OAuth2 and REST
Securing REST-ful Web Services with OAuth2
Dave Syer, 2012
Twitter: @david_syer
Email: dsyer@vmware.com
Agenda
Why would I use OAuth2?
If I was going to use Spring how would that look?
What's the easiest way to get something working?
Blog: http://blog.cloudfoundry.org/2012/10/09/oauth-rest/
Introduction
There is a strong trend distributed systems with lightweight architectures
So what are people doing about security in such systems?
What is a Lightweight Service?
HTTP transport.
Text-based message content, usually JSON.
Small, compact messages, and quick responses.
REST-ful, or at least inspired by the REST
Some degree of statelessness
Interoperability.
What Are the Security Requirements
Identity and permissions:
how is identity and permission information conveyed to a service?
how is it decoded and interpreted?
what data are needed to make the access decision (user accounts, roles, ACLs etc.)?
how is the data managed: who is responsible for storing and retrieving it?
HTTP Basic Authentication
something of a lowest common denominator
supported on practically all servers natively and out of the box
ubiquitous support on the client side in all languages
Example:
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$ curl "https://$username:$password@myhost/resource"
So what's wrong with that?
Nothing, but...
Where do you get the credentials (the username and password)?
Fine for systems where all participants can share secrets securely
In practice that means small systems
Only supports username/password
Only covers authentication
User or Client Permissions
Finer-grained information about the authenticated party
Role-based access: very common, sometimes available in server/container
Need to categorize user accounts, e.g. USER and ADMIN
Often business requirements are more complex
Identity Management: Three Corners
Identity Management: Four Corners
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Centralized Identity Management
Centralized Identity Management
Centralized: scales better than peers sharing secrets
Peer-to-peer: N(N-1)/2 pairs
Centralized: N pairs
For user accounts the scalability benefit is even bigger
OAuth2
Centralizing accounts and secrets is great, but what about permissions?
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OAuth 2.0 adds an extra dimension - more information for the access decision
Standards always help in security
Lightweight - easy to curl
Requires HTTPS for secure operation, but you can test with HTTP
Quick Introduction to OAuth2
A Client application, often web application, acts on behalf of a User, but with the User's approval
Authorization Server
Resource Server
Client application
Common examples of Authorization Servers on the internet:
Facebook - Graph API
Google - Google APIs
Cloud Foundry - Cloud Controller
Typical Web Application Client
OAuth2 and the Lightweight Service
Example command line Client:
$ curl -H "Authorization: Bearer $TOKEN" https://myhost/resource
is a Resource Server
https://myhost
TOKENis a Bearer Token
it came from an Authorization Server
Role of Client Application
Register with Authorization Server (get a client_id and maybe a client_secret)
Do not collect user credentials
Obtain a token (opaque) from Authorization Server
On its own behalf - client_credentials
On behalf of a user
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Use it to access Resource Server
Obtaining a Client Credentials Token
A client can act in its own right (not on behalf of a user):
$ curl "https://myclient:mysecret@uaa.cloudfoundry.com"
-d grant_type=client_credentials -d client_id=myclient
Result:
{
access_token: FUYGKRWFG.jhdfgair7fylzshjg.o98q47tgh.fljgh,
expires_in: 43200,
client_id: myclient,
scope: uaa.admin
}
OAuth2 Key Features
Extremely simple for clients
Access tokens carry information (beyond identity)
Resource Servers are free to interpret tokens
Example token contents:
Client id
Resource id (audience)
User id
Role assignments
UAA Bearer Tokens
OAuth 2.0 tokens are opaque to clients
But they carry important information to Resource Servers
Example of implementation (from Cloud Foundry UAA, JWT = signed, base64-encoded, JSON):
{ "client_id":"vmc",
"exp":1346325625,
"scope":["cloud_controller.read","openid","password.write"],
"aud":["openid","cloud_controller","password"],
"user_name":"vcap_tester@vmware.com",
"user_id":"52147673-9d60-4674-a6d9-225b94d7a64e",
"email":"vcap_tester@vmware.com",
"jti":"f724ae9a-7c6f-41f2-9c4a-526cea84e614" }
Web Application Client Again
The Client wants to access a Resource on behalf of the User
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Obtaining a User Token
A client can act on behalf of a user (e.g. authorization_code grant):
Authorization Code Grant Summary
1. Authorization Server authenticates the User
2. Client starts the authorization flow and obtain User's approval
3. Authorization Server issues an authorization code (opaque one-time token)
4. Client exchanges the authorization code for an access token.
Role of Resource Server
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1. Extract token from request and decode it
2. Make access control decision
Scope
Audience
User account information (id, roles etc.)
Client information (id, roles etc.)
3. Send 403 (FORBIDDEN) if token not sufficient
Role of the Authorization Server
1. Grant tokens
2. Interface for users to confirm that they authorize the Client to act on their behalf
3. Authenticate users (/authorize)
4. Authenticate clients (/token)
#1 and #4 are covered thoroughly by the spec; #2 and #3 not (for good reasons).
Client Registration and Scopes
For secure channels a client has to authenticate itself to obtain a token, so it has to be known to the Authorization
Server. Registration provides at a mimimum:
authentication (shared secret)
registered redirect URI (optional but essential to prevent attacks)
allowed scopes (clients are not permitted access to all resources)
Also useful:
a way to identify which resources can be accessed
ownership information (which user registered the client)
More on Scopes
Per the spec they are arbitrary strings. The Authorization Server and the Resource Servers agree on the content
and meanings.
Examples:
Google: https://www.googleapis.com/auth/userinfo.profile
Facebook: email, read_stream, write_stream
UAA: cloud_controller.read, cloud_controller.write, scim.read, openid
Authorization Server has to decide whether to grant a token to a given client and user based on the requested
scope (if any).
UAA Scopes
UAA scopes are actually Groups in the User accounts
GET /Groups, Get /Users/{id}
{
"id": "73ba999e-fc34-49eb-ac26-dc8be52c1d82",
"meta": {...},
"userName": "marissa",
"groups": [
...
{
"value": "23a71835-c7ce-43ac-b511-c84d3ae8e788",
"display": "uaa.user",
"membershipType": "DIRECT"
}
],
}
Special Mention for Vmc
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The UAA authenticates requests from vmc in a special way:
$ curl https://uaa.cloudfoundry.com/oauth/authorize
-d response_type=token -d client_id=vmc
-d redirect_uri=https:uaa.cloudfoundry.com/redirect/vmc
-d source=credentials
-d username=$username -d password=$password
Result:
302 FOUND
...
Location: https:uaa.cloudfoundry.com/redirect/vmc#access_token=FUYGKRWFG.jhdfgair7fylzshjg.o98q47tgh.fljgh...
Authentication and the Authorization Server
Authentication (checking user credentials) is orthogonal to authorization (granting tokens)
They don't have to be handled in the same component of a large system
Authentication is often deferred to existing systems (SSO)
Authorization Server has to be able to authenticate the OAuth endpoints ( /authorize and /token)
It does not have to collect credentials (except for grant_type=password)
Cloud Foundry UAA Authorization Server
Cloud Foundry Login Server
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Role of Login Server
Authenticate users and collect user approvals for OAuth2 scopes
Send authenticated user info in trusted channel to UAA
Maintain SSO state (e.g. session cookie)
Branded UI
OAuth2 endpoints - delegate (pass through) to UAA
Cloud Foundry UAA as a General Purpose Solution
User Account and Authentication Service is part of Cloud Foundry
open source and fairly generic
sample apps (including login server)
wrapper for Spring Security OAuth
runs in a servlet container (e.g. tomcat)
easy for Spring developers to install and customize
look for UAA blogs at http://blog.cloudfoundry.org (and .com)
UAA OAuth Implementation
UAA makes some explicit choices where the spec allows it, and also adds some useful features:
Client registration validation, e.g. implicit has no secret
Client has separate allowed scopes for user tokens and client tokens (if allowed).
User account management: groups = scopes, period-separated
JWT tokens, signed but not encoded, includes audience (a.k.a. resource_id)
/userinfo endpoint for remote authentication (SSO)
Auto-approve for client apps that are part of platform
Special authentication channels for /authorize:
source=credentials - used by vmc
source=login - used by Login Server
(Login Server) autologin via code=...
UAA Resources (Endpoints)
Brief list of all the UAA endpoints (with valid scopes if it is an OAuth2 resource):
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OAuth2 Authorization Server: oauth/authorize and /oauth/token
User info endpoint (for SSO): /userinfo, scope openid
Token decoding endpoint: /check_token
Login info endpoint (open to anyone): /login
SCIM user account management: /Users, scopes [scim.read, scim.write].
Password changes: /Users/{id}/password, scope password.write
UAA Resources (continued)
Token management, e.g. cancelling an approval: /oauth/users/{id}/tokens and /oauth/clients/{id}/tokens, scopes
[tokens.read, tokens.write]
Client registration: /oauth/clients, scopes [clients.read, clients.write, clients.secret]
Password strength meter: /password
Management endpoints, used by the Cloud Foundry platform internally: /health and /varz
Alternatives to OAuth2
OAuth 1.0a
SAML
Custom solution, e.g. HMAC signed requests
Extensions to OAuth2
In Conclusion
Lightweight services demand lightweight infrastructure
Security is important, but should be unobtrusive
OAuth 2.0 is a standard, and has a lot of useful features
Spring Security OAuth aims to be a complete solution at the framework level
Cloud Foundry UAA adds some implementation details and makes some concrete choices
Links
http://github.com/springsource/spring-security-oauth
http://github.com/cloudfoundry/uaa
http://blog.cloudfoundry.org
http://blog.cloudfoundry.com
http://blog.springsource.org
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