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FreeBSD-SA-00:46                                           Security Advisory
                                                                FreeBSD, Inc.

Topic:          screen port contains local root compromise

Category:       ports
Module:         screen
Announced:      2000-09-13
Affects:        Ports collection prior to the correction date.
Corrected:      2000-09-01
Credits:        Jouko Pynnönen <jouko@SOLUTIONS.FI>
Vendor status:  Updated version released
FreeBSD only:   NO

I.   Background

screen is a popular application that multiplexes a physical terminal
between several processes.

II.  Problem Description

The screen port, versions 3.9.5 and before, contains a vulnerability
which allows local users to gain root privileges.  This is
accomplished by inserting string-formatting operators into
configuration parameters, which may allow arbitrary code to be
executed.

The screen port is not installed by default, nor is it "part of
FreeBSD" as such: it is part of the FreeBSD ports collection, which
contains over 3800 third-party applications in a ready-to-install
format. The ports collections shipped with FreeBSD 3.5.1 and 4.1
contain this problem since it was discovered after the releases.

FreeBSD makes no claim about the security of these third-party
applications, although an effort is underway to provide a security
audit of the most security-critical ports.

III. Impact

Local users can obtain root privileges.

If you have not chosen to install the screen port/package, then your
system is not vulnerable to this problem.

IV.  Workaround

Remove the setuid bit on the program: execute the following command as
root:

chmod 555 /usr/local/bin/screen-3.9.5

Note that this should be considered a temporary measure and may affect
the behaviour of the screen program.

V.   Solution

One of the following:

1) Upgrade your entire ports collection and rebuild the screen port.

2) Deinstall the old package and install a new package dated after the
correction date, obtained from:

ftp://ftp.FreeBSD.org/pub/FreeBSD/ports/i386/packages-3-stable/misc/screen-3.9.8.tgz
ftp://ftp.FreeBSD.org/pub/FreeBSD/ports/i386/packages-4-stable/misc/screen-3.9.8.tgz
ftp://ftp.FreeBSD.org/pub/FreeBSD/ports/alpha/packages-4-stable/misc/screen-3.9.8.tgz
ftp://ftp.FreeBSD.org/pub/FreeBSD/ports/i386/packages-5-current/misc/screen-3.9.8.tgz
ftp://ftp.FreeBSD.org/pub/FreeBSD/ports/alpha/packages-5-current/misc/screen-3.9.8.tgz

NOTE: It may be several days before updated packages are available.

3) download a new port skeleton for the screen port from:

http://www.freebsd.org/ports/

and use it to rebuild the port.

4) Use the portcheckout utility to automate option (3) above. The
portcheckout port is available in /usr/ports/devel/portcheckout or the
package can be obtained from:

ftp://ftp.FreeBSD.org/pub/FreeBSD/ports/i386/packages-3-stable/devel/portcheckout-2.0.tgz
ftp://ftp.FreeBSD.org/pub/FreeBSD/ports/i386/packages-4-stable/devel/portcheckout-2.0.tgz
ftp://ftp.FreeBSD.org/pub/FreeBSD/ports/alpha/packages-4-stable/devel/portcheckout-2.0.tgz
ftp://ftp.FreeBSD.org/pub/FreeBSD/ports/i386/packages-5-current/devel/portcheckout-2.0.tgz
ftp://ftp.FreeBSD.org/pub/FreeBSD/ports/alpha/packages-5-current/devel/portcheckout-2.0.tgz

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