From: PortSwigger (mailportswigger.net)
Date: Tue Aug 12 2003 - 15:29:37 CDT

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I have recently looked at a web application which maintains session state
using a custom token stored in a hidden form field. Each page within the
application is accessed using a POST request which includes the token.

Several locations within the authenticated areas of the application exhibited
XSS-like behaviour (i.e. client data submitted in form and URL querystring
fields is returned unsanitised to the browser). However (as far as I could
see) there was no way to exploit this to attack other users. Any request not
containing a valid session token results in a redirect to the login page. And
so assuming that only the user knows their own session token, only they could
frame a malicious request that would succeed in injecting arbitrary
HTML/JavaScript into their browser.

The relevant difference from conventional session tokens (cookies) is that the
custom token is not automatically stored and resubmitted by the browser as
the user moves between pages. Rather, the browser only submits the token with
any given request because the application has set the token within the form
that generates that request. If the user initiates an arbitrary request
(induced by an attacker) then it will not contain a valid token, and they are
returned to the login page.

Although the developers had chosen this means of maintaining session state for
different reasons (to do with load-balancing across multiple web servers), it
effectively reduces both the scope and impact of any XSS vulnerabilities:

1. Users can only be attacked using XSS vulnerabilities which appear within
the unauthenticated area of the app, where a valid session token isn't
required. If this area consists of a single login page, the task of
addressing XSS problems is considerably reduced.

2. Attacking a logged-in user using any XSS vulnerabilities within the
unauthenticated area of the app could not straightforwardly be used to
capture their session token. Although the token is cached in the browser
request history, it is not stored in 'document.cookie', nor could it be
obtained using an XST-style trick. (Of course, there is plenty an attacker
can do with XSS other than cookie-stealing, but this does prevent one of the
easier attacks.)

Of course, there may be downsides to this approach to session management, and
I wouldn't recommend it without question, but I think it's worthy of
consideration.

Any thoughts?

PortSwigger

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