Azure AD B2C Security

Implementing Advanced Input Validation

Input validation is a critical aspect of security and user experience in any application. Azure Active Directory B2C (Azure AD B2C) provides robust mechanisms to validate user inputs during sign-up and profile editing, ensuring data integrity and preventing malicious entries.

This tutorial dives into advanced techniques for input validation beyond the basic constraints, leveraging custom policies and Azure Functions to achieve sophisticated checks.

Why Advanced Input Validation?

• Data Integrity: Ensure the data entered by users conforms to expected formats and business rules.
• Security: Prevent common attacks like SQL injection, cross-site scripting (XSS), and other vulnerabilities by sanitizing and validating inputs.
• User Experience: Provide clear, immediate feedback to users on input errors, guiding them to correct their entries.
• Compliance: Meet regulatory requirements for data handling and privacy.

Techniques for Advanced Validation

1. Using Custom Policies (Identity Experience Framework)

Custom policies offer the highest degree of flexibility in Azure AD B2C. They allow you to define complex validation logic within your identity flows.

Leveraging Claims Transformations

Custom policies use the Identity Experience Framework (IEF) to define your identity workflows. Within these policies, you can use Claims Transformations to perform various checks on user-provided data:

• Regular Expressions: Validate formats like email addresses, phone numbers, or custom ID patterns.
• String Manipulation: Check for specific keywords, length, or casing.
• Mathematical Operations: Validate numerical ranges.
• Lookup/Existence Checks: Verify if an input exists in a predefined list or against an external system (often via API calls).

Here's a conceptual example of a validation step within a custom policy's technical profile:

<TechnicalProfile Id="AAD-UserReadUsingObjectId">
  <DisplayName>Azure Active Directory</DisplayName>
  <Protocol Name="Proprietary" Handler="Web.TPEngine.Providers.AzureActiveDirectoryProvider, Web.TPEngine, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null"/>
  <OutputClaims>
    <OutputClaim ClaimTypeReferenceId="objectId"/>
    <OutputClaim ClaimTypeReferenceId="displayName"/>
    <OutputClaim ClaimTypeReferenceId="userPrincipalName"/>
  </OutputClaims>
  <ValidationClaimsTransformations>
    <ClaimsTransformation ReferenceId="VerifyEmailFormat"/>
    <ClaimsTransformation ReferenceId="CheckPasswordStrength"/>
  </ValidationClaimsTransformations>
  <.-- Other configurations -->
</TechnicalProfile>

<ClaimsTransformation Id="VerifyEmailFormat" Method="IsMatchRegex">
  <InputClaims>
    <InputClaim ClaimTypeReferenceId="emailAddress" />
  </InputClaims>
  <InputParameters>
    <InputParameter Name="Regex" Value="^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$" />
  </InputParameters>
  <OutputClaims>
    <OutputClaim ClaimTypeReferenceId="emailAddressIsValid" />
  </OutputClaims>
</ClaimsTransformation>

2. Integrating Azure Functions for Real-time Validation

For complex validation logic that cannot be easily expressed in custom policies (e.g., checking against a large database, performing computationally intensive checks, or calling third-party APIs), Azure Functions are an excellent choice. You can trigger these functions from your Azure AD B2C custom policies.

Workflow:

1. The user enters data on the Azure AD B2C sign-up or profile editing page.
2. When a specific field is validated (e.g., on blur or form submit), a request is sent to a pre-registered Azure Function endpoint.
3. The Azure Function performs the complex validation (e.g., checking if a username is already taken in your application's database).
4. The Azure Function returns a success or failure response.
5. Azure AD B2C processes the response and either allows the user to proceed or displays an error message.

Example Azure Function (Node.js):

// Example: Check if username is available
exports.handler = async (event) => {
    const username = event.body.username; // Assuming username is passed in the request body

    // In a real scenario, this would involve a database lookup
    const isUsernameTaken = await checkUsernameInDatabase(username);

    if (isUsernameTaken) {
        return {
            statusCode: 400, // Bad Request
            headers: { "Content-Type": "application/json" },
            body: JSON.stringify({ message: "Username is already taken." })
        };
    } else {
        return {
            statusCode: 200, // OK
            headers: { "Content-Type": "application/json" },
            body: JSON.stringify({ message: "Username is available." })
        };
    }
};

async function checkUsernameInDatabase(username) {
    // Placeholder for actual database check logic
    console.log(`Checking availability for username: ${username}`);
    // Simulate a lookup: if username is 'admin', it's taken
    return username.toLowerCase() === 'admin';
}

3. Client-Side Validation (JavaScript)

While server-side validation is paramount for security, client-side validation using JavaScript enhances the user experience by providing immediate feedback. This should be used in conjunction with server-side validation, as client-side checks can be bypassed.

Using Azure AD B2C Extensions

Azure AD B2C's User Flows and custom policies allow for the integration of custom JavaScript. This JavaScript can interact with the HTML form elements to perform validation before submitting the form.

// Example: Simple JavaScript for email format check
document.addEventListener('DOMContentLoaded', () => {
    const emailInput = document.getElementById('email'); // Assuming an input with id="email"
    const emailErrorSpan = document.getElementById('email-error'); // Assuming a span for error messages

    if (emailInput && emailErrorSpan) {
        emailInput.addEventListener('blur', () => {
            const emailValue = emailInput.value;
            const emailRegex = /^[^\s@]+@[^\s@]+\.[^\s@]+$/;

            if (emailValue && !emailRegex.test(emailValue)) {
                emailErrorSpan.textContent = 'Please enter a valid email address.';
                emailInput.classList.add('invalid');
            } else {
                emailErrorSpan.textContent = '';
                emailInput.classList.remove('invalid');
            }
        });
    }
});

Best Practices for Input Validation

• Validate Everything: Never trust user input. Validate on both the client-side and server-side.
• Be Specific with Error Messages: Provide clear, actionable feedback to users about what needs to be corrected. Avoid generic messages that could be exploited.
• Use Whitelisting: Prefer validating against known good patterns (whitelisting) rather than trying to block known bad patterns (blacklisting).
• Sanitize Output: When displaying user-provided data, always sanitize it to prevent XSS attacks.
• Regularly Review and Update: Security threats evolve. Regularly review your validation rules and update them as needed.
• Rate Limiting: Implement rate limiting on sensitive operations (like password resets or sign-ups) to prevent brute-force attacks.

Conclusion

Implementing robust input validation is a cornerstone of securing your Azure AD B2C applications. By leveraging the power of custom policies, Azure Functions, and client-side JavaScript, you can create secure, user-friendly, and reliable identity experiences.

Remember that security is an ongoing process. Continuous monitoring and adaptation are key to staying ahead of potential threats.