How Can I Modify Request Parameters in a Servlet Filter for Security Enhancement?

Modifying Request Parameter with Servlet Filter

Developers often encounter situations where they require modifying request parameters before their processing by web applications, particularly when faced with legacy applications that are vulnerable to security issues like XSS. Modifying the request parameter can protect against malicious input and enhance the application's security.

Issue:

While attempting to implement a Servlet filter to sanitize an incoming request parameter for a vulnerable page in an existing web application running on Tomcat 4.1, a developer encounters the limitation that HttpServletRequest does not provide a setParameter method.

Solution:

The solution involves creating a custom HttpServletRequestWrapper subclass that overrides the getParameter method:

<code class="java">import javax.servlet.*;
import javax.servlet.annotation.WebFilter;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletRequestWrapper;

@WebFilter(filterName = "XSSFilter")
public class XssFilter implements Filter {

    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
        // Custom request wrapper to sanitize parameter
        HttpServletRequest wrappedRequest = new HttpServletRequestWrapper((HttpServletRequest) request) {
            @Override
            public String getParameter(String name) {
                // Sanitize the value here
                String sanitizedValue = sanitize(super.getParameter(name));
                return sanitizedValue;
            }
        };

        chain.doFilter(wrappedRequest, response);
    }

    private String sanitize(String value) {
        // Implement your sanitization logic here
        return value;
    }
}</code>

Instead of passing the original request to the filter chain, this filter utilizes the wrapped request, which intercepts and sanitizes the parameter before it reaches the application.

Alternative Solution:

Alternatively, to avoid using request wrappers, developers can opt to modify the servlet or JSP that processes the parameter, making it expect a request attribute instead. In this approach, the filter examines the parameter, sanitizes it, and sets the attribute on the request object using request.setAttribute. This solution is more elegant but requires modifications to other parts of the application.

By implementing either solution, developers can effectively modify request parameters before their processing by vulnerable parts of the application, preventing malicious input and enhancing the application's security.

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