How to Modify Request Parameters in Servlet Filters for Security Enhancements?

Modifying Request Parameters with Servlet Filters

Problem:

A security vulnerability (XSS) exists in an existing web application hosted on Tomcat 4.1. Due to limitations in modifying the source code, a servlet filter is being considered to sanitize sensitive parameters before passing them to vulnerable pages. However, the ServletRequest interface lacks a setParameter method to alter parameter values.

Solution:

Option 1: HttpServletRequestWrapper Subclass

Although HttpServletRequest lacks the desired method, the HttpServletRequestWrapper class allows wrapping of one request with another. By subclassing this class and overriding the getParameter method, you can return the sanitized parameter value. This modified request can then be passed down the filter chain instead of the original request.

Code Snippet:

<code class="java">import javax.servlet.*;
import javax.servlet.http.*;

public class XssFilter implements Filter {

    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
            throws IOException, ServletException {
        // Create a wrapped request with sanitized parameter
        HttpServletRequest wrappedRequest = new HttpServletRequestWrapper((HttpServletRequest) request) {
            @Override
            public String getParameter(String parameterName) {
                String originalValue = super.getParameter(parameterName);
                return sanitize(originalValue);
            }
        };

        chain.doFilter(wrappedRequest, response);
    }

    ...
}</code>

Option 2: Use Request Attributes

A more refined approach involves modifying the vulnerable servlet or JSP to expect a request attribute instead of a parameter. The filter examines the parameter, makes necessary modifications, and sets the sanitized value as an attribute using request.setAttribute().

Code Snippet for JSP:

<code class="jsp"><usebean id="myBean" ...>
<setproperty name="myBean" property="sanitizedParam" value="${requestScope['sanitizedParam']}"></setproperty></usebean></code>

Code Snippet for Servlet:

<code class="java">import javax.servlet.*;
import javax.servlet.http.*;

public class MyServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws IOException, ServletException {
        // Retrieve the sanitized parameter from the attribute
        String sanitizedParam = (String) request.getAttribute("sanitizedParam");
    }
}</code>

Notes:

• The HttpServletRequestWrapper solution requires adherence to the Servlet specification, as some containers may raise errors if the wrapped request is not provided.
• The request attribute approach offers increased flexibility but requires modifications to other application components.

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