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Authentication mechanism absence or evasion - Response tampering

Need​

Implementation of a secure and reliable authentication mechanism

Context​

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

Description​

Non compliant code​

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class OTPValidation {
    public void validateOTP(HttpServletRequest request, HttpServletResponse response) {
        String otp = request.getParameter("otp");
        String successMessage = "OTP validation success";
        
        if (otp.equals("123456")) {
            response.getWriter().write(successMessage);
        }
    }
}

The above code represents a servlet that handles OTP validation. The validateOTP method retrieves the OTP from the request parameters and checks whether it equals "123456". If it does, the method writes a success message to the response.

The vulnerability lies in the way the OTP validation is performed. The OTP check is done directly against a hardcoded string "123456", and the success message is sent in the response. An attacker could potentially intercept the response and modify it to include the success message, bypassing the actual OTP check. This could allow the attacker to continue with the flow and perform unauthorized actions.

Specifically, the absence of a secure authentication mechanism and the potential for response tampering make this code vulnerable. There is no mechanism to ensure that the OTP check is not tampered with, and the response can be manipulated to trick the system into thinking the OTP validation was successful when it was not.

To mitigate this vulnerability, the application should implement secure OTP validation that cannot be bypassed by response tampering. This could involve using secure session management, encrypting the response, or implementing a secure server-side OTP validation mechanism.

Steps​

  • Implement a secure authentication mechanism
  • Use a secure and strong OTP validation process
  • Do not rely solely on the response of the request for OTP validation
  • Perform server-side validation of the OTP
  • Implement proper error handling and response messages

Compliant code​

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

public class OTPValidation {
    private static final String REAL_OTP = "123456"; // This should be dynamically generated and stored securely in a real application

    public void validateOTP(HttpServletRequest request, HttpServletResponse response) {
        HttpSession session = request.getSession();
        String otp = request.getParameter("otp");
        String successMessage = "OTP validation success";
        String failureMessage = "OTP validation failed";

        // Perform server-side validation of the OTP
        if (otp != null && otp.equals(session.getAttribute("REAL_OTP"))) {
            response.getWriter().write(successMessage);
        } else {
            response.getWriter().write(failureMessage);
        }
    }
}

The updated code now includes a secure OTP validation process. The OTP is stored in the server-side session and is compared with the OTP received from the client. This way, even if an attacker tries to tamper with the response, the server-side validation will fail if the OTPs do not match.

The REAL_OTP is a constant in this example, but in a real application, it should be dynamically generated and stored securely. The OTP should also be invalidated after a certain period of time or after it has been used to prevent replay attacks.

The code also includes proper error handling and response messages. If the OTP validation fails, a failure message is sent to the client. This gives the client clear feedback on the result of the OTP validation.

This solution mitigates the risk of an attacker bypassing the OTP validation by tampering with the response. The OTP validation is now performed securely on the server side, which is much harder for an attacker to manipulate.

References​

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