[mcstempel]

Happy to dive into that. For those unfamiliar, XSS attacks are a type of vulnerability where an attacker tricks a website into running the attacker’s code on an end user’s browser. For example, an attacker might set their profile picture URL to be `”/img><script src=”attacker.com/payload.js” />`, which if not properly sanitized would trigger the script to fire when inserted into the DOM. Credential exfiltration is the process of abusing an existing XSS vector to collect user sessions for later use. HTTPOnly cookies are an additional layer of security that can prevent exfiltration by preventing client-side JS from accessing the user session token. However, if the application has an XSS vector, the application is already severely compromised.

The approach our SDK takes (using client-side JS to write to storage) does not offer HTTPOnly protection. This pattern mirrors the approach taken by Firebase, which stores access tokens in local storage. In order to mitigate XSS impact, we have a few mechanisms available. The session JWT itself is only valid for 5 minutes (there is also a longer-lived opaque token, which is valid for longer and is rotated on its own cadence). We’re also introducing support for risk-based controls such as device fingerprinting.

That being said, there are designs that allow 3rd party APIs like ours to set HTTPOnly cookies, by proxying the 3rd party APIs as subdomains. In the future, we'll likely also offer a HTTPOnly session management offering in the SDK to interested customers.

A final note - for integrations with backend-as-a-service that require passing the JWT in a header (hasura [1], mongo atlas [2]), it's impossible to keep the JWT httponly. You can have multiple audiences but the JWT must be exposed to application code in order for the application code to send it somewhere else.

[1] https://hasura.io/docs/latest/auth/authentication/jwt/#heade... [2] https://www.mongodb.com/docs/realm/web/authenticate/#custom-...

[stytchthrowaway]

Of course, I am not concerned that a 5 minute JWT is not HttpOnly. I did not intend to imply that.

However, I am concerned that the refresh mechanism is also not HttpOnly.

Firebase storing access tokens in client-side storage is an example of the former, not the latter. Are they also storing refresh tokens client-side?

FWIW - I am surprised that you would conflate access tokens and refresh tokens like this.

[mcstempel]

Yes, Firebase also stores refresh tokens client-side [1]. The trade-off that both Firebase and Stytch are managing when we follow this pattern is the following:

- You can provide a significantly better developer experience and set-up with this architecture. While there are designs that allow 3rd party APIs like ours to set HTTPOnly cookies by proxying the 3rd party APIs as subdomains, this creates new burdens on the developer for minimal gain considering that a XSS attack vector indicates a severe compromise of the application.

- Today, customers that feel strongly about using HTTPOnly session management will opt for a direct integration with our API using one of our back-end client libraries rather than our JS SDK. While we have interest in providing a HTTPOnly solution in the future to interested customers, we’ve decided the default behavior of the existing SDK is better suited for most developers.

[1] https://github.com/firebase/firebase-js-sdk/blob/0b3ca78eb97...

[stytchthrowaway]

That's really surprising, thank you for following up long after this post has been flagged. That snippet certainly shows the refresh token is accessible client-side.

I remain shocked that an auth company CEO would push a solution without HttpOnly protection. This would not get by our security audits, and Auth0 and many open source tools I've used do not have the same limitation (Auth0 sets it in the SDK rather than a proxy).

OWASP and NIST are aligned that HttpOnly cookie should be used:

[1] https://cheatsheetseries.owasp.org/cheatsheets/Session_Manag...

[2] https://pages.nist.gov/800-63-3/sp800-63b.html#711-browser-c...

[mcstempel]

No problem – I’m happy to engage in good-faith discussions like this one when there are valid nuances to explore.

One callout I’d like to make is that there are two kinds of SDKs. Client-side ones (like Javascript SDKs) and Server-side ones (NodeJS, Go, Python, etc.). The server-side ones are capable of setting HttpOnly cookies, because the server-side ones run on the server and not in a browser context. This is true for both Auth0 and Stytch, and someone using any of the Stytch server-side SDKs will have HttpOnly protection.

Client-side only SDKs are never capable of setting first-party HttpOnly cookies without the aid of a proxy. In fact, there is no truly secure storage mechanism addressable by clientside javascript. All writable storage is accessible to all javascript loaded in the domain - that is to say that if at any point the Auth0 SDK has access to a token, any XSS attack running in the same document will also have access.

Auth0 has numerous clientside SDKs, but we’ll look at their most popular one - @auth0/auth0-spa-js. This SDK stores refresh tokens in a cache [1] in a few ways:

- By default, in memory, which makes exfiltration harder but still very possible via client-side JS. Wrapping a token in a closure doesn’t mean it isn’t addressable - a hacker can monkeypatch and listen to window.fetch for example. This also means that login state is not preserved across tabs or page refreshes, which is quite frankly extremely frustrating to both developers and users

- Auth0 also supports an iframe based flow, which breaks on browsers that use ITP2 such as Safari [2] - so 20% of all users on desktop and 25% of all users on mobile.

- Finally, for customers who do not want the above restrictions, Auth0 allows localstorage [3] to be configured as a cache storage. Local storage is just as open to XSS exfiltration as non-HTTPOnly cookies.

So while yes, Auth0 does not set cookies, their refresh tokens are still accessible client-side in many common deployment scenarios, and are still vulnerable to the same XSS exfiltration vulnerability that HTTPOnly cookies protect against.

Overall, the main reason that Google’s security team, Auth0’s security team, and our security team are comfortable with offering a non-HTTPOnly session management solution in a JS SDK comes down to:

1. HTTPOnly as a security layer can help prevent exfiltration, but if an app already has an XSS vector, it’s already severely compromised, making such a layer moot.

2. As an auth company (whether you’re Stytch vs. Auth0 vs. Google Firebase), you need to make a decision on how much flexibility you want to offer developers. Our stance is that when additional flexibility and an improved developer experience do not create any practical security risk, we should provide that better developer experience to our customers.

[1] https://github.com/auth0/auth0-spa-js/blob/0de9c6bf61d37fc21...

[2] https://community.auth0.com/t/silent-authorization-not-worki...

[3] https://github.com/auth0/auth0-spa-js/blob/0de9c6bf61d37fc21...