Is there any maximum number of requests to get a brand new access token using a refresh token?
Yes. 25 according to Google's developer documentation
Access tokens have a lifetime of 3600 seconds. Requesting a new access token before that time is up works fine but it's inefficient for your app to do so often. I don't think Google publishes a limit on how many non-expired access tokens a refresh token can have but you may get errors and it's non-optimal.
For most apps, refreshing an access token 10 minutes before the current access token expires (later if your not to worried about clock drift) works well.
The number of requests depend on the authorization server security policy.
Some servers allow only one request (the refresh token is invalidate after the request), other servers may allow 10, 100 or even unlimited requests.
Related
I'm creating an app that integrates with several 3rd-party Oauth2 providers (think Zapier). The user adds a connection through Oauth2 which gives me a refresh token and access token and which I'm storing in a database.
What is the best practice to keep the access token fresh? Should I be running an async job (e.g. cron) that refreshes the access token every 30 minutes for every connection? If the user doesn't use my app for several days I'd like to still be able to access his 3rd-party data without having him to go through the Oauth2 consent.
What is the best practice to keep the access token fresh? Should I be running an async job (e.g. cron) that refreshes the access token every 30 minutes for every connection?
Not necessarily. Wait till your API call fails. Check for a proper response such as "401 Unauthorized" which hints your access token is invalidated/expired. Once this happens use refresh token to renew the access token. If refresh token fails, then you have to fall back again and ask user to login again.
A refresh token can have a varying life time. It can be from few days to few months. For example check Google's explanation mentioning long lived refresh tokens and possible expiry of them. Also check how Azure AD mention about configurations related to token lifetimes.
So user not using app for few days (ex:- leave it and return after weekend) can be handled through proper validity configurations of tokens lifetimes. But be mindful about threats that could occur from long-lived, unmanaged tokens (ex:- due to token stealing).
The Oauth website has a pretty informative answer
The “expires_in” value is the number of seconds that the access token
will be valid. It’s up to the service you’re using to decide how long
access tokens will be valid, and may depend on the application or the
organization’s own policies. You could use this timestamp to
preemptively refresh your access tokens instead of waiting for a
request with an expired token to fail. Some people like to get a new
access token shortly before the current one will expire in order to
save an HTTP request of an API call failing. While that is a perfectly
fine optimization, it doesn’t stop you from still needing to handle
the case where an API call fails if an access token expires before the
expected time. Access tokens can expire for many reasons, such as the
user revoking an app, or if the authorization server expires all
tokens when a user changes their password.
If you make an API request and the token has expired already, you’ll
get back a response indicating as such. You can check for this
specific error message, and then refresh the token and try the request
again.
TLDR: I would only refresh the token when a request fails
I would like to request Uber cabs on behalf of an Uber user, but first the user needs to permit the app to have access to his profile and permit the app to send requests on his behalf. However, the returned authorization code that I can use to get the access token in order to send user requests on their behalf is only valid for 10 minutes and my requirement needs me to send requests even 24 hours later.
Is there a way to get a permanent authorization code or access token that never expires or at least one that lasts for a long period, e.g. a month?
RFC 6749 says as follows in 4.1.2. Authorization Response:
code
REQUIRED. The authorization code generated by the authorization server. 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. The client MUST NOT use the authorization code
Therefore, it is hopeless to expect an authorization code with longer lifetime than 10 minutes. So, you should exchange an authorization code for an access token at the token endpoint immediately using the way described in 4.1.3. Access Token Request.
If Uber's authorization server issues a refresh token when it issues an access token, you can expect it has longer lifetime than an access token. You can use a refresh token at the token endpoint in order to get a new access token. See 6. Refreshing an Access Token for details.
If lifetime of access tokens and refresh tokens issued by Uber is less than 24 hours, you have to change the flow of your application.
I am creating an Oath 2.0 system on my server allowing users to log into their account on my server from the app without having to give the username and password to the app itself. Which as I understand is the purpose of Oath, and it seems to work pretty well, the system is built on compliance with all Oath 2.0 specifications and is fully functional. But what I don't understand is why I have to have tokens expire... I mean I provide a refresh uri and they can easily at any time and at no cost renew the token (or get a new valid one). I dont see any purpose in this, why not just make the token never expire. I don't see the security benefits or any purpose for that matter for token expiration. Can someone explain to me why I have to have my tokens expire and why they can't just be good indefinitely?
In a word, for more safe.
In your question, you said at no cost renew the token, actually, when you refresh token, you need to authenticate with the authorization server(provide your client credential). So refresh token is not equal with access token. It can not give you the access to resource.
Why have token expire?
As OAuth 2.0 Threat Model and Security Considerations says:
3.1.2. Limited Access Token Lifetime
The protocol parameter "expires_in" allows an authorization server
(based on its policies or on behalf of the end user) to limit the
lifetime of an access token and to pass this information to the
client. This mechanism can be used to issue short-lived tokens to
OAuth clients that the authorization server deems less secure, or
where sending tokens over non-secure channels.
5.1.5.2. Determine Expiration Time
Tokens should generally expire after a reasonable duration. This
complements and strengthens other security measures (such as
signatures) and reduces the impact of all kinds of token leaks.
Depending on the risk associated with token leakage, tokens may
expire after a few minutes (e.g., for payment transactions) or stay
valid for hours (e.g., read access to contacts).
I found another use for it. When I store a token in my database at every login. If someone no longer uses a device or deletes their token (uninstalls an app or clears a cache) for any reason without properly "logging out" (logging out removes it from the database). Then there is a token stored on the database that will never be used again. After a while this becomes cumbersome taking up valuable space on the database and slowing down query executions for unused tokens. With token expirations I can run a cron job to scan the database for expired tokens every 15 minutes or so and remove them. Having to refresh tokens does put some strain on the server and the client but not as much as having potentially millions of unused tokens.
I need to understand if a refresh token from OAuth 2.0 for Google data expires or not if unused for more than 6 months?
At many places it is called out that it doesn't expire and at some places it is said that it will expire if unused for 6 months.
For example, in this question, the accepted answer says that it never expires but one of the comments suggest that it can expire if unused for 6 months.
This Google doc calls out that refresh token will never expire while this doc says it can expire if not used for 6 months.
Although I am using a safety check for the cases where refresh token is expired due to any reason, but still want to make sure I understand the behavior correctly.
You are correct the two pages Using OAuth 2.0 for Web Server Applications and Using OAuth 2.0 to Access Google APIs don't have the same information.
Using OAuth 2.0 to Access Google APIs is correct.
You should write your code to anticipate the possibility that a
granted token might no longer work. A token might stop working for one
of these reasons:
The user has revoked access.
The token has not
been used for six months.
The user account has exceeded a certain
number of token requests.
There is currently a 25-token limit per Google user account. If a user
account has 25 valid tokens, the next authentication request succeeds,
but quietly invalidates the oldest outstanding token without any
user-visible warning.
If you need to authorize multiple programs, machines, or devices, one
workaround is to limit the number of clients that you authorize per
user account to 15 or 20. If you are a Google Apps admin, you can
create additional admin users and use them to authorize some of the
clients.
A refresh token that hasn't been used for 6 months will expire.
I have sent in a bug report on the first URL. Google should clear up the discrepancy the next time they go though the documents.
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.