Need

Implementation of robust data validation for OTP tokenCode parameter

Context

• Usage of C# 7.0 for modern language features and enhancements
• Usage of Microsoft.AspNetCore.Mvc for building web applications using the MVC pattern

Description

Non compliant code

        [HttpPost]
public IActionResult VerifyOTP(string tokenCode)
{
    // Assume this method checks the tokenCode against the database
    var isValid = CheckTokenCodeInDatabase(tokenCode);

    if (isValid)
    {
        // Continue with the process
        return Ok();
    }
    else
    {
        return Unauthorized();
    }
}
        
        

In the code snippet above, we have an API endpoint VerifyOTP which accepts a tokenCode as a parameter and verifies it using the CheckTokenCodeInDatabase method.

The vulnerability here is that there is no validation of the tokenCode before it is passed to the CheckTokenCodeInDatabase method. This means that an attacker could potentially send any random value as the tokenCode, and the server would still process it. This could potentially lead to brute force attacks where an attacker tries multiple random values until they get a valid one.

Furthermore, there is no limit on the number of attempts that a client can make to this endpoint. This means an attacker could make unlimited attempts to guess the tokenCode, which significantly increases the risk of a successful brute force attack.

Another point to consider is that, if the CheckTokenCodeInDatabase method is not properly implemented, it could potentially throw an exception or return an error that could reveal sensitive information about the system, further increasing the potential for a successful attack.

In conclusion, the lack of input validation and rate limiting makes this piece of code vulnerable to brute force attacks, and potentially information disclosure if the CheckTokenCodeInDatabase method is not properly implemented.

Steps

• Implement input validation to ensure that the tokenCode parameter is not empty or null.
• Validate the format of the tokenCode parameter to ensure it meets the expected criteria.
• Implement server-side validation to check if the tokenCode exists in the database.
• Consider implementing rate limiting or throttling mechanisms to prevent brute-force attacks.
• Consider implementing additional security measures such as IP whitelisting or user session validation.

Compliant code

        [HttpPost]
public IActionResult VerifyOTP(string tokenCode)
{
    // Validate the input
    if (string.IsNullOrEmpty(tokenCode) || !Regex.IsMatch(tokenCode, @"^\\d{6}$"))
    {
        return BadRequest("Invalid token code.");
    }

    // Assume this method checks the tokenCode against the database
    var isValid = CheckTokenCodeInDatabase(tokenCode);

    if (isValid)
    {
        // Continue with the process
        return Ok();
    }
    else
    {
        // Consider implementing rate limiting or throttling mechanisms here
        return Unauthorized();
    }
}
        
        

The updated code now includes input validation for the tokenCode parameter. It first checks if the tokenCode is null or empty. If it is, the method immediately returns a BadRequest response, indicating that the client sent an invalid request.

Next, it uses a regular expression to validate the format of the tokenCode. The regular expression @"^\\d{6}$" checks if the tokenCode is a six-digit number. If the tokenCode does not match this format, the method again returns a BadRequest response.

After validating the input, the method proceeds as before, checking if the tokenCode exists in the database and returning an Ok response if it does, or an Unauthorized response if it does not.

The comment in the else block suggests a place where you could implement rate limiting or throttling mechanisms to prevent brute-force attacks. This would involve tracking the number of failed attempts and limiting the number of attempts allowed in a certain time period.

Additional security measures such as IP whitelisting or user session validation could also be implemented to further secure the OTP verification process.

References

• 341. Lack of data validation - OTP