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Ärende: CERT Advisory CA-2000-06
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CERT Advisory CA-2000-06 Multiple Buffer Overflows in Kerberos
Authenticated Services
Original release date: May 17, 2000
Last revised: --
Source: The MIT Kerberos Team, CERT/CC
A complete revision history is at the end of this file.
Systems Affected
* Systems running services authenticated via Kerberos 4
* Some systems running services authenticated via Kerberos 5
* Systems running the Kerberized remote shell daemon (krshd)
* Systems with the Kerberos 5 ksu utility installed
* Systems with the Kerberos 5 v4rcp utility installed
Overview
The CERT Coordination Center has recently been notified of several
buffer overflow vulnerabilities in the Kerberos authentication
software. The most severe vulnerability allows remote intruders to
gain root privileges on systems running services using Kerberos
authentication. If vulnerable services are enabled on the Key
Distribution Center (KDC) system, the entire Kerberos domain may be
compromised.
I. Description
There are at least four distinct vulnerabilities in various
versions and implementations of the Kerberos software. All of
these vulnerabilities may be exploited to obtain root privileges.
Buffer overflow in krb_rd_req() library function
This vulnerability is present in version 4 of Kerberos. It is also
present in version 5 (in the version 4 compatibility code). This
vulnerability can be exploited in services using version 4 or 5
when they perform version 4 authentication. This vulnerability may
also be exploited locally via the v4rcp setuid root program of
Kerberos 5.
This vulnerability may be exploitable in version 4. This
vulnerability is exploitable in version 5 in conjunction with the
krb425_conv_principal() vulnerability, described below.
Buffer overflow in krb425_conv_principal() library function
This vulnerability is present in version 5's backward compatibility
code. This vulnerability is known to be exploitable in version 5 in
conjunction with an exploit of the krb_rd_req() vulnerability.
Buffer overflow in krshd
This vulnerability is only present in version 5. This
vulnerability is not related to the previous two vulnerabilities.
Buffer overflow in ksu
This vulnerability is only present in version 5, and is corrected
in krb5-1.1.1 and krb5-1.0.7-beta1. The ksu vulnerability is
unrelated to the other vulnerabilities.
The MIT Kerberos Team Advisory
The MIT Kerberos Team described these vulnerabilities in detail in
an advisory they recently issued. The text of this advisory is
included below.
|
SUMMARY
Serious buffer overrun vulnerabilities exist in many
implementations of Kerberos 4, including implementations included
for backwards compatibility in Kerberos 5 implementations. Other
less serious buffer overrun vulnerabilities have also been
discovered. ALL KNOWN KERBEROS 4 IMPLEMENTATIONS derived from MIT
sources are believed to be vulnerable.
IMPACT
* A remote user may gain unauthorized root access to a machine
running services authenticated with Kerberos 4.
* A remote user may gain unauthorized root access to a machine
running krshd, regardless of whether the program is configured to
accept Kerberos 4 authentication.
* A local user may gain unauthorized root access by exploiting v4rcp
or ksu.
DETAILS
The MIT Kerberos Team has been made aware of a security
vulnerability in the Kerberos 4 compatibility code contained
within the MIT Kerberos 5 source distributions. This vulnerability
consists of a buffer overrun in the krb_rd_req() function, which
is used by essentially all Kerberos-authenticated services that
use Kerberos 4 for authentication. It is possible for an attacker
to gain root access over the network by exploiting this
vulnerability.
An exploit is known to exist for the Kerberized Berkeley remote
shell daemon (krshd) for at least the i386-Linux platform, and
possibly others. The extent of distribution of this exploit is
unknown at this time.
Other buffer overruns have been discovered as well, though with
less far-reaching impact.
The existing exploit does not directly use the buffer overrun in
krb_rd_req(); rather, it uses the buffer that was overrun by
krb_rd_req() to exploit a second overrun in
krb425_conv_principal(). The krb_rd_req() code itself might not
be exploitable once the overrun in krb425_conv_principal() is
repaired, though it is likely that some other method of exploit
may be found that does not require that an overrun exist in
krb425_conv_principal().
VULNERABLE DISTRIBUTIONS AND PROGRAMS
Source distributions which may contain vulnerable code include:
* MIT Kerberos 5 releases krb5-1.0.x, krb5-1.1, krb5-1.1.1
* MIT Kerberos 4 patch 10, and likely earlier releases as well
* KerbNet (Cygnus implementation of Kerberos 5)
* Cygnus Network Security (CNS -- Cygnus implementation of Kerberos
4)
Daemons or services that may call krb_rd_req() and are thus
vulnerable to remote exploit include:
krshd
klogind (if accepting Kerberos 4 authentication)
telnetd (if accepting Kerberos 4 authentication)
ftpd (if accepting Kerberos 4 authentication)
rkinitd
kpopd
In addition, it is possible that the v4rcp program, which is usually
installed setuid to root, may be exploited by a local user to gain
root access by means of exploiting the krb_rd_req vulnerability.
The ksu program in some MIT Kerberos 5 releases has a
vulnerability that may result in unauthorized local root
access. This bug was fixed in krb5-1.1.1, as well as in
krb5-1.0.7-beta1. Release krb5-1.1, as well as krb5-1.0.6 and
earlier, are believed to be vulnerable.
There is an unrelated buffer overrun in the krshd that is
distributed with at least the MIT Kerberos 5 source
distributions. It is not known whether an exploit exists for this
buffer overrun. It is also not known whether this buffer overrun
is actually exploitable.
WORKAROUNDS
Certain daemons that are called from inetd may be safe from
exploitation if their command line invocation is modified to
exclude the use of Kerberos 4 for authentication. Please consult
the manpages or other documentation for your Kerberos distribution
in order to determine the correct command line for disabling
Kerberos 4 authentication. Daemons for which this approach may
work include:
krshd (*)
klogind
telnetd
(*) The krshd program may still be vulnerable to remote attack if
Kerberos 4 authentication is disabled, due to the unrelated buffer
overrun mentioned above. It is best to disable the krshd program
completely until a patched version can be installed.
The v4rcp program should have its setuid permission removed, since
it may be possible to perform a local exploit against it.
The krb5 ksu program should have its setuid permission removed, if
it was not compiled from krb5-1.1.1, krb5-1.0.7-beta1, or later
code. Merely replacing the ksu binary with one compiled from
krb5-1.1.1 or krb5-1.0.7-beta1 should be safe, provided that it is
not compiled with shared libraries (the vulnerability is related
to some library bugs). If ksu was compiled with shared libraries,
it may be best to install a new release that has the library bug
fixed.
In the MIT Kerberos 5 releases, it may not be possible to disable
Kerberos 4 authentication in the ftpd program. Note that only
releases krb5-1.1 and later will have the ability to receive
Kerberos 4 authentication.
FIXES
The best course of action is to patch the code in the krb4
library, in addition to patching the code in the krshd
program. The following patches include some less essential patches
that also affect buffer overruns in potentially vulnerable code,
but for which exploits are somewhat more difficult to construct.
Please note that there are two sets of patches in this file that
apply against identically named files in two different
releases. You should separate out the patch set that is relevant
to you prior to applying them; otherwise, you may inadvertently
patch some files twice.
MIT will soon release krb5-1.2, which will have these changes
incorporated.
PATCHES AGAINST krb5-1.0.x
The following are patches against 1.0.7-beta1 (roughly). The most
critical ones are:
appl/bsd/krshd.c
lib/krb4/rd_req.c
lib/krb5/krb/conv_princ.c
The rest are not as important but you may wish to apply them
anyway out of paranoia. These patches may apply with a little bit
of fuzz against releases prior to krb5-1.0.7-beta1, but there
likely have not been significant changes in the affected
code. These patches may also apply against KerbNet. The
lib/krb4/rd_req.c patch may also apply against CNS and MIT
Kerberos 4.
[Patches to correct this issue in Kerberos version 5-1.0.x were
included at this point in the MIT advisory. The CERT Coordination
Center has made these patches available at the following link:
http://www.cert.org/advisories/CA-2000-06/mit_10x_patch.txt
-- CERT/CC]
|
PATCHES AGAINST krb5-1.1.1
The following are patches against 1.1.1. The most critical ones
are:
appl/bsd/krshd.c
lib/krb4/rd_req.c
lib/krb5/krb/conv_princ.c
IMPORTANT NOTE: If you are upgrading to krb5-1.1.1 (or krb5-1.1,
but we recommend krb5-1.1.1 if you are going to upgrade at all)
and compile the source tree with the --without-krb4 option, then
you will also want to install the patch to login.c that is also
provided below.
The rest are not as important but you may wish to apply them anyway
out of paranoia.
[Patches to correct this issue in Kerberos version 5-1.1.1 were
included at this point in the MIT advisory. The CERT Coordination
Center has made these patches available at the following link:
http://www.cert.org/advisories/CA-2000-06/mit_111_patch.txt
-- CERT/CC]
|
ACKNOWLEDGMENTS
Thanks to Jim Paris (MIT class of 2003) for pointing out the
krb_rd_req() vulnerability.
Thanks to Nalin Dahyabhai of Redhat for pointing out some other
buffer overruns and coming up with patches.
The full text of the MIT Kerberos Team advisory is also available
from:
http://web.mit.edu/kerberos/www/advisories/krb4buf.txt
II. Impact
The most significant impact of these vulnerabilities may allow a
remote intruder to gain root access to systems running vulnerable
services, including the KDC for the domain.
Buffer overflow in krb_rd_req() library function
This vulnerability may be exploited by remote users to gain root
privileges on systems running services linked against the
vulnerable library. As MIT indicated, these services include (but
may not be limited to):
krshd
klogind (if accepting Kerberos 4 authentication)
telnetd (if accepting Kerberos 4 authentication)
ftpd (if accepting Kerberos 4 authentication)
rkinitd
kpopd
Local users can execute arbitrary code as root on systems where
v4rcp is installed setuid root.
Buffer overflow in krb425_conv_principal() library function
This vulnerability can be exploited by remote users in conjunction
with the krb_rd_req vulnerability to gain root privileges on
systems running services linked against the vulnerable library.
Buffer overflow in krshd
Remote users may be able to execute arbitrary code as root on
systems running a vulnerable version of krshd.
Buffer overflow in ksu
Local users can can gain root privileges by exploiting the buffer
overflow in ksu.
III. Solution
Apply a patch from your vendor
Appendix A contains information provided by vendors for this
advisory. We will update the appendix as we receive more
information. If you do not see your vendor's name, the CERT/CC did
not hear from that vendor. Please contact your vendor directly.
Apply the MIT patches
If you are running the Kerberos 5 distribution from MIT, and can
rebuild your binaries from source, you can apply the source code
patches from MIT to correct these problems.
If you are running Kerberos version 4, you may be able to patch
your source code based on the version 5 patch provided by
MIT. Only the patches for the krb_rd_req() vulnerability need to
be applied to version 4 to address the issues described in this
advisory.
With either version, you will need to recompile the libraries and
the vulnerable programs (krshd and ksu). You will also need to
recompile any programs that have been statically linked with the
vulnerable libraries. In version 4, you should also recompile the
KDC server software.
These patches are available at:
http://www.cert.org/advisories/CA-2000-06/mit_10x_patch.txt
http://www.cert.org/advisories/CA-2000-06/mit_111_patch.txt
Disable version 4 authentication in version 5 if possible
As suggested by MIT, version 4 authentication in some daemons can
be disabled at run time by supplying command line options to these
programs when started by inetd. This approach may work for the
following daemons:
krshd
klogind
telnetd
This addresses the krb_rd_req() and krb425_conv_principal()
vulnerabilities. Note that krshd may still be vulnerable to the
krshd specific vulnerability described in this document.
Upgrade to MIT Kerberos 5 version 1.2
The vulnerabilities described in this advisory will be addressed in
Kerberos 5 version 1.2. This version will be available from the MIT
Kerberos web site:
http://web.mit.edu/kerberos/www/
Appendix A. Vendor Information
Microsoft Corporation
No Microsoft products are affected by this vulnerability.
MIT Kerberos
The MIT Kerberos Team advisory on this topic is available from:
http://web.mit.edu/kerberos/www/advisories/krb4buf.txt
NetBSD
NetBSD has two codebases for crypto software, a legacy of the US's
export laws until recently (and also some patent issues).
The crypto-intl tree intended for use by those outside the US was
not affected.
For the crypto-us tree,
* krb5 was not affected
* krb4 was affected, and has been fixed in NetBSD-current since
Jeff's announcement; this fix is making it's way into the 1.4.x
release branch. We will release an advisory and patches shortly.
In summary, users of NetBSD releases 1.4.2 and earlier or -current
up until yesterday, who have installed the crypto-us "secr" set
and who have enabled kerberos4, are vulnerable.
OpenBSD
OpenBSD uses the KTH Kerberos distribution, which has been
reported to be not vulnerable.
Washington University
We do not distribute any "default" binaries which uses Kerberos. In
order to get Kerberos support, you must rebuild the software
specifically to use Kerberos (the default build will not use
Kerberos).
We believe that the University of Washington IMAP and POP3 servers
are not vulnerable. The message from MIT specifically stated that
the problem was in the Kerberos 4 routines from MIT.
Kerberos support in these servers is based upon Kerberos 5, not
Kerberos 4. UW imapd/ipop3d only uses GSSAPI and Kerberos 5 calls;
Kerberos 4 routines are never called.
There is an unsupported, contributed code, module for Kerberos 4
available in our software, but that is client only. We are not aware
of the existence of any Kerberos 4 server code for UW imapd/ipop3d.
_________________________________________________________________
The CERT Coordination Center thanks Jeff Schiller and the MIT Kerberos
Team for notifying us about this problem and their help in developing
this advisory.
_________________________________________________________________
Cory Cohen and Jeff Havrilla were the primary authors of the
CERT/CC portions of this document.
______________________________________________________________________
This document is available from:
http://www.cert.org/advisories/CA-2000-06.html
______________________________________________________________________
CERT/CC Contact Information
Email: cert@cert.org
Phone: +1 412-268-7090 (24-hour hotline)
Fax: +1 412-268-6989
Postal address:
CERT Coordination Center
Software Engineering Institute
Carnegie Mellon University
Pittsburgh PA 15213-3890
U.S.A.
CERT personnel answer the hotline 08:00-20:00 EST(GMT-5) /
EDT(GMT-4) Monday through Friday; they are on call for emergencies
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If you prefer to use DES, please call the CERT hotline for more
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______________________________________________________________________
NO WARRANTY
Any material furnished by Carnegie Mellon University and the Software
Engineering Institute is furnished on an "as is" basis. Carnegie
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_________________________________________________________________
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Copyright 2000 Carnegie Mellon University, portions copyright MIT
University.
Revision History
May 17, 2000: Initial release
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