microsoft-authentication-library-for-dotnet: [Bug] Uno - Exchange authorization code for bearer token blocked by CORS in browser

Which Version of MSAL are you using ? 4.14

Platform WebAssembly

What authentication flow has the issue?

• Desktop / Mobile
  • Interactive
  • Integrated Windows Auth
  • Username Password
  • Device code flow (browserless)
• Web App
  • [ X ] Authorization code
  • OBO
• Web API
  • OBO

Other? - please describe;

Is this a new or existing app? This is a new app or experiment

Expected behavior Successfully exchange authorization code for bearer token

Actual behavior Exchange blocked by CORS

Additional context/ Logs / Screenshots I’m using the Uno platform to write an Azure B2C authenticated multi-platform app using auth code flow via PublicClientApplication. While most of the heads (i.e. Android, UWP, etc) work fine using this approach, I’m hitting an issue with the web assembly (WASM) head.

In the WASM project I’m able to show the sign-in page and successfully sign-in, but then hit a CORS error (“Access to fetch at ‘https://{TENANT}.b2clogin.com/tfp/{TENANT}.onmicrosoft.com/b2c_1_signupandsignin/oauth2/v2.0/token’ from origin ‘http://localhost:5000’ has been blocked by CORS policy”) when the PublicClientApplication endeavours to swap the authorization code for an access token.

Now given the WASM head is effectively a Single-Page Application, I tried following the instructions for Migrat[ing] a JavaScript single-page app from implicit grant to auth code flow. I added a new Application to my Azure B2C tenant and configured the “Single-page application” platform such that I now see the “Your Redirect URI is eligible for the Authorization Code Flow with PKCE.” notice in the “Grant types” section. It is my understanding that this configuration enables CORS but, even after these changes, I still hit exactly the same CORS issue within my application.

Capturing the session during login reveals that, as per the CORS protocol, the browser is making an OPTIONS request to the token endpoint which is responding with a 404 thereby preventing the POST to the token endpoint for authorization code exchange.

Here’s the session (with the Azure B2C Tenant replaced with ‘{TENANT}’, tokens replaced with ‘{TOKEN}’ and state replaced with ‘{STATE}’):

Method	Url	Result
GET	https://{TENANT}.b2clogin.com/tfp/{TENANT}.onmicrosoft.com/b2c_1_signupandsignin/oauth2/v2.0/authorize?scope=https%3A%2F%2F{TENANT}.onmicrosoft.com%2Fbackends%2Fread.only+offline_access+openid+profile&response_type=code&client_id=32bfdaf8-cc9f-4011-942d-851911aef544&redirect_uri=http%3A%2F%2Flocalhost%3A5000%2Fauthentication%2Flogin-callback.htm&client-request-id=293915f0-a719-46a8-8003-bba230c4a49c&x-client-SKU=MSAL.UnoWasm&x-client-Ver=4.4.0.0&x-client-OS=web&prompt=select_account&code_challenge=mTjqNj7n3fMaon2_7HrIzpfGhwLPIwqrpMRCBt47-I4&code_challenge_method=S256&state={STATE}	200 OK
POST	https://{TENANT}.b2clogin.com/{TENANT}.onmicrosoft.com/B2C_1_signupandsignin/SelfAsserted?tx=StateProperties={STATE}&p=B2C_1_signupandsignin	200 OK
GET	https://{TENANT}.b2clogin.com/{TENANT}.onmicrosoft.com/B2C_1_signupandsignin/api/CombinedSigninAndSignup/confirmed?csrf_token={TOKEN}&tx=StateProperties={STATE}&p=B2C_1_signupandsignin&diags=%7B%22pageViewId%22%3A%228a316cef-e06e-42c2-97a8-9dc9cfcd5d5a%22%2C%22pageId%22%3A%22CombinedSigninAndSignup%22%2C%22trace%22%3A%5B%7B%22ac%22%3A%22T005%22%2C%22acST%22%3A1591363250%2C%22acD%22%3A2%7D%2C%7B%22ac%22%3A%22T021%20-%20URL%3Ahttps%3A%2F%2F{TENANT}.b2clogin.com%2Fstatic%2Ftenant%2Ftemplates%2FAzureBlue%2Funified.cshtml%3Fslice%3D001-000%26dc%3DDB3%22%2C%22acST%22%3A1591363250%2C%22acD%22%3A55%7D%2C%7B%22ac%22%3A%22T029%22%2C%22acST%22%3A1591363250%2C%22acD%22%3A13%7D%2C%7B%22ac%22%3A%22T004%22%2C%22acST%22%3A1591363250%2C%22acD%22%3A3%7D%2C%7B%22ac%22%3A%22T019%22%2C%22acST%22%3A1591363250%2C%22acD%22%3A25%7D%2C%7B%22ac%22%3A%22T003%22%2C%22acST%22%3A1591363250%2C%22acD%22%3A52%7D%2C%7B%22ac%22%3A%22T002%22%2C%22acST%22%3A0%2C%22acD%22%3A0%7D%5D%7D	302 Found
GET	http://localhost:5000/authentication/login-callback.htm?state={STATE}&code={TOKEN}	200 OK
OPTIONS	https://{TENANT}.b2clogin.com/tfp/{TENANT}.onmicrosoft.com/b2c_1_signupandsignin/oauth2/v2.0/token	404 Not Found

I believe this is likely to be an issue with the Azure B2C service (and I will report an issue to them as soon as I can find an appropriate forum) but was wondering if the above flow looks correct to you?

Thanks, Ian