> On Jul 8, 2019, at 7:10 PM, Leo Tohill <leotoh...@gmail.com> wrote: > Re 8. Refresh Tokens > > "For public clients, the risk of a leaked refresh token is much > greater than leaked access tokens, since an attacker can potentially > continue using the stolen refresh token to obtain new access without > being detectable by the authorization server. " > > (first, note the typo "stoken".) > > Is it always "higher risk"? I could even argue that leakage of a refresh > token is lower risk. As a bearer document, a leaked access token allows > access to resources until it expires. A leaked refresh token, to be useful, > requires an exchange with the AS, and the AS would have the opportunity to > check whether the refresh token is still valid (has not been revoked). (of > course revocation might NOT have happened, but then again, it might have.)
I agree (with caveats, of course). Access tokens and refresh tokens may or may not be attached (by policy) to an authentication session lifetime. It is far easier to picture refresh tokens which are not attached to an authentication session (sometimes called ‘offline’ access) being inappropriate for a browser-based app, which is nearly always a client that the resource owner is interacting with. Variants that may want offline tokens are less easy to imagine - perhaps browser extensions? I believe the language currently there is due to AS implementations predominantly treating refresh tokens as being for offline access, and access token lifetime being short enough to not outlast an authentication session. > Furthermore, since the access token is transmitted to other servers, the risk > of exposure is greater, due to possible vulnerabilities in those called > systems (e.g., logs). Isn't this the reason that we have refresh tokens? > Don't refresh tokens exist because access tokens should have short TTL, > because they are widely distributed? Yes. Once you acknowledge the existence of ‘online’ refresh tokens, they become a strong security component: - Refresh tokens let you shorten the access token lifetime - A shorter access token lifetime lets you have centralized policy to invalidate access without needing to resort to token introspection/revocation - Token refresh can theoretically be used to represent other policy changes by both the client (creating tokens targeting a new resource server or with reduced scopes) and server (changing entitlements and attributes/claims embedded within a structured token) - Refresh tokens can be one-time-use, as recommenced by the security BCP. A exfiltrated refresh token will result in either the attacker or the user losing access on the next refresh, and a double refresh is a detectable security event by the AS. > "Additionally, browser-based applications provide many attack vectors by > which a refresh token can be leaked." > > The risks of leaking a refresh token from the browser are identical to the > risks of leaking an access token, right? This sentence could be changed to > "... by which a token can be leaked." > > A refresh token is "higher risk" because its TTL is usually greater than the > access token's TTL. But if our advice here leads to people using > longer-lived access tokens (because of the problems with getting a new access > token without involving the user), then the advice will be counter > productive. The longer life gives more time for the usefulness of a > browser-side theft, and more time for the usefulness of a server-side theft. > > Which scenario is safer? > A) using an access token with a 10 minute TTL, accompanied by a refresh token > with a 1 hour TTL > B) using an access token with a 1 hour TTL, and no refresh token. Given tokens that track authentication lifetime, it is hard to make a case that refresh tokens which last for the authentication session are a greater security risk than opaque access tokens (requiring token introspection) that will last the same time. Typically an AS (or OP) would issue a structured access token with a lifetime expected to expire before the authentication session, with new tokens issued via requests made in an embedded, iframe (hidden, prompt=none). There may be benefits here of user cookies (or perhaps managed-device information) against an authorization endpoint being used to make decisions that could not be made by a refresh against the token endpoint. I’d be interested in hearing how strong of an implementation issue this might be for deployments - I could see a non-security argument that the BCP should only have one recommended approach here, and that there are deployments needing the iframe approach. -DW
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