How does Postman handle localhost OAuth 2 redirects? - postman

Question

When using Postman to fetch an access token via Authorization Code, one of the fields I need to enter is for the Callback URL, aka the redirect URI query param when it's making the request to the authorization endpoint. I understand this URL needs to be registered/whitelisted within the OAuth provider, but my question is how does postman actually handle/intercept that request/redirect back when it's localhost-based? For example, if I already had a local server running on http://locahost:8090, and I told postman to use http://localhost:8090 for that callback, how does Postman end up seeing that request/redirect back (to exchange the auth code for an access token) instead of my local web server handling that request?

Answer 1

TL;DR: Postman basically ignores the callback URL when processing the response.
The Long Story
It does need it, but only for the request. As you say, it needs to be correct - exactly matching the IdP client application config - but that's it.
Postman is just helping you acquire the token, it doesn't need to provide it to the consuming application, which is the whole point of the redirect URL - a static path known by the client app and the OAuth client application that makes sure an evil website / intermediary doesn't steal tokens by abusing the redirection flows.
Since it's not meant to work on a browser on the internet, Postman can ignore the redirect. Once the IdP responds with the token then, as far as Postman is concerned, it's good to go. It can save the token in the local token store and use it to make API requests.
Implicit Flow
This is set up to get a token from an Okta endpoint:
When I click "Request token", Postman makes a request like this:
GET https://exampleendpoint.okta.com/oauth2/default/v1/authorize?nonce=heythere&response_type=token&state=state&client_id={the_client_id}&scope=profile%20openid&redirect_uri=http%3A%2F%2Flocalhost%3A8080%2Fimplicit%2Fcallback
Postman pops a browser to make this request to the /authorize endpoint, at which the IdP then either creates the token (if the browser already has a cookie), or performs various redirects to authenticate the user and then create the token.
At the end of this flow, Postman will receive the 302 from the IdP that contains the token (on the location header). The target of that redirect is the redirect URL configured in the IdP:
302
Location: http://localhost:8080/implicit/callback#access_token=eyJraWQiOiJxOGRmTGczTERCX3BEVmk4YVBrd3JBc3AtMFU1cjB6cXRUMFJveFZRUVVjIiwiYWxnIjoiUlMyNTYifQ.{the_rest_of_the_token}&token_type=Bearer&expires_in=3600&scope=profile+openid&state=state
At this point Postman grabs the token from the #access_token parameter and it's good to go.
Auth Code Flow
Auth Code flow comes in 2 flavours:
Auth Code (Classic)
Auth Code + PKCE
Auth Code flow has been seen as "better" than the implicit flow because it requires a 2nd step in the process to get an access token. You hit authorize which gives the client a code and the code is then exchanged for the tokens. This code for token gives more chances for the server side components to do more stuff - extra checks, enrich tokens and various other things.
Q: Why are there 2 Auth Code flows?
A: The problem with this was that it required a server side component, which many SPA's and/or mobile apps didn't want to host. The endpoint that receives the code and gets the token(s) had to maintain credentials - a client id and client secret - which are required by the IdP when creating the token. PKCE is an extension that removes the requirement for a trusted server. It's adds computed hash to the /authorize call, which the IdP remembers, and then on the subsequent call to /token the client provides the source value of the hash. The server does the same computation, checks it's the same as that on the original request and is then satisfied that it's not handing out tokens to a bad guy.
Auth Code with PKCE
In terms of redirects, this is exactly the same as implicit. But for requests, it needs to make the second request to exchange the code for the tokens. The main differences here are
the access token URL, which is where to send the code and get tokens in response.
the code challenge and verifier, which are PKCE requirements for generating and computing the hash
The requests are now as follows:
The GET to /authorize
GET https://exampleendpoint.okta.com/oauth2/default/v1/authorize?nonce=heythere&response_type=code&state=state&client_id={client_id}&scope=profile%20openid&redirect_uri=http%3A%2F%2Flocalhost%3A8080%2Fimplicit%2Fcallback&code_challenge=E7YtiHqJRuALiNL_Oc5MAtk5cesNh_mFkyaOge86KXg&code_challenge_method=S256
Postman will pop the browser (and the IdP will redirect it through login if required)
The eventual code response is also 302, however the location header contains the code rather than tokens:
location: http://localhost:8080/implicit/callback?code=J3RlQqW122Bnnfm6W7uK&state=state
So now the client needs to call the endpoint defined in the "Access Token URL" field to get tokens:
POST https://exampleendpoint.okta.com/oauth2/default/v1/token
Body:
grant_type: "authorization_code"
code: "J3RlQqW122Bnnfm6W7uK"
redirect_uri: "http://localhost:8080/implicit/callback"
code_verifier: "Fqu4tQwH6bBh_oLKE2zr0ijArUT1pfm1YwmKpg_MYqc"
client_id: "{client_id}"
client_secret: ""
And the response is a good old 200 that doesn't redirect - the authorize call sends the client back to the final redirect landing page, and the POST is just a normal request with the tokens on the the response
{"token_type":"Bearer","expires_in":3600,"access_token":"eyJraWQiOiJxOGRmTGczTERCX3BEVmk4YVBrd3JBc3AtMFU1cjB6cXRUMFJveFZRUVVjIiwiYWxnIjoiUlMyNTYifQ.*******","scope":"profile openid","id_token":"eyJraWQiOiJxOGRmTGczTERCX3BEVmk4YVBrd3JBc3AtMFU1cjB6cXRUMFJveFZRUVVjIiwiYWxnIjoiUlMyNTYifQ.********"}