Need

Secure and reliable authentication mechanism

Context

• Usage of C# for building robust and scalable applications
• Usage of Microsoft.AspNetCore.Mvc for building web applications using the ASP.NET Core MVC framework
• Usage of System.Threading.Tasks for asynchronous programming in .NET

Description

Non compliant code

        [HttpPost]
public async Task ValidateOtp([FromBody] OtpModel model)
{
    var result = await _otpService.ValidateOtpAsync(model);
    return Ok(result);
}
        
        

In the above code snippet, we have a method ValidateOtp in an ASP.NET Core controller. This method takes in a model OtpModel as input which contains the OTP entered by the user. The method then calls a service _otpService.ValidateOtpAsync(model) to validate the OTP. The response of this service is directly sent as the HTTP response.

The vulnerability exists because the response is not verified or sanitized before being sent. An attacker can potentially modify the response to include a success message even when the OTP validation fails. This could allow the attacker to bypass the OTP validation step and gain unauthorized access to the system.

Steps

• Implement a secure authentication mechanism, such as using a secure token-based authentication or session-based authentication.
• Ensure that the authentication mechanism includes proper validation and verification of the OTP.
• Do not solely rely on the response of the request to validate the OTP. Perform additional server-side validation to ensure the integrity of the OTP validation process.
• Implement measures to prevent response tampering, such as using secure communication protocols (e.g., HTTPS) and implementing message integrity checks (e.g., digital signatures).
• Regularly update and patch the authentication mechanism to address any known vulnerabilities or weaknesses.

Compliant code

        [HttpPost]
[ValidateAntiForgeryToken]
public async Task ValidateOtp([FromBody] OtpModel model)
{
    if (!ModelState.IsValid)
    {
        return BadRequest(ModelState);
    }

    var result = await _otpService.ValidateOtpAsync(model);

    if (result == null)
    {
        return Unauthorized();
    }

    return Ok(result);
}
        
        

The updated code now includes a few additional security measures to prevent the OTP validation from being tampered with.

1. AntiForgeryToken: This attribute helps to prevent cross-site request forgery attacks. In a CSRF attack, an innocent end user is tricked by an attacker into submitting a web request that they did not intend. This attribute adds a token to the user's session, which must be sent with every POST request. This ensures that the user is who they say they are.

2. ModelState.IsValid: This checks if the received OTP model is valid or not. If the model is not valid, it will return a BadRequest response. This is a simple way to ensure that the server only processes valid requests.

3. Null Check on Result: After the OTP is validated, the result is checked for null. If the result is null, it means that the OTP validation failed and an Unauthorized response is returned. This prevents an attacker from being able to proceed with the flow even if they tamper with the response of the request.

These measures, combined with secure communication protocols (HTTPS) and regular updates and patches, will help to ensure the integrity of the OTP validation process and prevent response tampering.

References

• 365. Authentication mechanism absence or evasion - Response tampering