    }

    public void destroy() {
    }
}

package pl.garciapl.xss;

import org.jsoup.Jsoup;
import org.jsoup.safety.Whitelist;
import org.owasp.esapi.ESAPI;

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletRequestWrapper;

public class XSSRequestWrapper extends HttpServletRequestWrapper {

    public XSSRequestWrapper(HttpServletRequest request) {
        super(request);
    }

    @Override
    public String[] getParameterValues(String parameter) {
        String[] values = super.getParameterValues(parameter);

        if (values == null) {
            return null;
        }

        int count = values.length;
        String[] encodedValues = new String[count];
        for (int i = 0; i < count; i++) {
            encodedValues[i] = stripXSS(values[i]);
        }

        return encodedValues;
    }

    @Override
    public String getParameter(String parameter) {
        String value = super.getParameter(parameter);
        return stripXSS(value);
    }

    @Override
    public String getHeader(String name) {
        String value = super.getHeader(name);
        return stripXSS(value);
    }

    private String stripXSS(String value) {
        if (value != null) {
            // It's highly recommended to use the ESAPI to avoid encoded attacks.
            value = ESAPI.encoder().canonicalize(value);
            // Avoid null characters
            value = value.replaceAll("", "");

            value = Jsoup.clean(value, Whitelist.none());
        }
        return value;
    }

}