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Re: Session data pollution vulnerabilities in web applications
From: Yvan Boily <yboily () gmail com>
Date: Fri, 13 Jan 2006 07:41:10 -0600

There is nothing that you have provided that indicates that this is a new
class of vulnerability; this is a classic state management issue.  The
application is designed to have a password reset function, and in order for
that function to behave properly application state must be enforced.

Essentially the issue here is that the application is not maintaining its
state properly, and the result is that an attacker can force the application
to engage in some unplanned functionality.  Any web application must
leverage some form of state management, and this is pretty easy for most
developers to learn.  What most developers have difficulty with is the idea
that process flow should be enforced; if you have a 'Registration' module
with 3 states, and a 'Password Reset' module with 4 states, then any state
transition from the Registration module into the Password Reset module
should result in the use being forced to start at Password Reset
modules initial state.

Without a strong means of enforcing the application logic it is trivial to
move an application from a known to an unknown state, and this is never a
good thing in an application, let alone an application that must make
decisions about security based on application state.

The reuse of the 'login' session variable in your example is still a case of
poor software development, but if application state was properly maintained,
then it wouldn't be an issue.

On 1/13/06, Alla Bezroutchko <alla () scanit be> wrote:

Frank Knobbe wrote:
The proposed fix is -- besides being only specific to this example --
equally flawed. The underlying issue is that you trust user supplied
data. When a user supplies a user name for login purposes, you should
only use that input to perform a search in your database. If a match has
been found, take a *trusted* value from your database, for example an
index ID, and carry that in the session object to identify the user.

Suppose in my example resetpw3.php instead of doing

$_SESSION['login'] = $_POST['login'];

did

$_SESSION['login'] = $db->getOne("SELECT login FROM users WHERE login=?
AND secret_answer=?", array($_POST['login'], $_POST['secret_answer']));

As you suggest it takes a trusted value from the database. It is still
does not prevent using register2.php to initialize $_SESSION['login']
and then jump to resetpw4.php and have it use the value to change
password.

In other words, don't accept any user input (even after proper input
validation) and carry it as trusted data in your session object. Don't
base further decisions on this data. Since it is user supplied it can
not be trusted.

Are you saying that user supplied data should never ever be stored in
the session? Where do you suggest to keep the data that needs to be
passed between different pages in the application?

Say you have a form that is filled in in several steps, each step being
a separate page. Where would you store the data that the user entered to
have it all available on the last page? I can only think of carrying it
around in hidden form fields, and this is not always possible.

Your example is further flawed in that it allows the change of a
password without being properly authenticated. Just having a valid
'login' session object present doesn't indicate that the user is
authenticated.

I think it does, if 'login' session object is only ever set by the
authentication procedure after verifying user's credentials. I believe
the usual approach to authentication in web applications is to check
user's credentials and then store something in the session that
indicates that the credentials were correct. Usually it is something
like a user ID or an object or JavaBean storing user information. Then
every pafge that needs to check auth just checks if the session object
is present and valid.

How do applications that you encounter check if the user is authenticated?

I really, really hope this was just an example you made
up, and not something you actually saw being used. If so, go back to
that web site with a clue-by-four and give it a few whacks.

I work for a company that does penetration testing, vulnerability
assessments, and stuff like that. The contracts we usually sign do not
authorise us to apply blunt instruments to customers' web sites. :)

This particular example is made up. Giving real ones would violate
non-diclosure agrements. It is similar to the real ones in the way that
it has the same problem - one storage location is used to keep both
trusted and untrusted data. I used the password reset example because
the security implications are clear and one don't need application
specific context to understand it. The bug does not have to do anything
with authentication as such.

And, in case you are wondering, the real ones were just as bad. :)

Alla.
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--
____
ygjb
Computer Science is no more about computers than astronomy is about
telescopes. E. W. Dijkstra
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