OAuth 2.0 is not backward compatible with 1.0. Do not use 1.0.
Internet logins began with the standard "form login": username and password home-grown login on
a website that stashed session info in a cookie after successful login. The website and owner are
1. Security: Have to be aware of best practices and how they change over time
2. Maintenance: Testing auth system over time
OAuth and Open ID Connect are industry best practices at the time of writing to overcome the above
Pre mobile era commonly identity use cases included
1. Simple login (forms and cookies)
2. Single Sign On (SSO) using a protocol called (SAML)
Now, post 2010, two more use cases arise:
1. Mobile app login
2. Delegated authentication.
The client should never get the resource owner's password. This is because the password is generally
the "keys to the kingdom". For example, an app that wants to share a post on my facebook page. If I
give it my FB password it can share the post, but it can also get my friends list, send them messages,
block them, make other unauthoized posts and critically access ANY other service which I access using
my FB credentials!
So, I must avoid giving any third party my password. This is where OAuth comes in.
DELEGATED AUTHENTICATION is where you allow a 3rd party access to your data/platform, probably in
a restricted manner, WITHOUT giving the 3rd party you password.
Example of the "good old bad old days" were websites that asked for you email account password
so that they could send invite emails to your friends. This was GIVING AWAY THE KEYS TO THE
CASTLE! E.g. your bank password reset flow probably goes back there!!
What we want to do is say "Email provider, I authorize ABC.com to send emails on my behalf from
my account, as long as you authenticate them when they try to do this". This is delegated
authentication, and is what OAuth came out of. So ABC can send emails from my account, but,
importantly wouldn't be authorized to read my emails, for example, or add/delete contacts etc.
ABC says login to your email account -> Redirect me to email account. I trust my email provider
more than ABC so I am happy(er) to log into my email provider. The email provider then
authenticates me (makes sure its me and not Stevil). It then should warn me about the permissions
that ABC is seeking. If I say its OK it should then accept the instruction that it
should authorise ABC to send emails. It then returns a token to ABC, which ABC can then use
in requests to send emails that it sends to my email provider.
From Udemy Course https://www.udemy.com/course/learn-oauth-2:
The OAuth standard ensures that there is no unintended leakage of information about the resource owner
to the app. The app may access only specific resources with the explicit consent of the resource owner.
The app does not get the resource owner's credentials, which would be wildcard access to all of the user's
data. By protecting the resource owner's data, especially the personal and profile data of the resource
owner, the OAuth standard can be used to ensure the privacy of the resource owner.
- Resource Owner - You - The person who owns the data that the application wants access to. e.g. You
own your email resource - see above example.
- Client - Just refers to the application that wants access to your data.
- Authorization Server - System the resource owner uses to authorize the client - to say "yes".
- Resource Server - API that holds the data that the client wants to get to. Sometimes resource
server and auth server are same but oftentimes they are seperate.
- Authorization Grant - The "thing" that prooves that the resource owner has consented to a certain
level of client access to the resource. This authorization grant allows the client to go
back to the resource server and get the access token. The authorization code is exchanged for
the access token.
- Redirect URI - When auth server redirects back to the client application it redirects to the
- Access Token - The client needs an access token - the key they use to get into the data resource
owner granted access to.
See also https://www.youtube.com/watch?v=BNEoKexlmA4:
The access token is like the electronic RFID key card for a hotel room. You sign in at the front
desk, which is where you validate using your credentials, and once you have the key you only
need to present that to get into your room (the resource).
- Scope - The subset of permissions that the access token gives the client
Auth server will have a list of scopes it understands, e.g. contacts.read. Any types of
permissions that make sense for the particular resource being accessed.
E.g. Google scopes tend to be really long URL strings.
- Back Channel - Highly secure channel. API req or HTTP req from my server to Google over HTTPS
SSL/TLS encrypted - this is a back channel. From my backend server to another system. We
completely trust our server and code.
- Front Channel - Less secure than back channel.
E.g. Browser requested website. They can see my JS/HTML code or someone could look over my
shoulder and see my password as I type it in. We don't completely trust the browser.
Why Exchange Access Code For Access Token - Why The Extra Step?
Why do we get a code rather than the token right away? The reason for the extra step is to take
advantage of the "best things about the front channel and the best things about the back chanell".
The authorisation code is returned to the browser, i.e., over the front end channel. You can
see the code openly in the redirect URL.
This code is then given to the client backend, which then gets the actual access token using this
code over the more secure backend channel. That is why there is the second step so that the thing
that really grants the client access to a resource is only ever communicated over the more secure
and thus more trusted channel.
One Downside To OAuth
One downside to OAuth is that the login page can be easily "spoofed". Thus it is up to the user
to figure out whether the login page they are redirected to is genuine, e.g. by examining the
page address. But this can be harder/impossible if the login is embedded in another page.
From RFC6749 (The OAuth 2.0 Authorization Framework), section 9:
An embedded user-agent poses a security challenge because resource
owners are authenticating in an unidentified window without access
to the visual protections found in most external user-agents. An
embedded user-agent educates end-users to trust unidentified
requests for authentication (making phishing attacks easier to
And also in section 10.11 (Phishing Attacks):
Wide deployment of this and similar protocols may cause end-users to
become inured to the practice of being redirected to websites where
they are asked to enter their passwords. If end-users are not
careful to verify the authenticity of these websites before entering
their credentials, it will be possible for attackers to exploit this
practice to steal resource owners' passwords.
Service providers should attempt to educate end-users about the risks
phishing attacks pose and should provide mechanisms that make it easy
for end-users to confirm the authenticity of their sites. Client
developers should consider the security implications of how they
interact with the user-agent (e.g., external, embedded), and the
ability of the end-user to verify the authenticity of the
Flow In Brief
App wants to access my FB account
* It asks the OAuth server for a token
* OAuth asks me (the resource owner) if I want to give permission for a SUBSET of operations on
my resource (FB account)
* I authorize by giving my user name and password to the OAuth servier
OAuth servier validates this information either itself or by asking another third party
(OAuth server and resource sever can be seperate)
* Token is sent back to the app (client)
Thus the client never sees our password. It only gets a "passport" or "permissions slip"
that it can give to FB to access a SUBSET of the services that I can access on FB - only
those services I want the client to access
* Client asks the resource server for access to a service and gives token to resource server.
Resource server doesn't know anything about the token.
It contacts the Oauth server, gives it the token, and asks "is this token valid?".
OAuth server looks it up - is it still active? has it been revoked? etc etc
Tells the resource server whether it is valid or not, and WHAT TYPE OF ACCESS
* If token valid resource server gives client access to the requested service.
OAuth is used for DELEGATION and not authorization or authentication. It is the resource server that does
the authorization as to what resources can be accessed based on the OAuth token presented. Authentication
is done by the external component such as the login page - OAuth server triggers authentication but does
not do it itself. Thus everything is delegated.
SEE the RFC here: https://tools.ietf.org/html/rfc6749
The following is an annotated version of the abstract protocol flow from the RFC:
Actor #1: OAuth Provider
OAuth server has 3 components:
Login page etc. Identity provider Oauth is the front end of the IAM
Get consent for the delegation of access rights to the client
User is logged in. This page says "do you really want to provide this subset
to the app
Token management infra structure
Basically the token DB
The OAuth server generally provides 2 enpoints: their function is standard but the URL
need not be.
1. Authorization endpoint
For "auth code grant" gives AUTHORIZATION CODE
For "implicit grant" gives ACCESS TOKEN
2. Token endpoint
It is a protected enpoint so requires the client ID and secret, that would
have been delivered to the client via the autorize process. Uses HTTP BASIC
Produces an ACCESS TOKEN and a REFRESH TOKEN for "auth code grant", "client
credentials grant", "resource owner credentials grant".
Actor #2: Resource Provider (Server)
Makes a protected resource available over HTTPS. Often via a RESTful API which ensures only allowed
clents can access the data (access token), which it does via the OAuth token.
Provides the resources owned by the resource owner, that are requested by the client (3rd party)
using the access tokens provided by the Oauth provider.
Provides the resource EP
Actor #3: Resource Owner
Owner of the protected resource. S/he can access their data directly. Wih OAuth the resource owner
is giving a client permissions to indirectly access their data via the OAuth authentication mechanism
(owner delegates his access rights, or a subset of them, to the client) and can specify restricted
access to the protected resource, i.e., only allow access to a subset of the resource or restrict the
things that can be done with or to that resource.
Actor #4: Client
Third party tring to access protected resources of the resource owner. E.g. an app trying to access
my FB page etc.
The OAuth server will provide the client with an ID and a secret by which the client can then
Client provides the redirect EP.
Clients come in two types:
1. CONFIDENTIAL - Client can securely store client credentials or is capable of secure
client authentication using other means.
2. PUBLIC - Incapable of maintaining the confidentiality of their credentials. E.g. client
app in web browser, and incapable of secure client authentication via any other means.
A component can have the role of several actors. Eg Words with friends wants access to my FB friends list.
"Words with friends" is the client. I am the resource owner, FB is both the resource provider and the
Another example. I want to login to PrintMyPhotos.com. FB is the OAuth provider. PrintMyPhotos.com is
both the client. But who is the resource owner? It is PrintMyPhotos.com... in this case the OAuth
provider is just authenticating that I am who I say I am. The protected resource is the photos stored
1. Authorization EP (OAuth server): The authorization endpoint is used to interact with the
resource owner and obtain an authorization grant.
Verify identity of resource owner.
Because requests to the authorization endpoint result in user authentication and the
transmission of clear-text credentials (in the HTTP response), the authorization server MUST
require the use of TLS.
2. Token EP (OAuth server): Used by the client to obtain an access token by presenting its
authorization grant or refresh token.
3. Redirect EP (Client)
4. Resource EP(s) (Resource server)
CAUTION WITH OAUTH TOKENS: Tokens grant access but do NOT verify who the user is! So, if Bob has access
to Alice's token, he will be able to access everything Alice can, whether or not Alice has given him
permission or not (imagine Bob somehow stole the token, for example!).
Thus it is important to keep any kind of token CONFIDENTIAL!
This means USE TLS EXCLUSIVELY for any token transmission
Access Tokens (AT):
Used by client to access resources. AT valididty is time limited. Stored and sent
by the client. Sent to resource server generally.
The holder of the token has the access rights associated with the token BUT the token
holder is not authenticated after the initial issue, so tokens must be kept
Refresh Token (RT):
Time limited validity, longer than AT validity. Used to request new AT after AT expired.
When RT used the credentials of the resource owner do NOT have to be checked again.
Stored and sent by the client. Never sent to the resource server, only to OAuth server.
Thus, only used to refresh ATs and never used to actually access resources themselves.
Authorization Code (AC):
Send by auth server to client after authenticating the resource owner (i.e. getting
the consent of the resource owner for delegating the access). The AC is just a code
that represents the authentication and consent of the resource owner that the client
should be able to access the resource. It is not, the AT! At AC validity is normally
only valid for a couple of minutes. The AC can then be used to get an AT!
When new client wants access to resources. OAuth server needs some information about the client and
returns, in exchange, a client ID and a client secret.
When new client is registered, must provide
1. Redirect URI
2. Required scopes (what type/subset of resource is going to be used)
Dynamic client registration protocol: https://tools.ietf.org/html/rfc7591
Explanation taken from https://ldapwiki.com/wiki/OAuth%202.0%20Client%20Registration:
OAuth Clients must register with the Authorization Server before any transactions
may occur. Before an OAuth Client can request access to Protected Resource on a
Resource Server, the OAuth Client must first register with the Authorization Server
associated with the Resource Server.
OAuth 2.0 Client Registration is typically a one-time task. Once registered, the
registration remains valid, unless the OAuth Client registration is revoked.
At OAuth 2.0 Client Registration the OAuth Client is assigned a Client_id and a
Client Secret (password) by the Authorization Server.
The Client_id and Client Secret is unique to the OAuth Client on that Authorization Server.
If a OAuth Client registers with multiple Authorization Servers (e.g. both Facebook, Twitter
and Google), each Authorization Server will probably issue a different and unique Client ID
to the OAuth Client application.
Whenever the OAuth Client requests access to resources stored on that same Resource Server,
the OAuth Client needs to Authenticate itself by sending the Client ID and the Client Secret
to the Authorization Server.
During the registration the OAuth Client also registers a redirect_uri. This redirect_uri is
used when a Resource Owner grants Authorization to the OAuth Client. When a Resource Owner
has successfully Authorized the OAuth Client via the Authorization Server, the Resource Owner
is redirected back to the OAuth Client's redirect_uri.
OAuth 2.0 Client Registration must be done outside of the The OAuth 2.0 Authorization
OAuth Flows Overview
To request an access token, the client obtains authorization from the
resource owner. The authorization is expressed in the form of an
authorization grant, which the client uses to request the access
1. Authorizaton Grant Flow
A.k.a. "three-legged ouath"
Most secure flow
Client must be able to securley store client ID/secret and tokens
2. Implicit Grant Flow
Used when client can NOT securley store client ID/secret or OAuth tokens
Cons: Short validity of tokens and refreshing AT very difficult
3. Client Credential Flow
A.k.a "two-legged OAuth"
Used when client is also the resource owner
Uses ONLY the /token EP
4. Resource Owner Password Credentials Flow
When resource owner can entrust password to client
Authorization Grant Flow
Checks the identity of the 3 involved actors (why its called "3-legged"). The authorization code
grant type is used to obtain both access tokens and refresh tokens and is optimized for
The authorization code grant type is used to obtain both access
tokens and refresh tokens and is optimized for confidential clients.
Since this is a redirection-based flow, the client must be capable of
interacting with the resource owner’s user-agent (typically a web
browser) and capable of receiving incoming requests (via redirection)
from the authorization server.
OAuth server authenticates the resource owner using his/her username/password that
are provided interactively via the login mechanism that the OAuth server provides.
OAuth server authenticates the client using its client ID and secret.
Identity of the Auth server is checked by its certificate.
Flow should only be used when CLIENT PROVIDES SECURE STORAGE FOR ID, SECRET AND
Resource ------------> OAuth <----------- Resource -----------> Resources
Owner <------------ Server -----------> Server <----------/
/ \ |
/auth /token |
^ ^ |
| | |
v v |
Relatively high security level
AT does NOT flow through browser
Username/pwd of resource owner not known to client
Convenience - Use of RT allows access for longer periods of time without
Identities of all 3 participants is ensured
Implicit Grant Flow
From the RFC:
The implicit grant is a simplified authorization code flow optimized
for clients implemented in a browser ...
... In the implicit flow, instead of issuing the client
an authorization code, the client is issued an access token directly
(as the result of the resource owner authorization). The grant type
is implicit, as no intermediate credentials (such as an authorization
code) are issued (and later used to obtain an access token).
The implicit grant type does not include client authentication, and
relies on the presence of the resource owner and the registration of
the redirection URI. Because the access token is encoded into the
redirection URI, it may be exposed to the resource owner and other
applications residing on the same device.
Used for clients that cannot securely store the client ID and client secret as well as the refresh
token: no refresh tokens are issued in this flow.
Does not used /token EP.
/auth EP gives the access token directly - dont need the token EP - but no refresh token.
The access tokens have a SHORT VALIDITY time. This is because the client has not been
authenticated and a short validity period tries to hedge against the danger this introduces.
Thus implicit grant has a REDUCED LEVEL OF SECURITY COMPARED TO THE AUTHORIZATION GRANT from
the previous section.