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Logout mechanism

Russ Allbery eagle at
Wed Dec 12 09:21:30 PST 2012

Klaus Hueck <klaus.hueck at> writes:

> concerning the missing logout feature in WebAuth we came up with a
> logout mechanism which might work:

>  - The Weblogin server creates yet another cookie (WebloginTrackCookie) in
> which all services are listed which requested a token
>  - If the user clicks the logout button in any of the services he is
> logged in to, he will be forwarded to the logout site (WebAuthDoLogout on)
> of the service. This logout site again forwards the user via a 303 http
> forward to the logout url of the Weblogin server.
>  - The weblogin server then looks up the WebloginTrackCookie to check if
> there are still other services the user is logged in to.
>  - If so, the user will be forwarded once more via a 303 forward to the
> next logout site and so on and so forth.

> Of course this scheme requires that all services have their own logout
> site but - at least in our usage scenario - this is feasible and the
> logout mechanism described above implemented here in our testing system
> works like a charm.

> If I remember right, there was already such a scheme proposed but it was
> implemented by means of JavaScript instead of http forwards.

> What do you guys think about such an logout mechanism?

I think it sounds like a basically sound idea that would at least get us
most of the way there.  The one thing that I'm worried about is the
maximum cookie length.  WebAuth can already break due to exceeding the
maximum cookie length on the WebLogin server with Active Directory
Kerberos tickets with a large PAC.  Adding another long cookie could be a

One of the motivating reasons for using JavaScript is that JavaScript lets
you use web storage, which gets you out of having to create another long
cookie at the cost of losing support for IE 7 and a few other older
browsers.  I don't think you can examine web storage contents without
using JavaScript.

Russ Allbery <eagle at>
Technical Lead, ITS Infrastructure Delivery Group, Stanford University

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