Serious Security Issue in Windows XP SP2's Firewall

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Date: Sun, 19 Sep 2004 11:23:11 +0200
To: bugtraq@securityfocus.com

Hello,

this might be interesting for you (see below): Please note that all
screenshots and more details can be found in the German article only (see
links), the English one is slightly shortened.

cheers,
Andreas Marx

PC-WELT discovers and fixes serious security issue in Windows XP SP2
by Andreas Kroschel and Thorsten Eggeling; Sep 15, 2004

English version: <http://www.pcwelt.de/know-how/extras/103039/>
German version: <http://www.pcwelt.de/news/sicherheit/103013/>

Windows XP Service Pack 2 with Advanced Security Technologies helps you
protect your PC against viruses, hackers, and worms." - this is how
Microsoft promotes its Service Pack 2 on its website. What the company does
not say: Instead of viruses, worms, and hackers, the supposedly safe SP2
for Windows XP invites any Internet user to have a look around your PC.

As soon as you install SP2 on a Windows XP PC with a certain configuration,
your file and printer sharing data are visible worldwide, despite an
activated Firewall. This also applies to all other services. The PC only
has to provide sharing for an internal local network and connect to the
Internet via dial-up or ISDN. Users of DSL services are also affected, if a
firewall is not integrated into the DSL modem or a common modem instead of
a DSL router is used. Additionally, Internet Connection Sharing of the PC
has to be disabled.

A number of test scans run by PC-Welt revealed that this in fact is a
common configuration and not a rare sight. Without great effort, we were
able to discover private documents on easily accessible computers on the
Internet. It must be assumed, that these users wrongly believe they are
safe and that their sharing configurations are only visible in their
network at home: Often, we did not even encounter password protection.

Already Windows 95 affected by a similar problem

Experienced Windows users may remember that there was a similar problem in
the past, specifically with Windows 95. Back then, Microsoft forgot to
separate file and printer sharing from the dial-up network adapter when
such a connection was configured.

In other words, this caused the service to be released worldwide through
the dial-up connection as soon as you were connected to the Internet.
Microsoft at that time issued an update to patch the bug. The fact that
file and printer sharing since then is not connected to the dial-up
connection anymore, can easily be seen on your system: Right-click on the
symbol "My Network Places" and select "Properties". Repeat the right-click
and selection with the icon of your dial-up connection and select the tab
"Settings". If there is no check at "File and Printer Sharing", it
indicates that this service should not be made available through your
dial-up connection.

This in fact is true for Windows XP without Service Pack. Since SP1, this
configuration is hardly more than cosmetics and does not serve any purpose
anymore. This means, the file and printer sharing service is connected in
general, also to the dial-up network adapter. This in itself is a serious
bug, since your shared data potentially could be seen on the Internet.
However, there are no catastrophic effects, as every dial-up connection is
configured with an activated firewall by default.

If you intended to deactivate this firewall, Windows displayed an easily
recognizable dialog, that this choice would allow access to your computer.
Despite the bug in SP1, the configuration of the firewall was worked out in
a clean way: You were able to run the dial-up connection with a firewall
and the internal network card without, because the latter was supposed to
enable access through the Windows network.

SP1 + SP2 leads to a catastrophic error

Due to the bug carried over from SP1 as well as a new bug, the firewall
configuration with SP2 has a catastrophic effect. The SP2 installation
simply uses the previous configuration of the firewall: If it was active
for the dial-up connection, now it also has been activated for the network
adapter.

At the same time, an exception is determined for file and printer sharing:
For the internal network card - and astonishingly also for all adapters.

With the first use of the dial-up connection after installing SP2, all of
your shared data are available on the Internet. Now, other users can start
guessing your passwords for administrator and guest and you basically are
no more secure than the first Windows 95 users with an Internet connection
- thanks to Service Pack 2.

How to correct the problem

It is not advisable to keep this defective default configuration. However,
the previous environment cannot be restored: The configuration for the
firewall was changed, which does not allow the setting of active or
inactive conditions or exceptions for each network adapter anymore. Now
this only works for network areas.

Choose "Windows Firewall" in the in the Windows Control Panel and the there
the tab "Exceptions". Select "File and Print Services" and click on "Edit".
Now you can see four ports which are used by the file and print sharing
service.

To lock the service to the outside and keep it open for the internal LAN,
you have to individually select and change its area with the respective
button. Our reader Yves Jerschov notified us of another bug: The value for
the area set by default "Only for own network (Subnet)" only works, if the
Internet Connection Sharing is activated. If this is not the case, your
shared data are visible worldwide. This error can be corrected by choosing
"User defined List" and entering the IP addresses that are supposed to have
access - the IP addresses of your LAN. A whole range of an IP area can be
entered as "192.168.x.0/255.255.255.0", if the respective addresses start
with 192.168.x.

After these measures, you can be sure to be as safe as you were with SP1.
Great, don't you think?

-- 
AV-Test GmbH, Klewitzstr. 7, 39112 Magdeburg, Germany
Phone: +49 (0)391 6075466, <http://www.av-test.org>

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