How does a short-lived Access Token add security? - oauth-2.0

In the answer to the question Why do access tokens expire?, the first point provided states:
[Bearer-Tokens are] short-lived and requiring refresh, they limit the time an attacker can abuse a stolen token.
But when an Access-Token is used (From a native-app), the Client uses the Refresh-Token to get a new Access Token, and sends that new token back to the requestor. So if an attacker uses somebody else's access token, he'll just be sent a brand new access token every time.
So who cares about how long the token lasts? If an attacker gets it, they have taken over the session For As Long As The Refresh Token Lasts
I already know a dozen answers to my question, but I have questions to each answer. My original question was so long because I explained every scenario and how they are inconsequential or false (as far as I know). So please try to help me understand and I'll comment if I think the answer has caveats.
Addition/Edit - Hoping for more answers with my additional information
Web Page calls a Client with Resource Owner (User) Credentials
Client calls the Auth Server and gets an Access and Refresh Token. The Access Token will expire in 5 minutes, the Refresh Token will expire in hours or days or whatever.
Client sends the Access Token to the Web Page
Resource Owner (User) uses the web page
Web Page sends the Access Token to the Client
Client sends the Access Token to the Resource Server
Resource Server checks the Access Token in any number of ways
Resource Server sends Resources to the Client
Client sends Resources to the Resource Owner
Resource Owner (user) Continues to use the Web Page
The Client, either during Each Request or every 4 minutes and 30 seconds, uses the Refresh Token to get a new Access Token
The Client, either during Each Request or every 4 minutes and 30 seconds, sends a New Access Token to the Active Resource Owner
Yes? No? Since the Resource Owner is Actively using the web site, the web site is in constant communication with the Client, and the Client gets a new Access Token (Using the Refresh Token) and sends it back to the web site, so the Active user can continue using the site without being kicked out every 5 minutes.
Therefore, if ANY Person gets ahold of that Access Token, and are hitting the Client with it, the Client will continue to send new Access Tokens to whoever has that Access Token. Granted: After a single refresh, one of those two people will have a bad Access Token and be booted, but not necessarily the right person.

Your point seems to be that if an attacker can take over your browser session then they will be able to access the third-party resource for the entire length of the refresh token, as you've described. So what's the point of having a short-lived access token?
There's certainly some truth to that, but there are two general responses:
Using a refresh token makes it practicable to invalidate the user if the third-party figures out that the session has been taken over by an attacker.
The access token / refresh token system is used to prevent or limit other kinds of attacks that you haven't mentioned.
Regarding the first point, remember that the access token is sent to the resource server, not the authorization server. Although there's nothing in the specification that prevents the resource server from checking the validity of the user, in practice that could present performance issues. Access tokens are typically designed to be self-validating, without requiring access to some external resource.
Given that, the only way to invalidate a user is to do it on the authorization server when the refresh token is sent in. The authorization server sees that this user has been marked as compromised, and refuses to send a new access token.
Regarding the second point, there are plenty of other security scenarios that OAuth is designed to protect against other than a user's browser session being taken over by an attacker. What if someone is able to get ahold of the access token in some other way? Since the access token itself isn't generally used to get access to the client (see below), they won't be able to get the client to refresh the token for them, and therefore the fact that the access token is short-lived will be a security advantage.
As a reference, both of these points are made succinctly in this email to the Oauth Working Group mailing list.
Looking specifically at the flow you described in your post, I think your confusion is rooted in the idea that the client (web server) sends the user agent (the browser) the access token (your step 3), and that that token (in the form of a cookie) is what the client uses to authenticate the user agent. Although it's possible for a web framework to do those things, neither one is a part of OAuth (nor generally true of web frameworks, in my experience).

Access tokens are short-lived, refresh tokens are long-lived. Access tokens are presented to the Resource Server (and only the Resource Server) that hosts the protected content to get access. Refresh tokens are presented only to the Authorization Server, never to the Resource Server. So when an attacker obtains an access token, he can use it for the lifetime of the access token to get access to the protected content.
When the access token expires there's no way to get a new one unless he has obtained the refresh token as well. Besides that, when using the refresh token the client typically authenticates itself to the Authorization Server, so even when the attacker got the refresh token, he would also need the Client's credentials to use it to get a new access token.

Related

Why do you need authorization grant when you can just give the token out directly?

Watching this video, it details in OAuth2 that the client application first has to get the authorization grant from the Authorization server and then use that grant to get a token before being able to access the resource server. What purpose does the grant serve? Why not give the client the token right away after the user signs on with his/her username and password?
Because it is more secure, for some application types.
What you describe is so called authorization-code-flow. It is normally used for "classical" web applications, where only the backend needs to access resource server. The exchange of authorization code to access token happens on the backend and access token never leaves it. Exchange can be done only once and in addition client id and secret (stored on the backend) are necessary.
Single-Page-Applications often use implicit-flow where access token is delivered to the frontend directly in the URL.
See more here:
IdentityServer Flows
EDIT: Q: "I still don't see how it is more secure given that you have to have the grant in order to get the token. Why need 2 things instead of just 1 thing to access the resource? If someone steals the token, they can access the resource anyway – stackjlei"
"Stealing" access token will work independent on how your application acquires it. However, stealing access token on the backend is much more difficult than on the frontend.
Authorization code is delivered to the backend also over the frontend but the risk that someone intercepts and uses it is tiny:
It can be exchanged only once.
You need client-id and client-secret in order to exchange it. Client-secret is only available on the backend.
Normally, authorization code will be exchanged by your backend to access-token immediately. So the lifetime of it is just several seconds. It does not matter if someone gets hold of used authorization code afterwards.
In your scenario there could be two servers, an Authorization and a Resource one.
It could be only one as well, but let's imagine this scenario.
The purpose of the Authorization Server is to issue short lived access tokens to known clients. The clients identify themselves via their CLientID and CLientSecret.
The Authorization Server ( AS ) holds the list of clients and their secrets and first checks to make sure the passed values match its list. If they do, it issues a short lived token.
Then the client can talk to the Resource Server ( RS ), while the token is valid. Once the token expires, a new one can be requested or the expired one can be refreshed if that is allowed by the Authorization Server.
The whole point here is security, Normally, the access tokens are passed in the Authorization header of the request and that request needs to be over https to make sure that the data can't be stolen. If, somehow, someone gets hold of an access token, they can only use it until it expires, hence why the short life of the tokens is actually very important. That's why you don't issue one token which never expires.
You have different type of OAuth. On type doesn't require to use the 'grant' authorization. It depend who are the user/application, the ressource owner and the server API.
This way, you - as a user - don't send the password to the application. The application will only use the grant token to gain access to your ressources.
I think this tuto is a pretty good thing if you want more details
https://www.digitalocean.com/community/tutorials/an-introduction-to-oauth-2

Refreshed Token could be revoked 50 times per account

I tried to add YouTube Video from the third party and After one day, I got the success in doing so. But While uploading a video the access token is required and in order to get that access token the user must be logged in. And the expiration time for that access token is 3600 seconds( 1 hr).
Now, There are some of my questions regarding this.
Is there anyway, by which I can refresh access token.
If some one has G Suite account, then Is there any special values for expiration time, or it remains the same?
As per the documentation, I can have maximum 50 tokens, So is there any alternative for it, So that I can get valid token after 50 requests.
To answer your question for number 1, you can check the documentation here.
Access tokens periodically expire. You can refresh an access token
without prompting the user for permission (including when the user is
not present) if you requested offline access to the scopes associated
with the token.
If you use a Google API Client Library, the client object refreshes the access token as needed as long as you configure that
object for offline access.
If you are not using a client library, you need to set the access_type HTTP query parameter to offline when redirecting the
user to Google's OAuth 2.0 server. In that case, Google's
authorization server returns a refresh token when you exchange an
authorization code for an access token. Then, if the access token
expires (or at any other time), you can use a refresh token to obtain
a new access token.
Requesting offline access is a requirement for any application that
needs to access a Google API when the user is not present. For
example, an app that performs backup services or executes actions at
predetermined times needs to be able to refresh its access token when
the user is not present. The default style of access is called online.
About the G Suite account, it was stated 24 Hours in the documentation. Note:
In this SO post answer, the function of Access Token and Refresh Token was discussed.
I am not sure if there are ways to alter the limits because of security reasons.
To clearly differentiate these two tokens and avoid getting mixed up,
here are their functions given in The OAuth 2.0 Authorization
Framework:
Access Tokens are issued to third-party clients by an authorization server with the approval of the resource owner. The
client uses the access token to access the protected resources hosted
by the resource server.
Refresh Tokens are credentials used to obtain access tokens. Refresh tokens are issued to the client by the authorization server
and are used to obtain a new access token when the current access
token becomes invalid or expires, or to obtain additional access
tokens with identical or narrower scope.

Are refresh tokens necessary for online applications

Per Google's docs it would seem refresh tokens are only necessary for offline applications (applications that may run into an expired access token when the user isn't around).
Access tokens periodically expire. You can refresh an access token
without prompting the user for permission (including when the user is
not present) if you requested offline access to the scopes associated
with the token.
...
Requesting offline access is a requirement for any application that
needs to access a Google API when the user is not present. For
example, an app that performs backup services or executes actions at
predetermined times needs to be able to refresh its access token when
the user is not present. The default style of access is called online.
However, a description of refresh tokens in general and this question in particular both seem to imply that refresh tokens are needed anytime you want to request a new access token.
I think I would agree with Google's explanation and not use refresh tokens. My experience with OIDC providers has been that refresh works as follows:
User requests protected resource from client server
Client server determines access token has expired.
Client server redirects user to OP auth endpoint
OP authenticates user without interaction due to cookies stored on user's browser with OP's domain.
Client server finishes the request.
The user might see a few redirects but other than that the re-authentication went by without any interaction from them. Given this, is it necessary to bother with refresh tokens if the user will always be present at the application?
My biggest concern with using refresh tokens for online apps is that it takes away transparency from the user.
Refresh tokens facilitate long term access and should be stored safely. But they also don't provide a natural way to "sign out", and (most importantly) it becomes completely opaque how, when and from where your data is accessed, as the often used scope name offline_access suggests.
OIDC offers a front channel mechanism prompt=none that largely leads to the same effect (i.e. new tokens), and without needing intermediate redirects if the re-authentication is performed inside an iframe.
Hence in my opinion you and Google are right and the answer must be: No, don't use refresh tokens if the user is present.
No, it is not necessary to bother with refresh tokens if the user will always be present at the application. The reasoning is largely the OP describes.
But there are reasons why one may still want a refresh token:
as the OP mentions the user might see a few redirects and both the UI expert and the branding guy on your team will hate this
when an access token expires in the middle of an HTML Form POST action, the redirect may have lost the context/POST-data on return; you may want to minimize this or you'll have to take appropriate (complex) POST-data-save actions
if your access token expiry is really short, the redirects create a lot of overhead and nuisance; you may not be able to control access token expiry when dealing a Providers in a different domain and when dealing with multiple Providers it will vary across them
when refreshing the access token with a redirect your application now depends on the Provider keeping an SSO session; not all Providers may do this and if they do they may do it in different ways: the SSO session duration may vary between them and the authentication method may vary; as an example: a Provider that doesn't keep an SSO session but does use 2-factor authentication will have large impact on the user experience
Imagine a scenario where you want to use the access token to update user information in almost real-time from the user info endpoint but the access token expiry is relatively short. Either you'll have to perform a lot of redirects with the nuisance as described, or you can use a refresh token.
Refresh token is essentialy a credential reference, that your client can exchange for access token, when there is no active user session.
For example if you want to periodicaly sync issues from Github with your inhouse system.
It is often misused like some kind of session. It is essential to diffirentiate those things. And scope name offline_access is there for a reason.
So in simple cases - you just rely on OP session and get new token with authorize/token endpoints combo. You should not be prompted to provide credentials as long as session is alive and consent is given for that particular app.
If you need to do some backgound stuff - ask for refresh token also.
As for question: no.
EDIT(More in-depth explanation):
if we are talking about web there are two main cases:
Client that can securely store secrets like usual web app with server page rendering and clients, that cant store secrets, like SPA apps. From that perspective there are two main flows (omitting hybrid to not over-complicate): Authorization Code Flow and Implicit Flow respectively.
Authorization Code Flow
On first request your app checks it own session(client session) and if there is none - redirects to external OP(OpenID Connect provider) authorize url. OP authenticates user according to requirements expressed in request, gathers consent and other stuff and returns authorization code. Then client asks token endpoint with it and receives access_token/id_token pair with optional refresh token if user granted offline access consent. This is important, because user can deny it for your app. After this client can request userInfo endpoint to get all user claims that were granted during consent. Those claims represent user identity and do not contain stuff like authentication method, acr etc. Those claims present in id_token alongside with expiration for example. After that client starts it own session and have option to set its lifetime equal to id_token lifetime or use it own to provide smooth UX for example. At this point you can discard access_token and id_token at all if you don't need access to other APIs(like all scopes in access_token are specific to OP and subject). If you need access to some API you can store access_token and use it for access. It becomes invalid - redirect to OP for new one. Expiration can be more lax here, because of more secure environment on server. So even 1hr is an option. No refresh tokens used at all.
Implicit Flow
In this case your lets say Angular app redirects to OP, gets its id_token and optional access_token from authorize endpoint directly and uses it to access some APIs. On every request expiration is checked an if needed, client sends request to OP in hidden iFrame, so there won't be any visible redirects as long as OP session is alive. There are some great libs for that like openid-client.js. No refresh is allowed here at all.
It is important to differentiate client session from OP session, token lifetime and session lifetime.
To address some specific needs there is Hybrid Flow. It can be used to get authorization code and id_token for your session in one request. No chit chat over network.
So when you think about refresh token just check your needs and map them to a spec :) And if you need it anyway - store it as secure as you can.
Refresh tokens are useful for applications that keep access tokens in a server session. For example if a web application doesn't call a protected service using JavaScript XHR, but calls its backend and the backend calls the service. In this scenario, it's easier to get a new access token whenever it's needed than asking a user for a new one.
In JavaScript applications running in browsers, refresh tokens cannot be used, because you need a client secret to get an access token from the /token endpoint and you cannot keep the secret safe in such applications.
The process for getting new access tokens you described can be improved - an application may ask for a new access token just before the current one expires, so the user doesn't get redirected to the OAuth2 server, but the application calls the /auth endpoint with prompt=none parameter in an iframe.

Spring oauth2 - authorization code deleted after consumed

maybe i'm misunderstanding the authorization code grant type, but i thought that after issuing a code spring will keep the code in oauth_code table in DB forever till it will be revoked by user , but the code is deleted right after it is consumed (first time you exchange this for token),
so what is the flow here - should my clients keep the refresh tokens for getting access token later on (or when user is offline)
the spec say the return of refresh tokens is optional as written
The authorization server authenticates the client, validates the
authorization code, and ensures that the redirection URI
received matches the URI used to redirect the client in
step (C). If valid, the authorization server responds back with
an access token and, optionally, a refresh token.
so it the authorization server decide not to give refresh token there is no way for clients to offline request access token and they need to go though the authorization code process all over again ?
thanks
Shlomi
The authorization code can only be used once, and it must have a short lifetime. If you read section 4.1.2 of the spec it says
The authorization code MUST expire
shortly after it is issued to mitigate the risk of leaks. A
maximum authorization code lifetime of 10 minutes is
RECOMMENDED.
So once a code has been used there is not real reason to retain it (other than possibly to detect further attempts to use it).
The refresh token allows the client to obtain another access token. Otherwise, as you say, it needs to re-execute the authorization code flow to get one from scratch, which requires the interaction of the resource owner (the user). Usually the system would be set up to issue a refresh token or not to that client depending on whether it actually requires offline access to the resource, or whether the resource owner (user) will always be interating with it directly.

Why do access tokens expire?

I am just getting started working with Google API and OAuth2. When the client authorizes my app I am given a "refresh token" and a short lived "access token". Now every time the access token expires, I can POST my refresh token to Google and they will give me a new access token.
My question is what is the purpose of the access token expiring? Why can't there just be a long lasting access token instead of the refresh token?
Also, does the refresh token expire?
See Using OAuth 2.0 to Access Google APIs for more info on Google OAuth2 workflow.
This is very much implementation specific, but the general idea is to allow providers to issue short term access tokens with long term refresh tokens. Why?
Many providers support bearer tokens which are very weak security-wise. By making them short-lived and requiring refresh, they limit the time an attacker can abuse a stolen token.
Large scale deployment don't want to perform a database lookup every API call, so instead they issue self-encoded access token which can be verified by decryption. However, this also means there is no way to revoke these tokens so they are issued for a short time and must be refreshed.
The refresh token requires client authentication which makes it stronger. Unlike the above access tokens, it is usually implemented with a database lookup.
A couple of scenarios might help illustrate the purpose of access and refresh tokens and the engineering trade-offs in designing an oauth2 (or any other auth) system:
Web app scenario
In the web app scenario you have a couple of options:
if you have your own session management, store both the access_token and refresh_token against your session id in session state on your session state service. When a page is requested by the user that requires you to access the resource use the access_token and if the access_token has expired use the refresh_token to get the new one.
Let's imagine that someone manages to hijack your session. The only thing that is possible is to request your pages.
if you don't have session management, put the access_token in a cookie and use that as a session. Then, whenever the user requests pages from your web server send up the access_token. Your app server could refresh the access_token if need be.
Comparing 1 and 2:
In 1, access_token and refresh_token only travel over the wire on the way between the authorzation server (google in your case) and your app server. This would be done on a secure channel. A hacker could hijack the session but they would only be able to interact with your web app. In 2, the hacker could take the access_token away and form their own requests to the resources that the user has granted access to. Even if the hacker gets a hold of the access_token they will only have a short window in which they can access the resources.
Either way the refresh_token and clientid/secret are only known to the server making it impossible from the web browser to obtain long term access.
Let's imagine you are implementing oauth2 and set a long timeout on the access token:
In 1) There's not much difference here between a short and long access token since it's hidden in the app server. In 2) someone could get the access_token in the browser and then use it to directly access the user's resources for a long time.
Mobile scenario
On the mobile, there are a couple of scenarios that I know of:
Store clientid/secret on the device and have the device orchestrate obtaining access to the user's resources.
Use a backend app server to hold the clientid/secret and have it do the orchestration. Use the access_token as a kind of session key and pass it between the client and the app server.
Comparing 1 and 2
In 1) Once you have clientid/secret on the device they aren't secret any more. Anyone can decompile and then start acting as though they are you, with the permission of the user of course. The access_token and refresh_token are also in memory and could be accessed on a compromised device which means someone could act as your app without the user giving their credentials. In this scenario the length of the access_token makes no difference to the hackability since refresh_token is in the same place as access_token. In 2) the clientid/secret nor the refresh token are compromised. Here the length of the access_token expiry determines how long a hacker could access the users resources, should they get hold of it.
Expiry lengths
Here it depends upon what you're securing with your auth system as to how long your access_token expiry should be. If it's something particularly valuable to the user it should be short. Something less valuable, it can be longer.
Some people like google don't expire the refresh_token. Some like stackflow do. The decision on the expiry is a trade-off between user ease and security. The length of the refresh token is related to the user return length, i.e. set the refresh to how often the user returns to your app. If the refresh token doesn't expire the only way they are revoked is with an explicit revoke. Normally, a log on wouldn't revoke.
Hope that rather length post is useful.
In addition to the other responses:
Once obtained, Access Tokens are typically sent along with every request from Clients to protected Resource Servers. This induce a risk for access token stealing and replay (assuming of course that access tokens are of type "Bearer" (as defined in the initial RFC6750).
Examples of those risks, in real life:
Resource Servers generally are distributed application servers and typically have lower security levels compared to Authorization Servers (lower SSL/TLS config, less hardening, etc.). Authorization Servers on the other hand are usually considered as critical Security infrastructure and are subject to more severe hardening.
Access Tokens may show up in HTTP traces, logs, etc. that are collected legitimately for diagnostic purposes on the Resource Servers or clients. Those traces can be exchanged over public or semi-public places (bug tracers, service-desk, etc.).
Backend RS applications can be outsourced to more or less trustworthy third-parties.
The Refresh Token, on the other hand, is typically transmitted only twice over the wires, and always between the client and the Authorization Server: once when obtained by client, and once when used by client during refresh (effectively "expiring" the previous refresh token). This is a drastically limited opportunity for interception and replay.
Last thought, Refresh Tokens offer very little protection, if any, against compromised clients.
It is essentially a security measure. If your app is compromised, the attacker will only have access to the short-lived access token and no way to generate a new one.
Refresh tokens also expire but they are supposed to live much longer than the access token.
I've written a little about this because I was pondering the reasoning myself today.
https://blog.mukunda.com/cat/2023/refreshing-access-tokens.txt
Essentially, I think the main security boost is only there if the refresh token does not remain the same over its lifetime.
Let's say someone steals your tokens from your browser cookies because they had access to your device temporarily.
If they use the refresh token, and the refresh token changes, then you have feedback – you are logged out. That can seem rightfully suspicious to careful users who can then take action and revoke all tokens.
If the refresh token doesn't update upon each use, then it is harder to notice that someone has access in tandem. (Chances are, if does update, then it might update from your device automatically before the attacker can even get to use it.)
If the refresh token does not get updated each time you use it, then I don't see any boost in security from the strategy, since it will be right next to the access token and client secrets.
So, why access tokens? It is so you can check that your credentials are valid regularly.
Do refresh tokens expire? Yes, but usually after a few months if you have "remember me" ticked. There's no expiration time in the spec, so you just go until it fails. Services that require longer unmonitored sessions might have secret credentials so they can refresh their refresh token.
Update:
I also glossed through the OAuth 2.0 specification and see the same reasoning, though it emphasizes that the invalid authentication feedback can be caught on the server side. That is a great point – the server can automate revoking the token if it is compromised.
If a refresh token is compromised and subsequently used by both the attacker and the legitimate client, one of them will present an invalidated refresh token, which will inform the authorization server of the breach.

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