Why Does OAuth v2 Have Both Access and Refresh Tokens

Question

Section 4 2 of the draft OAuth 2 0 protocol indicates that an authorization server can return both an access token  which is used to authenticate oneself with a resource  as well as a refresh token  which is used purely to create a new access token   https   tools ietf org html rfc6749 section-4 2  Why have both  Why not just make the access token last as long as the refresh token and not have a refresh token

User · Answer

This answer is from Justin Richer via the OAuth 2 standard body email list. This is posted with his permission.

The lifetime of a refresh token is up to the (AS) authorization server — they can expire, be revoked, etc. The difference between a refresh token and an access token is the audience: the refresh token only goes back to the authorization server, the access token goes to the (RS) resource server.

Also, just getting an access token doesn’t mean the user’s logged in. In fact, the user might not even be there anymore, which is actually the intended use case of the refresh token. Refreshing the access token will give you access to an API on the user’s behalf, it will not tell you if the user’s there.

OpenID Connect doesn’t just give you user information from an access token, it also gives you an ID token. This is a separate piece of data that’s directed at the client itself, not the AS or the RS. In OIDC, you should only consider someone actually “logged in” by the protocol if you can get a fresh ID token. Refreshing it is not likely to be enough.

For more information please read http://oauth.net/articles/authentication/

User · Answer

While refresh token is retained by the Authorization server  Access token are self-contained so resource server can verify it without storing it which saves the effort of retrieval in case of validation  Another point missing in discussion is from rfc6749 page-55       For example  the authorization server could employ refresh token   rotation in which a new refresh token is issued with every access   token refresh response The previous refresh token is invalidated but   retained by the authorization server   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     I think the whole point of using refresh token is that even if attacker somehow manages to get refresh token  client ID and secret combination  With subsequent calls to get new access token from attacker can be tracked in case if every request for refresh result in new access token and refresh token

User · Answer

Despite all the great answers above  I as a security master student and programmer who previously worked at eBay when I took a look into buyer protection and fraud  can say to separate access token and refresh token has its best balance between harassing user of frequent username password input and keeping the authority in hand to revoke access to potential abuse of your service   Think of a scenario like this  You issue user of an access token of 3600 seconds and refresh token much longer as one day    The user is a good user  he is at home and gets on off your website shopping and searching on his iPhone  His IP address doesn t change and have a very low load on your server  Like 3-5 page requests every minute  When his 3600 seconds on the access token is over  he requires a new one with the refresh token  We  on the server side  check his activity history and IP address  think he is a human and behaves himself  We grant him a new access token to continue using our service  The user won t need to enter again the username password until he has reached one day life-span of refresh token itself  The user is a careless user  He lives in New York  USA and got his virus program shutdown and was hacked by a hacker in Poland  When the hacker got the access token and refresh token  he tries to impersonate the user and use our service  But after the short-live access token expires  when the hacker tries to refresh the access token  we  on the server  has noticed a dramatic IP change in user behavior history  hey  this guy logins in USA and now refresh access in Poland after just 3600s       We terminate the refresh process  invalidate the refresh token itself and prompt to enter username password again  The user is a malicious user  He is intended to abuse our service by calling 1000 times our API each minute using a robot  He can well doing so until 3600 seconds later  when he tries to refresh the access token  we noticed his behavior and think he might not be a human  We reject and terminate the refresh process and ask him to enter username password again  This might potentially break his robot s automatic flow  At least makes him uncomfortable    You can see the refresh token has acted perfectly when we try to balance our work  user experience and potential risk of a stolen token  Your watch dog on the server side can check more than IP change  frequency of api calls to determine whether the user shall be a good user or not   Another word is you can also try to limit the damage control of stolen token abuse of service by implementing on each api call the basic IP watch dog or any other measures  But this is expensive as you have to read and write record about the user and will slow down your server response

User · Answer

To clear up some confusion you have to understand the roles of the client secret and the user password  which are very different   The client is an app website program      backed by a server  that wants to authenticate a user by using a third-party authentication service  The client secret is a  random  string that is known to both this client and the authentication server  Using this secret the client can identify itself with the authentication server  receiving authorization to request access tokens   To get the initial access token and refresh token  what is required is    The user ID The user password The client ID The client secret   To get a refreshed access token however the client uses the following information    The client ID The client secret The refresh token   This clearly shows the difference  when refreshing  the client receives authorization to refresh access tokens by using its client secret  and can thus re-authenticate the user using the refresh token instead of the user ID   password  This effectively prevents the user from having to re-enter his her password   This also shows that losing a refresh token is no problem because the client ID and secret are not known  It also shows that keeping the client ID and client secret secret is vital

User · Answer

The link to discussion  provided by Catchdave  has another valid point  original  dead link   made by Dick Hardt  which I believe is worth to be mentioned here in addition to what s been written above      My recollection of refresh tokens was for security and revocation     lt           revocation  if the access token is self contained  authorization can be revoked by not issuing new access tokens  A resource does not need to query the authorization server to see if the access token is valid This simplifies access token validation and makes it easier to scale and support multiple authorization servers   There is a window of time when an access token is valid  but authorization is revoked     Indeed  in the situation where Resource Server and Authorization Server is the same entity  and where the connection between user and either of them is  usually  equally secure  there is not much sense to keep refresh token separate from the access token   Although  as mentioned in the quote  another role of refresh tokens is to ensure the access token can be revoked at any time by the User  via the web-interface in their profiles  for example  while keeping the system scalable at the same time   Generally  tokens can either be random identifiers pointing to the specific record in the Server s database  or they can contain all information in themselves  certainly  this information have to be signed  with MAC  for example    How the system with long-lived access tokens should work  The server allows the Client to get access to User s data within a pre-defined set of scopes by issuing a token  As we want to keep the token revocable  we must store in the database the token along with the flag  revoked  being set or unset  otherwise  how would you do that with self-contained token   Database can contain as much as len users  x len registered clients  x len scopes combination  records  Every API request then must hit the database  Although it s quite trivial to make queries to such database performing O 1   the single point of failure itself can have negative impact on the scalability and performance of the system   How the system with long-lived refresh token and short-lived access token should work  Here we issue two keys  random refresh token with the corresponding record in the database  and signed self-contained access token  containing among others the expiration timestamp field   As the access token is self-contained  we don t have to hit the database at all to check its validity  All we have to do is to decode the token and to validate the signature and the timestamp   Nonetheless  we still have to keep the database of refresh tokens  but the number of requests to this database is generally defined by the lifespan of the access token  the longer the lifespan  the lower the access rate    In order to revoke the access of Client from a particular User  we should mark the corresponding refresh token as  revoked   or remove it completely  and stop issuing new access tokens  It s obvious though that there is a window during which the refresh token has been revoked  but its access token may still be valid   Tradeoffs  Refresh tokens partially eliminate the SPoF  Single Point of Failure  of Access Token database  yet they have some obvious drawbacks    The  window   A timeframe between events  user revokes the access  and  access is guaranteed to be revoked   The complication of the Client logic   without refresh token   send API request with access token if access token is invalid  fail and ask user to re-authenticate   with refresh token   send API request with access token If access token is invalid  try to update it using refresh token if refresh request passes  update the access token and re-send the initial API request If refresh request fails  ask user to re-authenticate    I hope this answer does make sense and helps somebody to make more thoughtful decision  I d like to note also that some well-known OAuth2 providers  including github and foursquare adopt protocol without refresh tokens  and seem happy with that

User · Answer

This answer has been put together by the help of two senior devs  John Brayton and David Jennes   The main reason to use a refresh token is to reduce the attack surface  Let s suppose there is no refresh key and let   s go through this example  A building has 80 doors  All doors are opened with the same key  The key changes every 30 minutes  At the end of the 30 minutes I have to give the old key to the keymaker and get a new key  If I   m the hacker and get your key  then at the end of the 30 minutes  I   ll courier that to the keymaker and get a new key  I   ll be able to continuously open all doors regardless of the key changing  Question  During the 30 minutes  how many hacking opportunities did I have against the key  I had 80 hacking opportunities  each time you used the key  think of this as making a network request and passing the access token to identify yourself   So that   s 80X attack surface  Now let   s go through the same example but this time let   s assume there   s a refresh key  A building has 80 doors  All doors are opened with the same key  The key changes every 30 minutes  To get a new key  I can   t pass the old access token  I must only pass the refresh key  If I   m the hacker and get your key  I can use it for 30 minutes  but at the end of the 30 minutes sending it to the keymaker has no value  If I do  then the keymaker would just say  quot This token is expired  You need to  refresh the token  quot  To be able to extend my hack I would have to hack the courier to the keymaker  The courier has a distinct key  think of this as a refresh token   Question  During the 30 minutes  how many hacking opportunities did I have against the refresh key  80  No  I only had 1 hacking opportunity  During the time the courier communicates with the keymaker  So that   s 1X attack surface  I did have 80 hacking opportunities against the key  but they are no good after 30 minutes   A server would verify an access token based on credentials and signing of  typically  a JWT  An access token leaking is bad  but once it expires it is no longer useful to an attacker  A refresh token leaking is far worse  but presumably it is less likely   I think there is room to question whether the likelihood of a refresh token leaking is much lower than that of an access token leaking  but that   s the idea   Point is that the access token is added to every request you make  whereas a refresh token is only used during the refresh flow So less chance of a MITM seeing the token Frequency helps an attacker  Heartbleed-like potential security flaws in SSL  potential security flaws in the client  and potential security flaws in the server all make leaking possible  In addition  if the authorization server is separate from the application server processing other client requests then that application server will never see refresh tokens  It will only see access tokens that will not live for much longer  Compartmentalization is good for security  Last but not least see this awesome answer  What refresh token is NOT about  The ability to update revoke access level through refresh tokens is a byproduct of choosing to use refresh tokens  otherwise a standalone access token could be revoked or have its access level modified when it expires and users gets a new token

User · Answer

Let s consider a system where each user is linked to one or more roles and each role is linked to one or more access privileges  This information can be cached for better API performance  But then  there may be changes in the user and role configurations  for e g  new access may be granted or current access may be revoked  and these should be reflected in the cache   We can use access and refresh tokens for such purpose  When an API is invoked with access token  the resource server checks the cache for access rights  IF there is any new access grants  it is not reflected immediately  Once the access token expires  say in 30 minutes  and the client uses the refresh token to generate a new access token  the cache can be updated with the updated user access right information from the DB    In other words  we can move the expensive operations from every API call using access tokens to the event of access token generation using refresh token

User · Answer

To further simplify B T s answer  Use refresh tokens when you don t typically want the user to have to type in credentials again  but still want the power to be able to revoke the permissions  by revoking the refresh token    You cannot revoke an access token  only a refresh token

User · Answer

Assume you make the access token last very long  and don t have refresh token  so in one day  hacker get this access token and he can access all protected resources   But if you have refresh token  the access token s live time is short  so the hacker is hard to hack your access token because it will be invalid after short period of time  Access token can only be retrieved back by using not only refresh token but also by client id and client secret  which hacker doesn t have

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First  the client authenticates with the authorization server by giving the authorization grant  Then  the client requests the resource server for the protected resource by giving the access token  The resource server validates the access token and provides the protected resource  The client makes the protected resource request to the resource server by granting the access token  where the resource server validates it and serves the request  if valid  This step keeps on repeating until the access token expires  If the access token expires  the client authenticates with the authorization server and requests for a new access token by providing refresh token  If the access token is invalid  the resource server sends back the invalid token error response to the client  The client authenticates with the authorization server by granting the refresh token  The authorization server then validates the refresh token by authenticating the client and issues a new access token  if it is valid

User · Answer

Why not just make the access token last as long as the refresh token and not have a refresh token   In addition to great answers other people have provided  there is another reason why we would use refresh tokens and it s to do with claims  Each token contains claims which can include anything from the user s name  their roles  or the provider which created the claim  As a token is refreshed  these claims are updated  If we refresh the tokens more often  we are obviously putting more strain on our identity services  however  we are getting more accurate and up-to-date claims

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The idea of refresh tokens is that if an access token is compromised  because it is short-lived  the attacker has a limited window in which to abuse it   Refresh tokens  if compromised  are useless because the attacker requires the client id and secret in addition to the refresh token in order to gain an access token   Having said that  because every call to both the authorization server and the resource server is done over SSL - including the original client id and secret when they request the access refresh tokens - I am unsure as to how the access token is any more  compromisable  than the long-lived refresh token and clientid secret combination   This of course is different to implementations where you don t control both the authorization and resource servers   Here is a good thread talking about uses of refresh tokens  OAuth Archives   A quote from the above  talking about the security purposes of the refresh token      Refresh tokens    mitigates the risk of a long-lived access token leaking  query param in a log file on an insecure resource server  beta or poorly coded resource server app  JS SDK client on a non https site that puts the access token in a cookie  etc

User · Answer

Neither of these answers get to the core reason refresh tokens exist  Obviously  you can always get a new access-token refresh-token pair by sending your client credentials to the auth server - that s how you get them in the first place  So the sole purpose of the refresh token is to limit the use of the client credentials being sent over the wire to the auth service  The shorter the TTL of the access-token  the more often the client credentials will have to be used to obtain a new access-token  and therefore the more opportunities attackers have to compromise the client credentials  although this may be super difficult anyway if asymmetric encryption is being used to send them   So if you have a single-use refresh-token  you can make the TTL of access-tokens arbitrarily small without compromising the client credentials

User · Answer

Clients can be compromised in many ways  For example a cell phone can be cloned   Having an access token expire means that the client is forced to re-authenticate to the authorization server  During the re-authentication  the authorization server can check other characteristics  IOW perform adaptive access management    Refresh tokens allow for a client only re-authentication  where as re-authorize forces a dialog with the user which many have indicated they would rather not do   Refresh tokens fit in essentially in the same place where normal web sites might choose to periodically re-authenticate users after an hour or so  e g  banking site   It isn t highly used at present since most social web sites don t re-authenticate web users  so why would they re-authenticate a client

[security] Why Does OAuth v2 Have Both Access and Refresh Tokens?

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