Skip to main content

Authentication mechanism absence or evasion - Redirect

Need​

Implementation of a robust and secure authentication mechanism

Context​

  • Usage of Java 8 for developing applications with enhanced features and performance
  • Usage of javax.servlet-api for Java Servlet development

Description​

Non compliant code​

import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

public class VulnerableServlet extends HttpServlet {

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException {
        String username = request.getParameter("username");
        String redirectURL = request.getParameter("redirect");
        
        if (username != null && !username.isEmpty()) {
            response.sendRedirect(redirectURL);
        }
    }
}

The above code is a servlet that redirects the user based on a parameter provided in the request. This is a common practice in web applications, however, it can lead to vulnerabilities if not handled properly.

In this case, the servlet is vulnerable to an Authentication mechanism absence or evasion - Redirect attack. The attacker can gain access to the application by knowing a valid username and by changing the redirect URL with which the server responds.

The doGet method retrieves the username and redirect parameters from the request. If the username is not null and is not empty, the servlet sends a redirect response to the client using the URL provided by the redirect parameter.

The problem here is that the servlet does not validate the redirect parameter. An attacker can manipulate the redirect parameter to point to a malicious site, leading to a phishing attack. Additionally, the servlet does not authenticate the username before redirecting the user. This means that an attacker can enter a valid username and be redirected without having to provide a password or go through any other form of authentication.

This vulnerability can be exploited by an attacker to gain unauthorized access to the application, steal sensitive user data, or perform other malicious actions.

Steps​

  • Implement proper authentication mechanism
  • Validate user credentials before redirecting
  • Sanitize and validate input parameters
  • Use a whitelist approach for redirect URLs
  • Implement secure session management

Compliant code​

import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.util.List;

public class SecureServlet extends HttpServlet {

    private List<String> whitelistURLs = List.of("http://www.example1.com", "http://www.example2.com");

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException {
        String username = request.getParameter("username");
        String password = request.getParameter("password");
        String redirectURL = request.getParameter("redirect");

        if (username != null && !username.isEmpty() && password != null && !password.isEmpty()) {
            if (authenticate(username, password)) {
                if (whitelistURLs.contains(redirectURL)) {
                    response.sendRedirect(redirectURL);
                } else {
                    response.sendError(HttpServletResponse.SC_BAD_REQUEST, "Invalid redirect URL");
                }
            } else {
                response.sendError(HttpServletResponse.SC_UNAUTHORIZED, "Invalid credentials");
            }
        } else {
            response.sendError(HttpServletResponse.SC_BAD_REQUEST, "Missing credentials");
        }
    }

    private boolean authenticate(String username, String password) {
        // Implement your authentication logic here
        // This is just a placeholder
        return "admin".equals(username) && "password".equals(password);
    }
}

The above code fixes the vulnerability by implementing a proper authentication mechanism and validating user credentials before redirecting. It also sanitizes and validates input parameters and uses a whitelist approach for redirect URLs.

  1. Proper Authentication Mechanism: The code now requires both a username and password to be provided. These are then authenticated using a placeholder authenticate method. In a real-world application, this method would check the provided credentials against a database or other secure storage mechanism.

  2. Input Sanitization and Validation: The code checks that both a username and password have been provided before attempting to authenticate. If either is missing, it returns a 400 Bad Request error.

  3. Whitelist for Redirect URLs: The code now contains a whitelist of valid redirect URLs. When a redirect is requested, the provided URL is checked against this whitelist. If it is not found, a 400 Bad Request error is returned. This prevents an attacker from providing a malicious redirect URL.

  4. Secure Session Management: Although not shown in this code, in a real-world application, after successful authentication, a secure session should be established. This could involve setting a secure, HttpOnly cookie with a session ID that is tied to the user's authenticated state on the server.

References​

Last updated on