Identity Server 4能否为外部身份提供商支持Password授权类型？

Great question! Let’s break this down clearly based on how Azure AD and IdentityServer4 work together:

Azure AD Does NOT Natively Support the Password Grant Flow #

First, the hard truth: Azure AD intentionally avoids supporting the Resource Owner Password Credentials (ROPC) flow (the technical name for the "password" grant type) for most scenarios. Here’s why:

• Security risks: This flow requires your application to handle raw user passwords, which violates modern security best practices. Azure AD is designed to keep passwords out of third-party apps to prevent leaks and credential misuse.
• Limited exception: Microsoft does offer a restricted ROPC flow for Azure AD, but it’s only allowed for legacy applications that can’t use modern flows (like Authorization Code with PKCE), and it only works for pure Azure AD accounts (not Microsoft accounts, social logins, or external guests). Even then, Microsoft strongly discourages its use due to security concerns.

How to Handle This in IdentityServer4 #

Since you can’t directly use Azure AD for password-based validation in the Password Grant, here are your options:

1. The Recommended Approach: Stick to Modern Flows #

For Azure AD users, forget the Password Grant entirely. Use the flows that Azure AD supports natively (and securely):

• Authorization Code Flow with PKCE (for public clients like SPAs or mobile apps)
• Hybrid Flow (for server-side apps)
  These flows use HTTP redirects (which you already have working) and are far more secure than handling passwords directly.

2. If You Must Use Password-Like Validation (Not Recommended) #

If you have a legacy requirement that forces password-based validation for Azure AD users, you can wrap Azure AD’s limited ROPC flow in a custom validator:

• Implement IResourceOwnerPasswordValidator in your IdentityServer4 instance (just like you do for your local STS).
• In the validator’s ValidateAsync method, check if the user is trying to authenticate via Azure AD.
• For those users, call Azure AD’s token endpoint using the ROPC flow parameters:

  POST https://login.microsoftonline.com/{tenant-id}/oauth2/v2.0/token
  Content-Type: application/x-www-form-urlencoded
  
  client_id=your-client-id
  &username=user@domain.com
  &password=user-password
  &grant_type=password
  &scope=openid User.Read
  &client_secret=your-client-secret (if using a confidential client)
  

• If Azure AD returns a valid token, consider the user authenticated in IdentityServer4.
• Important caveats: This requires your Azure AD app to have the right permissions (e.g., User.Read delegated permission), and you’ll be taking on the security risk of handling passwords. Microsoft may also restrict or deprecate this flow in the future.

For Your Local Custom STS #

As you noted, implementing IResourceOwnerPasswordValidator is the perfect fit here. You can directly integrate your backend API’s password validation logic into this interface, which will work seamlessly with IdentityServer4’s Password Grant flow.

Final Takeaway #

Unless you have an unavoidable legacy constraint, avoid using the Password Grant with Azure AD. Modern flows are more secure, better supported, and align with Azure AD’s design principles. If you have no other choice, the custom validator wrapper is a possible (but risky) workaround.

内容的提问来源于stack exchange，提问作者Eugene S.