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Telnetd Environment Vulnerability (fwd)


If this has been covered already ignore me I am behind on mail again.. :)

CA-95:14                         CERT Advisory
                               November 1, 1995
                        Telnetd Environment Vulnerability

The CERT Coordination Center has been made aware of a vulnerability with
some telnet daemons. The daemons affected are those that support RFC 1408
or RFC 1572, both titled "Telnet Environment Option," running on systems
that also support shared object libraries.

To determine if your system is potentially vulnerable, refer to the
information we have received from vendors which is summarized in
Section III below; details are in Appendix A and reproduced in the
CA-95:14.README file. Note that if you installed a version of David 

Borman's telnet package that is older than October 23, 1995, your
system may be vulnerable even though it was not vulnerable as distributed
by the vendor.

If your vendor is not listed, you will need to determine if your system
may be vulnerable. First, determine if your telnet daemon is RFC 1408/1572
compliant. One indication that it is compliant is if your telnet(1)
program supports the "environ" command or your telnetd(8) program supports
the ENVIRON or NEW-ENVIRON options. Unless you are certain that your
telnet daemon is not RFC 1408/1572 compliant, you may wish to assume it is
to be safe. Second, determine if your system supports shared libraries. To
do this, consult the ld(1) manual page. If it describes dynamic or shared
objects, your system probably supports shared object libraries. A system
is potentially vulnerable if the telnet daemon supports RFC 1408/RFC 1572
and the system supports shared object libraries.

We recommend that you follow your vendor's directions for addressing this
vulnerability. Until you can install a patch, we recommend using the
workaround in Appendix B below. If you have previously installed David
Borman's telnet package on your system, we recommend that you obtain the
current version of telnet (see Section III.C).

As we receive additional information relating to this advisory, we will
place it in:


We encourage you to check our README files regularly for updates on
advisories that relate to your site.


I.   Description

     Some telnet daemons support RFC 1408 or RFC 1572, both titled "Telnet
     Environment Option." This extension to telnet provides the ability
     to transfer environment variables from one system to another. If
     the remote or targeted system, the one to which the telnet is
     connecting, is running an RFC 1408/RFC 1572-compliant telnet daemon
     *and* the targeted system also supports shared object libraries, then
     it may be possible to transfer environment variables that influence
     the login program called by the telnet daemon. By influencing that
     targeted system, a user may be able to bypass the normal login and
     authentication scheme and may become root on that system.

     Users with accounts on the targeted system can exploit this
     vulnerability. Users without accounts on that system can also
     exploit this vulnerability if they are first able to deposit an
     altered shared object library onto the targeted system. Therefore, a
     system may be vulnerable to users with and without local accounts.

     Not all systems that run an RFC 1408/RFC 1572-compliant telnet daemon
     and support shared object libraries are vulnerable. Some vendors have
     changed the trust model such that environment variables provided by
     the telnet daemon are not trusted and therefore are not used by the
     login program. Section III contains a summary of information vendors
     have reported as of the date of this advisory.

II.  Impact

     Local and remote users can become root on the targeted system.

III. Solution

     The general solution to this problem is to replace the telnet daemon 

     with one that changes the environment given to the login program. We
     recommend that you install a patch from your vendor if possible. If this
     is not possible, we recommend using the workaround in Appendix B until 

     you can install a patch. Finally, if you have previously installed Mr.
     Borman's telnet package, see Section C for how to get a new version that
     fixes the vulnerability. 

     A. Vendor Patches

        Below is a summary of the vendors listed in the current version of
        the CA-95:14.README file, and the status they have provided.  More
        complete information, including how to obtain patches, is provided
        in Appendix A of this advisory and reproduced in the README file.
        We will update the README file as we receive more information from

        If your vendor's name is not on this list, please contact the
        vendor directly.

        REMINDER: If you installed a version of David Borman's telnet package
                  that is older than October 23, 1995, your system may be
                  vulnerable even though it was not vulnerable as distributed 

                  by the vendor.

        Vendor or Source                  Status
        ----------------                  ------------
        Apple Computer                    not vulnerable
        Berkeley Software Design          not vulnerable
        Cray Research                     not vulnerable
        CYGNUS                            cns-95q1 - vulnerable
                                          cns-95q4 - not vulnerable
        Data General                      not vulnerable
        Digital Equipment                 Ultrix - not vulnerable
                                          OSF/1 - vulnerable
        FreeBSD                           vulnerable
        Harris                            not vulnerable
        Hewlett-Packard                   not vulnerable
        Linux                             Debian - vulnerable
                                          Red Hat - vulnerable
                                          Slackware - appears vulnerable
        MIT-distributed for Athena        vulnerable
        NetBSD                            1.0 - vulnerable
                                          current - not vulnerable
        NEC                               vulnerable
        Open Software Foundation          OSF/1 version 1.3 not vulnerable
        OpenVision                        OpenV*Secure 1.2 - vulnerable
        SCO                               not vulnerable
        SGI                               5.2, 5.3, 6.0.1, 6.1 - vulnerable
        Sony Corp.                        NEWS-OS 6.x - not vulnerable

     B. Workaround

        Until you can install a patch from your vendor, you can use
        the workaround provided in Appendix B.

     C. If you have installed a previous version of Mr. Borman's
        telnet package, note that he has fixed this problem in
        the version available at the following location:


                MD5 checksum 2e14879a5b0aa6dd855a17fa8a3086cf

The CERT Coordination Center staff thanks Eric Halil of AUSCERT, Wolfgang
Ley of DFNCERT, and Sam Hartman of the MIT Kerberos Development team for
their support in responding to this problem.

If you believe that your system has been compromised, contact the CERT
Coordination Center or your representative in the Forum of Incident
Response and Security Teams (FIRST).

If you wish to send sensitive incident or vulnerability information to
CERT staff by electronic mail, we strongly advise that the email be
encrypted.  The CERT Coordination Center can support a shared DES key, PGP
(public key available via anonymous FTP on info.cert.org), or PEM (contact
CERT staff for details).

Internet email: cert@cert.org
Telephone: +1 412-268-7090 (24-hour hotline)
           CERT personnel answer 8:30 a.m.-5:00 p.m. EST(GMT-5)/EDT(GMT-4),
           and are on call for emergencies during other hours.
Fax: +1 412-268-6989

Postal address:  CERT Coordination Center
                 Software Engineering Institute
                 Carnegie Mellon University
                 Pittsburgh, PA 15213-3890

CERT advisories and bulletins are posted on the USENET newsgroup
comp.security.announce. If you would like to have future advisories and
bulletins mailed to you or to a mail exploder at your site, please send mail
to cert-advisory-request@cert.org.

Past CERT publications, information about FIRST representatives, and
other information related to computer security are available for anonymous 

FTP from info.cert.org. 

Copyright 1995 Carnegie Mellon University
This material may be reproduced and distributed without permission provided it
is used for non-commercial purposes and the copyright statement is included.

CERT is a service mark of Carnegie Mellon University.

Appendix A: Vendor Information
Current as of November 1, 1995
See CA-95.14.README for updated information

Below is information we have received from vendors. If you do not see your
vendor's name below, contact the vendor directly for information.

Apple Computer, Inc.
Apple's A/UX is not vulnerable.

Berkeley Software Design, Inc.
BSDI's BSD/OS is not vulnerable.

Cray Research, Inc.
Cray's UNICOS is not vulnerable.

CYGNUS Network Security V4 Free Network Release
cns-95q1 is vulnerable.   cns-95q4 is not vulnerable.

Customers can use the following URL to obtain the patch:


If customers are unable to obtain the patch in this manner
or have any questions, send e-mail to kerbask@cygnus.com/

Note that while the URL and patch are already available, there
is no link to the page yet.  We will add a link once the
announcement has been made.

Data General Corporation
Data General believes the DG/UX operating system to be
NOT vulnerable to this problem.  This includes all supported
releases, DG/UX 5.4 Release 3.00, DG/UX 5.4 Release 3.10,
DG/UX Release 4.10 and all related Trusted DG/UX releases. 

Specifically, telnetd shipped in DG/UX does not support
environment options and does not support RFC 1572.

Digital Equipment Corporation
Digital's OSF/1: vulnerable
Digital's ULTRIX: not vulnerable

Digital has corrected this potential vulnerability. Patches containing
new images for Digital's OSF/1 platforms are being provided to your 

normal Digital Support channels beginning October 31 (U.S. time). The
kits may be identified as ECO SSRT0367 (telnetd) for DEC OSF/1  V2.0
thru V3.2c

This potential vulnerability is not present on Digital's ULTRIX systems. 

Digital distribution of this announcement will be via AES services  (DIA,
DSNlink FLASH etc.). Digital Equipment Corporation strongly urges Customers
to upgrade to a minimum of DEC OSF/1 V3.0, then apply this patch.

Vulnerable.  A patch has been applied to the current development FreeBSD
source tree which is not yet released.  This patch is slightly modified
compared to posted one, i.e. only variables which affects FreeBSD are
disabled. It is telnetd patch, not a login wrapper.

For the official patch, location please contact: 

Jordan Hubbard <jkh@FreeBSD.org>

Harris Computer Systems Corporation's Night Hawk is not vulnerable.

Hewlett-Packard Company
HP/UX is not vulnerable.

Linux (freely available software; not a vendor)
Debian GNU/Linux (From "Peter Tobias" <tobias@et-inf.fho-emden.de>):
The current version of the Debian GNU/Linux distribution (released 10/27/95)
is not vulnerable anymore. All Debian Installations that use a netstd
package version prior to v1.21-1 are vulnerable (telnetd is part of
the netstd package). netstd-1.21-1 and above are ok.

Patches are available. Peter fixed the bug last week and uploaded the
fixed version to our ftp site (ftp.debian.org). Binaries, sources and
the diffs against the bsd telnetd can be found there.  The URL for the
new binary package is:


and the sources and the diff against the bsd telnetd can be found at:


Red Hat Linux (From Erik Troan <ewt@redhat.com>):
Vulnerable.  A fix is now available at:



It will also be fixed in the upcoming Red Hat 2.1 release.

Slackware Linux (Alan Cox <alan@cymru.net>):
The telnetd distributed with Slackware Linux appears to be vulnerable,
although it has not been verified.

MIT-distributed Athena telnet/telnet95
Vulnerable. Patches available in:

beta4-3.patch is the patch versus the Beta 4 patchlevel 3 distribution
of Kerberos v5.

beta5.patch is the patch versus the Beta 5 distribution of Kerberos V5.

Both patches have been PGP signed by Ted Ts'o <tytso@MIT.EDU> using
detached signatures (beta4-3.patch.sig and beta5.patch.sig).

NetBSD 1.0 (the last official release) is vulnerable; NetBSD 1.1 (due
out in mid-November) will not be.  NetBSD-current is not vulnerable,
as of a week or so ago.

Patches: A source form patch has been developed.  A core team member will
have to make source and binary patches available and provide a location for

The login-wrapper given in the advisory can be compiled with NetBSD with:
        cc -o login-wrapper login-wrapper.c

NEC Corporation
Some NEC systems are vulnerable.  Here is their vulnerability matrix:

       OS               Version        Status
------------------   ------------   -------------------------------------
EWS-UX/V(Rel4.0)     R1.x - R6.x    not vulnerable

EWS-UX/V(Rel4.2)     R7.x - R10.x   not vulnerable

EWS-UX/V(Rel4.2MP)   R10.x          vulnerable
                                    patch available by the end of Nov, 1995

UP-UX/V              R2.x - R4.x    not vulnerable

UP-UX/V(Rel4.2MP)    R5.x - R7.x    vulnerable
                                    patch available by the end of Nov, 1995

UX/4800              R11.x          vulnerable
                                    patch available by the end of Nov, 1995
Contacts for further information:

Open Software Foundation
OSF/1 version 1.3 is not vulnerable.

This is from: Barry Jaspan <bjaspan@cam.ov.com>:
OpenVision has a patch for the telnetd in OpenV*Secure 1.2 and will
contact its customers directly.

Not believed to be vulnerable.

Silicon Graphics
IRIX 5.2, 5.3, 6.0.1, and 6.1 are vulnerable.  

SGI acknowledges the telnetd vulnerability reported by MIT and is
currently investigating.   No further information is available at
this time.

As further information becomes available, additional advisories will be

SGI Security Information/Contacts:

For obtaining security information, patches or assistance, please
contact your SGI support provider.

If there are questions about this document, email can be sent to
cse-security-alert@csd.sgi.com .

For reporting *NEW* SGI security issues, email can be sent to

Sony Corporation
Sony's NEWS-OS 6.x is not vulnerable.
Appendix B: login-wrapper Workaround

The login-wrapper program shown below is meant to be executed just before
the distributed login program. The wrapper cleans specific variables from
the environment before invoking the distributed login program.

------------------------cut here--8<------------------------
 * This is a login wrapper that removes all instances of
 * various variables from the environment.
 * Note: this program must be compiled statically to be
 * effective against exploitation.
 * Author:      Lawrence R. Rogers
 * 10/25/95     version 1.1     Original version
 * 10/26/95     version 1.2     ELF_ variables removed (Linux)
 * 10/27/95     version 1.3     ELF_ changed to ELF_LD_
 *                              Added AOUT_LD_ (Linux)

#include        <stdio.h>

#if !defined(_PATH_LOGIN)
# define                _PATH_LOGIN     "/bin/login.real"

main (argc, argv, envp)
int argc;
char **argv, **envp;
        register char **p1, **p2;

        for (p1 = p2 = envp; *p1; p1++) {
                if (strncmp(*p1, "LD_", 3) != 0 &&
                    strncmp(*p1, "_RLD", 4) != 0 &&
                    strncmp(*p1, "LIBPATH=", 8) != 0 &&
                    strncmp(*p1, "ELF_LD_", 7) != 0 &&
                    strncmp(*p1, "AOUT_LD_", 8) != 0 &&
                    strncmp(*p1, "IFS=", 4) != 0 ) {
                            *p2++ = *p1;
        *p2 = 0;
        execve(_PATH_LOGIN, argv, envp);
------------------------cut here--8<------------------------

The following two examples show how to compile the login-wrapper for SGI's
IRIX 5.3 and FreeBSD 2.x systems. The examples move the distributed login
program to a new location and install the wrapper in the standard location.
When executed, the wrapper first cleanses the environment and then calls
the relocated, distributed login program.

Note 1: The wrapper must be compiled statically. On SGI's IRIX system,
compiling statically requires that the non-shared versions of libraries
be installed. Consult your system documentation to determine how to do

Note 2: You may need to change the _PATH_LOGIN variable to define where
the real login program resides on your system. On some systems, login
resides in /usr/bin/login.

Compiling for IRIX 5.3
# uname -a
IRIX test 5.3 11091812 IP22 mips
# /bin/ls -lL /bin/login
-rwsr-xr-x    1 root     sys        65832 Sep  9 14:24 /bin/login
# /bin/cc -non_shared -O login-wrapper.c -o login-wrapper
# /bin/mv /bin/login /bin/login.real
# /bin/chmod 755 /bin/login.real
# /bin/mv login-wrapper /bin/login
# /bin/chmod 4755 /bin/login
# /bin/chown root /bin/login
# /bin/chgrp  sys /bin/login
# /bin/ls -lL /bin/login /bin/login.real
-rwxr-xr-x    1 root     sys        65832 Sep  9 14:24 /bin/login.real
-rwsr-xr-x    1 root     sys       213568 Oct 30 08:42 /bin/login

Compiling for FreeBSD 2.x
# /bin/ls -lg /usr/bin/login
-r-sr-xr-x  1 root  bin  20480 Jun 10 20:00 /usr/bin/login
# /usr/bin/cc -D_PATH_LOGIN=\"/usr/bin/login.real\" -static \
        -O login-wrapper.c -o login-wrapper
# /bin/mv /usr/bin/login /usr/bin/login.real
# /bin/chmod 555 /usr/bin/login.real
# /bin/mv login-wrapper /usr/bin/login
# /bin/chmod 4555 /usr/bin/login
# /usr/sbin/chown root.bin /usr/bin/login
# /bin/ls -lg /usr/bin/login /usr/bin/login.real
-r-sr-xr-x  1 root  bin  24885 Oct 25 22:14 /usr/bin/login
-r-xr-xr-x  1 root  bin  20480 Jun 10 20:00 /usr/bin/login.real

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