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, ie 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.

    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.

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