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

🔑
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:

1. User logs in via Azure AD
   B2C.
2. B2C calls your JIT web
   service with
   credentials.
3. Service validates credentials
   with the legacy identity provider.
4. 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

🧠 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:

• Protect your B2C_Extensions app and custom attributes.
• Leverage flexible app
  registration and reply URL policies.
• Enable SSO to create seamless user
  journeys.
• Carefully plan for pricing and
  ancillary costs.
• Use JIT, bulk, or dual-repo
  migration strategies based on your business case.

 💬 Got questions or need help with your Azure AD B2C
migration? Drop them in the comments or contact us
for a free consultation.