Azure AD B2C: Unable to get token with authorization code

Question

I am currently following this guide on setting up authorization for an azure function. I'm currently trying to test the auth portion in fiddler separately from my web app to ensure I can get a valid token. I am able to successfully retrieve an authorization code however after that, I need to post some data to exchange the authorization code for a token. When trying to do that, I am being returned a 502 with message: DNS Lookup for "mytenantname.onmicrosoft.com" failed. System.Net.Sockets.SocketException No such host is known DNS Lookup for "mytenantname.onmicrosoft.com" failed. System.Net.Sockets.SocketException No such host is known as if the host does not exist at all. My post to the endpoint looks as follows where B2C_1_FunctionsPlaygroundSignInRegister is the name of my user flow:

POST https://mytenantname.onmicrosoft.com/oauth2/v2.0/token?p=B2C_1_FunctionsPlaygroundSignInRegister HTTP/1.1

User-Agent: Fiddler

Content-Type: application/x-www-form-urlencoded

Host: mytenantname.onmicrosoft.com

Body:grant_type=authorization_code&client_id=MyApplicationIdForTheAzure-AD-B2C-ap-here&code=myCodeHere&redirect_uri=jwt.ms

For reference, here is the working get request I send that I can type in a browser, be prompted with a login/register screen, and be returned an authorization code after logging in:

https://mytenantname.b2clogin.com/tfp/mytenantname.onmicrosoft.com/B2C_1_FunctionsPlaygroundSignInRegister/oauth2/v2.0/authorize?client_id=yApplicationIdForTheAzure-AD-B2C-ap-here&nonce=2&redirect_uri=https://functionapi.azurewebsites.net/.auth/login/aad/callback&scope=https://mytenantname.onmicrosoft.com/api/read&response_type=code 

Can someone please point out where I'm going wrong here? I've now spent a great deal of time on this issue and am out of ideas.

Answer 1

Your POST url and Host header are wrong.

See this section -

OAuth 2.0 authorization code flow in Azure Active Directory B2C

POST fabrikamb2c.onmicrosoft.com/oauth2/v2.0/token?p=b2c_1_sign_in HTTP/1.1
Host: https://fabrikamb2c.b2clogin.com

Note the Host header. That means your Fiddler request should read something like this:

POST https://mytenantname.b2clogin.com/mytenantname.onmicrosoft.com/oauth2/v2.0/token?p=...
Host: mytenantname.b2clogin.com

PS This is way better than Fiddler for what you're trying to do - https://oauthdebugger.com/