Consider that we have:
An SPA or a statically generated JAMStack website.
A REST API.
The website is being served with nignx that also reverse proxies to our API.
--
It is required that a user should be able to register/authenticate with an identity provider (say, Google) through the OpenID Connect protocol. For the sake of simplicity, let us assume that the user has already registered with our API.
Talking about authentication using OIDC, from what I have read on the subject, the steps you take are the following:
Register the application with the IdP and receive a client id and a secret.
When the user initiates a login (with Google) request on the API ('/api/loginWithGoogle') the API sets a state variable on the request session (to prevent CSRF) and redirects the user-agent to the IdP's login page.
At this page, the user enters their credentials and if they are correct, the IdP redirects the user to the callback URL on the API callback (/api/callback).
The request received on the callback has the state parameter (which we should verify with the one we set on the session previously) and a code parameter. We exchange the code for the identity token with the authorization server/IdP (we also receive access/refresh tokens from the auth server, which we discard for now because we do not want to access any APIs on the behalf of the user).
The identity token is parsed to verify user identity against our database (maybe an email).
Assume that the identity is verified.
-- The next part is what's giving me trouble --
The documentation that I have read advises that from here we redirect the user to a URL (e.g. the profile page)and start a login session between the user agent and the API. This is fine for this specific architecture (with both the SPA/static-site being hosted on the same domain).
But how does it scale?
Say I want to move from a session based flow to a JWT based flow (for authenticating to my API).
What if a mobile application comes into the picture? How can it leverage a similar SSO functionality from my API?
NOTE: I have read a little on the PKCE mechanism for SPAs (I assume it works for JAMStack as well) and native mobile apps, but from what I gather, it is an authorization mechanism that assumes that there is no back-end in place. I can not reconcile PKCE in an authentication context when an API is involved.
Usually this is done via the following components. By separating these concerns you can ensure that flows work well for all of your apps and APIs.
BACKEND FOR FRONTEND
This is a utility API to keep tokens for the SPA out of the browser and to supply the client secret to the token service.
WEB HOST
This serves unsecured static content for the SPA. It is possible to use the BFF to do this, though a separated component allows you to serve content via a content delivery network, which some companies prefer.
TOKEN SERVICE
This does the issuing of tokens for your apps and APIs. You could use Google initially, though a more complete solution is to use your own Authorization Server (AS). This is because you will not be able to control the contents of Google access tokens when authorizating in your own APIs.
SPA CLIENT
This interacts with the Backend for Frontend during OAuth and API calls. Cookies are sent from the browser and the backend forwards tokens to APIs.
MOBILE CLIENT
This interacts with the token service and uses tokens to call APIs directly, without using a Backend for Frontend.
BUSINESS APIs
These only ever receive JWT access tokens and do not deal with any cookie concerns. APIs can be hosted in any domain.
SCALING
In order for cookies to work properly, a separate instance of the Backend for Frontend must be deployed for each SPA, where each instance runs on the same parent domain as the SPA's web origin.
UPDATE - AS REQUESTED
The backend for frontend can be either a traditional web backend or an API. In the latter case CORS is used.
See this code example for an API driven approach. Any Authorization Server can be used as the token service. Following the tutorial may help you to see how the components fit together. SPA security is a difficult topic though.
I need to build a REST API that will be used by one private SPA web application. The problem is that the REST API must be on a different server than the SPA, so it must allow CORS. For two layers of security, the API can require Basic Authentication over SSL plus check for the correct client IP address.
It will not be possible to have a user login process for the API, since it is a service used by the SPA. The user is already logged into the client that hosts the SPA. I will not have access to anything other than the user's ID, and the REST API will have no way to validate this user ID.
Because the user is logged into the client's server, the API can be restricted to requests from the SPA's IP address, but that still potentially could allow anyone on that server to use the API other than the SPA.
Is this adequate protection?
I am considering adding a third layer of protection in the way of an access token, but this would be also fairly simple to uncover. If you know the basic authentication information (easily obtainable) and your IP address matches the expected client IP, then you are able to call the API to get an access token.
Also, with an access token comes the complexity of dealing with expired tokens.
Is it impossible to completely secure an API that requires a web-based client?
How many layers of security are enough?
Preface
I'm developing several web services and a handful of clients (web app, mobile, etc.) which will interface with said services over HTTP(s). My current work item is to design an authentication and authorization solution for the product. I have decided to leverage external identity providers, such as Facebook, Google, Microsoft, Twitter, and the like for authentication.
I'm trying to solve the problem of, "when a request comes to my server, how do I know who the user is and how can I be sure?". More questions below as well...
Requirements
Rely on external identities to indicate who I'm dealing with ('userId' essentially is all I care about).
The system should use token-based authentication (as opposed to cookies for example or basic auth).
I believe this is the right choice for scaling across multiple clients and servers while providing loose coupling.
Workflow
Based on my reading and understanding of token-based authentication, the following is how I imagine the workflow to be. Let's focus for now on Facebook in a web browser. My assumption is that other external identity providers should have similar capabilities, though I have not confirmed just yet.
Note, as of writing, I'm basing the following off of Facebook login version 2.2
Client: Initiates login to Facebook using the JavaScript SDK
Facebook: User authenticates and approves app permissions (to access user's public profile for example)
Facebook: Sends response to client which contains user’s access token, ID, and signed request
Client: Stores user access token in browser session (handled by SDK conveniently)
Client: Makes a request to my web service for a secure resource by sending along the user’s access token in the authorization header + the user’s ID (in custom header potentially)
Server: Reads user access token from request header and initiates verification by sending a request to the debug_token graph API provided by Facebook
Facebook: Responds back to the server with the user access token info (contains appId and userId)
Server: Completes verification of the token by comparing the appId to what is expected (known to itself) and the userId to what was sent on the client request
Server: Responds to the client with the requested resource (assuming the happy authorization path)
I’m imagining steps 5-9 would be repeated for subsequent requests to the server (while the user’s access token is valid – not expired, revoked from FB side, app permissions changed, etc.)
Here's a diagram to help go along with the steps. Please understand this system is not a single page application (SPA). The web services mentioned are API endpoints serving JSON data back to clients essentially; they are not serving HTML/JS/CSS (with the exception of the web client servers).
Questions
First and foremost, are there any glaring gaps / pit falls with the described approach based on my preface and requirements?
Is performing an outbound request to Facebook for verifying the access token (steps 6-8 above) per client request required / recommended?
I know at the very least, I must verify the access token coming from the client request. However, the recommended approach for subsequent verifications after the first is unknown to me. If there are typical patterns, I’m interested in hearing about them. I understand they may be application dependent based on my requirements; however, I just don’t know what to look for yet. I’ll put in the due diligence once I have a basic idea.
For instance, possible thoughts:
Hash the access token + userId pair after first verification is complete and store it in a distributed cache (accessible by all web servers) with expiry equal to access tokens. Upon subsequent requests from the clients, hash the access token + userId pair and check its existence in the cache. If present, then request is authorized. Otherwise, reach out to Facebook graph API to confirm the access token. I’m assuming this strategy might be feasible if I’m using HTTPS (which I will be). However, how does performance compare?
The accepted answer in this StackOverflow question recommends creating a custom access token after the first verification of the Facebook user token is complete. The custom token would then be sent to the client for subsequent requests. I’m wondering if this is more complex than the above solution, however. This would require implementing my own Identity Provider (something I want to avoid because I want to use external identity providers in the first place…). Is there any merit to this suggestion?
Is the signedRequest field present on the response in step #3 above (mentioned here), equivalent to the signed request parameter here in the ‘games canvas login’ flow?
They seem to be hinted as equivalent since the former links to the latter in the documentation. However, I’m surprised the verification strategy mentioned on the games page isn’t mentioned in the ‘manually building a login flow’ page of the web documentation.
If the answer to #3 is ‘Yes’, can the same identity confirmation strategy of decoding the signature and comparing to what is expected to be used on the server-side?
I’m wondering if this can be leveraged instead of making an outbound call to the debug_token graph API (step #6 above) to confirm the access token as recommended here:
Of course, in order to make the comparison on the server-side, the signed request portion would need to be sent along with the request to the server (step #5 above). In addition to feasibility without sacrificing security, I’m wondering how the performance would compare to making the outbound call.
While I’m at it, in what scenario / for what purpose, would you persist a user's access token to a database for example?
I don’t see a scenario where I would need to do this, however, I may be overlooking something. I’m curious was some common scenarios might be to spark some thoughts.
Thanks!
From what you describe I'd suggest to use a server-side login flow as described in
https://developers.facebook.com/docs/facebook-login/manually-build-a-login-flow/v2.2
so that the token is already on your server, and doesn't need to be passed from the client. If you're using non-encrypted connections, this could be a security risk (e.g. for man-in-the-middle attacks).
The steps would be:
(1) Logging people in
You need to specify the permission you want to gather from the users in the scope parameter. The request can be triggered just via a normal link:
GET https://www.facebook.com/dialog/oauth?
client_id={app-id}
&redirect_uri={redirect-uri}
&response_type=code
&scope={permission_list}
See
https://developers.facebook.com/docs/facebook-login/manually-build-a-login-flow/v2.2#login
(2) Confirm the identitity
GET https://graph.facebook.com/oauth/access_token?
client_id={app-id}
&redirect_uri={redirect-uri}
&client_secret={app-secret}
&code={code-parameter}
https://developers.facebook.com/docs/facebook-login/manually-build-a-login-flow/v2.2#confirm
(3) Inspect the access token
You can inspect the token as you already said in your question via
GET /debug_token?input_token={token-to-inspect}
&access_token={app-token-or-admin-token}
This should only be done server-side, because otherwise you'd make you app access token visible to end users (not a good idea!).
See
https://developers.facebook.com/docs/facebook-login/manually-build-a-login-flow/v2.2#checktoken
(4) Extending the access token
Once you got the (short-lived) token, you can do a call to extend the token as described in
https://developers.facebook.com/docs/facebook-login/access-tokens#extending
like the following:
GET /oauth/access_token?grant_type=fb_exchange_token
&client_id={app-id}
&client_secret={app-secret}
&fb_exchange_token={short-lived-token}
(5) Storing of access tokens
Concerning the storing of the tokens on the server, FB suggests to do so:
https://developers.facebook.com/docs/facebook-login/manually-build-a-login-flow/v2.2#token
(6) Handling expired access tokens
As FB doesn't notify you if a token has expired (and if you don't save the expiry date and compare this to the current timestamp before making a call), it's possible that you receive error messages from FB if the token got invalid (after max. 60 days). The error code will be 190:
{
"error": {
"message": "Error validating access token: Session has expired at unix
time SOME_TIME. The current unix time is SOME_TIME.",
"type": "OAuthException",
"code": 190
}
}
See
https://developers.facebook.com/docs/facebook-login/access-tokens#expiredtokens
If the access token becomes invalid, the solution is to have the person log in again, at which point you will be able to make API calls on their behalf once more. The login flow your app uses for new people should determine which method you need to adopt.
I dont' see any glaring gaps / pit falls, but I'm not a security expert.
Once your server has verified the given token (step 8), as you said:
The accepted answer in this StackOverflow question recommends creating a custom access token after the first verification of the Facebook user token is complete. The custom token would then be sent to the client for subsequent requests. I’m wondering if this is more complex than the above solution, however. This would require implementing my own Identity Provider (something I want to avoid because I want to use external identity providers in the first place…). Is there any merit to this suggestion?
IMHO is the way to go. I would use https://jwt.io/ which allows you to encode values (the userId for example) using a secret key.
Then your client attach this token to every request. So you can verify the request without need to a third party (you don't need database queries neither). The nice thing here is there is no need to store the token on your DB.
You can define an expiration date on the token, to force the client authenticate with the third party again when you want.
Let's say you want your server be able to do some action without the client interaction. For example: Open graph stories. In this scenario because you need to publish something in the name of the user you would need the access token stored on your DB.
(I can not help with the 3 and 4 questions, sorry).
Problem with Facebook is that they do not use OpenId connect on top of Oauth (https://blog.runscope.com/posts/understanding-oauth-2-and-openid-connect).
Thus resulting in their custom ways of providing Oauth authentification.
Oauth2 with OpenId connect identity services usually provide issuer endpoint where you can find URL (by appending ".well-known/openid-configuration") for jwk's which can be used to verify that JWT token and its contents were signed by the same identity service. (i.e access token originated from the same service that provided you jwk's)
For example some known openid connect identity providers:
https://accounts.google.com/.well-known/openid-configuration
https://login.microsoftonline.com/common/v2.0/.well-known/openid-configuration
(btw it is not a coincidence that Attlasian provides only these two services to perform external login)
Now as you mentioned, you need to support multiple oauth providers and since like Facebook not all providers use same configuration of oauth (they use different JWT attribute names, toke verification methods, etc. (Openid connect tries to unify this process)) i would suggest you to use some middleware identity provider like Oauth0 (service not protocol) or Keycloak. These can be used with external identity providers (Social pages as you mentioned) and also provides you with custom user store.
Advantage is that they unify authentication process under one type (e.g both support openid connect). Whereas when using multiple oauth providers with not unified authentication workflow you will end up with redudant implementations and need for merging different information's under one type (this is basically what mentioned middle-ware identity providers solve for you).
So if you will use only Facebook as identity provider in your app then go for it and make implementation directly for Facebook Oauth workflow. But with multiple identity providers (which is almost always case when creating public services) you should stick with mentioned workaround or find another one (or maybe wait till all social services will support Openid connect, which they probably wont).
There may be hope.. This year, Facebook have announced a "limited login" feature, which, if they were to add to their javascript sdks would certainly make my life easier:
https://developers.facebook.com/blog/post/2021/04/12/announcing-expanded-functionality-limited-login/
At the time of writing, I can only find reference to iOS and Unity SDKs, but it does seem to return a normal JWT, which is all I want!
I am creating a solution that will contains a website and mobile apps. I will use Zend-Framework 2 for the website.
So, to make it good, I am wondering if it would be a good idea to build :
A REST web service (using zf2)
Another website that will call the REST ws (using zf2)
The mobile apps that will call the REST ws
I will use OAuth for the autentication and security.
My question is, if my website gets the data by calling the REST ws, it will have to make a database request at each call to check the token whereas if I do a "normal" website, my app will be able to use session to store the information of the connected user.
Because, for what I have read, there is no such thing as session with OAuth/REST so for each call, I have one more sql request to check the token validity.
Is it still a good idea to make a full REST service, even for the website or to have a "normal" website and also a REST service API just for the mobile apps ?
Thanks
Oauth is a server to server authentication framework. Like it is between mobile app and your API server , website vs your API server etc. You can adopt an approach where , you generate only one access token for your website client instead of multiple access token for each user from the website. This access token is stored in your webserver vs user cookie in website.Ultimately the aim is to identify all the clients of your REST WS and your website is one of its client and a very trusted one.
This way you can cache the access token to avoid db calls (typically cache time can be equal to or less than token expiry time). Do explore the multiple grant types specified in the oauth spec for this
Regarding maintaining session for user in your website, it is not dependent on whether the back end is a REST WS or not, it can be handled in your website
When using User-agent flow with OAuth2 for mobile platform, there is no way for Authorization server to authenticate the client_id of the application.
So, anyone can impersonate my app by copying the client_id (and so get all access tokens on my behalf), and this is applicable to Facebook, Foursquare,...
This is not managed by OAuth2 ? Or I missed something ?
For Web applications (Web server flow), access token is stored on the server side, and the client is authenticated using secret key.
There's no good answer. Native app callbacks typically happen via custom registered URI schemes (e.g.: callback redirection URI is something like: myapp://oauth?code=xyz123). Unfortunately, any app can claim ownership of a given protocol scheme and receive the callback.
This problem is very synonymous with trying to lock down any protocol with "trusted clients". Think of the IM networks battle to lock out 3rd party clients (in early 2000's). Eventually they gave up - since whatever client & protocol endpoints are deployed could be reverse engineered by 3rd party developers.
Note: There is also some active discussion on this topic on the OAuth WG mailing list: http://www.ietf.org/mail-archive/web/oauth/current/msg08177.html
Normally client_id is associated with site's URL - OAuth responses/redirects will be sent only to the registrated Url. So attacker will not be able to receive results of the request on own site (unless somehow your and attacker pages are on the same domain).