KDE fixes screensaver security holes

On BUGTRAQ, Christian Esken
writes:

KDE Screensavers are usually running SUID root. Security issues
have been posted to Bugtraq on Nov 16 1998, under the subject “KDE
1.0’s klock can be used to gain root priveledges”. The KDE team has
now published a fix for the KDE1.0 branch and the current
branch.

With this change, screensavers and klock are not running SUID
anymore. This will solve every potential exploit, like misuse of
buffer overruns to gain root rights or executing a wrong executable
under SUID rights.

The following text explains the technique used to solve the
problem. An advisory for distributors, users and administrators
follows the technical description.

Technique
———

An authentification program, kcheckpass, has been introduced.
This is a separate, helper program, that runs SUID and is called
each time a password has to be checked. The password is passed via
STDIN to the program and the result of the authentification process
is returned in the return code of the program. This program is
small and supposed to be free from security hazzles.

Advisory
——–

Administrators should remove any SUID bit from KDE
executables.

After updating to the fixed KDE1.0 tree or to the current KDE,
administrators should

1. check the access rights of the installed executables. The
   screensavers must not be installed SUID anymore. If in doubt,
   remove the SUID bits manually by a suitable command, like “chmod -s
   *.kss klock” under Linux.
2. check the access rights of the kcheckpass program. This program
   should only be installed SUID root under certain authentification
   systems, like the shadow password suite.
3. Distributions using the shadow password system can be made more
   secure by creating a “shadow” group and setting the access rights
   of /etc/shadow and kcheckpass like in the following example:

 
 -rw-r-----   1 root     shadow        746 Sep  2 21:35 /etc/shadow
 -rwxr-sr-x   1 root     shadow       4720 Nov 17 22:32 /opt/kde/bin/kcheckpass

Distributors are strongly encouraged to follow this scheme. This
way, the kcheckpass is running under the effective user id of the
user itself and the effective group “shadow”. With this, kcheckpass
has only one additional right against regular users: The right to
read /etc/shadow. Attackers won’t be able to make wider use of
“kcheckpass”.

Availability of the fix
———————–

The patches are already integrated in the KDE1.0 and the KDE1.1
branches. You can use cvs/cvsup to get current sources. You can
also get the patch from KDE’s ftp Server ftp://ftp.kde.org and its mirrors, which
you can apply against a clean KDE1.0 kdebase package.

It has been uploaded under the name kdebase1.0-klock-patch and
should show up soon on a suitable place on KDE’s ftp Server. The
precise location will be announced later, for example http://www.kde.org/news_dyn.html
will provide this information as soon as it available.