[OAUTH-WG] OAuth 2.0 for Browser-Based Apps - On the usefulness of refresh token rotation

Philippe De Ryck <philippe@pragmaticwebsecurity.com> Sat, 16 May 2020 15:08 UTC

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From: Philippe De Ryck <philippe@pragmaticwebsecurity.com>
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Date: Sat, 16 May 2020 17:08:48 +0200
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Subject: [OAUTH-WG] OAuth 2.0 for Browser-Based Apps - On the usefulness of refresh token rotation
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Hi all,

I am working on formulating developer guidelines on using Refresh Token Rotation (RTR), as required by "OAuth 2.0 for Browser-Based Apps". 

The protection offered by RTR kicks in the moment a refresh token is used twice, so the assumption is that the attacker has the ability to steal tokens from the client. In general, this means the attacker has malicious code running in the application (e.g., XSS, remote JS inclusion, ...). 

Within these constraints, I can think of a couple of malicious payloads that sidestep the protection offered by RTR:

1. Stealing access tokens in an online XSS attack
2. Stealing refresh tokens, but waiting to use the latest until the original client is no longer active
3. Running a silent authentication flow in an iframe to obtain a new and unrelated AT and RT, and use that until it expires

Scenario 1 is straightforward in most applications, but the attack requires the vulnerable application to remain online. Scenario 2 might be difficult if the RT is kept out of reach from the main application (e.g. in a worker thread). Scenario 3 is most dangerous, but also a bit tricky to implement as the payload needs to make sure the application's code does not interfere (however, the browser's Same-Origin Policy will not intervene). The specifics depend on the concrete implementation, but all three attacks are technically feasible.

With these attacks in mind, it seems that the use of the Authorization Code flow with RTR does not really add much improvement for security, if other best practices are followed (e.g., using HTTPS). RTR does a lot for usability and handling third-party cookie blocking scenarios though.

In this context, my advice to developers is to avoid handling tokens in the browser in security-sensitive scenarios. A Backend-for-frontend pattern gives a server-side component control over tokens, along with the ability to implement additional security measures.

Additionally, is there any official recommendation to link the validity of a refresh token to the lifetime of the user's session with the Authorization Server? Having that property gives RTR similar security properties as the silent renew scenario. 

Any feedback on this train of thought is more than welcome.

Philippe