A Complete Guide to Azure AD B2C: Custom Attributes, App Registration, SSO, Cost Planning, and User Migration

As businesses scale and prioritize secure, seamless user experiences, identity management becomes central. Microsoft Entra ID (formerly Azure AD B2C) provides powerful identity and access management (IAM) tailored for customer-facing apps. This blog walks you through everything you need to know about setting up and managing Azure AD B2C custom attributes, app registration, enabling SSO, planning cost-efficient deployments, and migrating users with minimal friction.

🔐 Understanding Azure AD B2C Custom Attributes

When integrating Azure AD B2C into your application, extending
user profiles with custom attributes
allows you to store business-specific data — like whether users have accepted
terms or subscribed to newsletters.

What Do Custom Attributes Look Like?

Here’s an example of
a custom attribute returned in a Microsoft Graph API call:

extension_ prefix: Identifies this as a custom attribute.
e5988430254…: Refers to the Application (Client) ID of the app used to create the attribute — typically the B2C_1A_Extensions app.
If this app is deleted, all
associated custom attributes will be lost, so it’s crucial to protect it.

🔑Azure AD B2C custom attributes, Graph API, extension attributes, B2C_1A_Extensions

📱 App Registration in Azure AD B2C

All applications using Azure AD B2C authentication must be registered individually.

Key App Registration Details:

Up to 250 apps per B2C tenant.
Each app must:

Provide its own Application ID.
Include Reply URLs (multiple allowed within the same domain).
Specify the user flow or custom policy to initiate.

Why It Matters:

Enables secure, consistent OAuth2/OpenID Connect flows.
Important for implementing multi-tenant apps.
Allows precise redirect URI control during sign-ins.

🔑 Azure AD B2C app registration, reply URLs, Application ID, policy-based authentication

🔁 Enabling Single Sign-On (SSO)

Single Sign-On (SSO) allows users to authenticate once and access multiple apps without re-logging in — a key aspect of modern, frictionless UX.

Azure AD B2C SSO Options:

Tenant-wide SSO
Policy-based SSO
App-specific SSO
Disabled SSO

🧠 Pro Tip: All apps using the same policy and sharing a domain can benefit from session tokens for SSO.

🔑 Keywords: Azure AD B2C SSO, seamless login, policy-based SSO, single sign-on configuration.

💰 Azure AD B2C Pricing and Cost Planning

🔄 Azure AD B2C User Migration Strategies

A successful migration ensures existing users continue accessing your services without disruption.

Pre-Migration Checklist:

Clean junk data (spam email addresses, invalid accounts)
Deduplicate based on emails, phone numbers
Review old/unused accounts for pruning

Migration Strategy Comparison

Strategy	Best For	Requirements
Authentication Middleware	Staged app migration	Legacy system federation (SAML, OAuth, etc.)
Bulk Migration	Few apps & clear-text passwords	Full database access
Just-In-Time (JIT) Migration	Preserving password experience	API to validate credentials
Dual Repositories	Gradual migration & coexistence	Sync strategy and authority source needed

🔑 Keywords: Azure AD B2C user migration, bulk migration, JIT migration, dual identity repository

⚙️ How Just-In-Time (JIT) Migration Works

This is the most user-friendly migration approach when users’ passwords are stored securely (e.g., hashed).

Flow:

User logs in via Azure AD B2C.
B2C calls your JIT web service with credentials.
Service validates credentials with the legacy identity provider.
If valid:

User is created in B2C using Graph API.
Login session (JWT) is returned to the app.

📂 Example implementation: GitHub JIT Migration Sample

🔑 Keywords: Azure AD B2C JIT migration, seamless user migration, authenticate legacy users

🛠 Maintaining Two User Repositories

In some scenarios, businesses must maintain both B2C and a legacy identity system.

Key Decisions:

Define the authoritative source.
Implement sync mechanisms (e.g., via JIT service).
Ensure profile updates and password resets are replicated.

Password Management Considerations:

Reset on login: Easy, secure, and enforces new complexity.
Migrate as-is: Requires clear-text passwords.
Flag weak passwords: Force reset on next change attempt.

🔑 Keywords: Azure AD B2C password migration, syncing user repositories, dual identity store

📊 Example User Workflow Table

Workflow	Azure AD B2C as Primary	Legacy IDP as Primary
Sign-In	B2C authenticates directly	B2C federates or calls REST API
Sign-Up	Handled by B2C, sync to legacy	Handled by legacy, sync to B2C
Profile Edit	B2C updates both stores	Updates via legacy, sync to B2C
Password Reset	B2C with optional API update	Handled by legacy, B2C updated via API

🧠 Final Thoughts

Migrating to Azure AD B2C is more than just a tech upgrade — it’s a transformation of how you handle identity, security, and customer experience. Whether you need custom attribute management, SSO, or a hybrid identity model, Azure AD B2C can handle it — but planning is everything.

Key Takeaways: