Russ Allbery rra at
Fri Jul 18 13:01:06 EDT 2008

"Sharad Desai" <ssdesai1 at> writes:

>> The only fly in the ointment here is that none of the WebSSO solutions
>> currently available can handle authenticating POST requests, where the
>> user hasn't previously authenticated to the service, due to their
>> requirement for redirects. For us, this was a small price to pay.
> I apologize, but can you elaborate on this?

WebSSO systems handle unauthenticated users by redirecting them to a
central login server as a response to an attempt to access a protected
resource.  The HTTP protocol, however, does not permit returning a
redirect as the result of a POST, nor is there any good way to stash the
data that comes along with a POST while bouncing the user through the
login server without application support for the SSO system (which is
contrary to a primary goal: ability to drop WebSSO in front of any
arbitrary web application without modifying the application).

As a result, when using a WebSSO, you have to ensure that the user has
authenticated at some point in the page flow before they do a POST.  You
can't authenticate them at the time of the POST; you need to have existing
credentials to use at that point.

This usually isn't much of a problem since it's considered best practice
for most applications using POST to force the user to authenticate prior
to the POST anyway (otherwise, some cross-site attacks and deceptive
tricks are easier to perform).

Russ Allbery (rra at             <>

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