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Subject: CERT Advisory CA-2002-36 Multiple Vulnerabilities in SSH Implementations
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CERT Advisory CA-2002-36 Multiple Vulnerabilities in SSH Implementations

Original issue date: December 16, 2002
Last revised: --
Source: CERT/CC

A complete revision history is at the end of this file.

Systems Affected

* Secure shell (SSH) protocol implementations in SSH clients and
servers from multiple vendors


Multiple vendors' implementations of the secure shell (SSH) transport
layer protocol contain vulnerabilities that could allow a remote
attacker to execute arbitrary code with the privileges of the SSH
process or cause a denial of service. The vulnerabilities affect SSH
clients and servers, and they occur before user authentication takes

I. Description

The SSH protocol enables a secure communications channel from a client
to a server. From the IETF draft SSH Transport Layer Protocol:

The SSH transport layer is a secure low level transport protocol.
It provides strong encryption, cryptographic host authentication,
and integrity protection.... Key exchange method, public key
algorithm, symmetric encryption algorithm, message authentication
algorithm, and hash algorithm are all negotiated.

Rapid7 has developed a suite (SSHredder) of test cases that examine
the connection initialization, key exchange, and negotiation phase
(KEX, KEXINIT) of the SSH transport layer protocol. The suite tests
the way an SSH transport layer implementation handles invalid or
incorrect packet and string lengths, padding and padding length,
malformed strings, and invalid algorithms.

The test suite has demonstrated a number of vulnerabilities in
different vendors' SSH products. These vulnerabilities include buffer
overflows, and they occur before any user authentication takes place.
SSHredder was primarily designed to test key exchange and other
processes that are specific to version 2 of the SSH protocol; however,
certain classes of tests are also applicable to version 1.

Further information about this set of vulnerabilities may be found in
Vulnerability Note VU#389665.

Rapid7 has published a detailed advisory (R7-0009) and the SSHredder
test suite.

Common Vulnerabilities and Exposures (CVE) has assigned the following
candidate numbers for several classes of tests performed by SSHredder:

* CAN-2002-1357 - incorrect field lengths
* CAN-2002-1358 - lists with empty elements or multiple separators
* CAN-2002-1359 - "classic" buffer overflows
* CAN-2002-1360 - null characters in strings

II. Impact

The impact will vary for different vulnerabilities and products, but
in severe cases, remote attackers could execute arbitrary code with
the privileges of the SSH process. Both SSH servers and clients are
affected, since both implement the SSH transport layer protocol. On
Microsoft Windows systems, SSH servers commonly run with SYSTEM
privileges, and on UNIX systems, SSH daemons typically run with root
privileges. In the case of SSH clients, any attacker-supplied code
would run with the privileges of the user who started the client
program, with the possible exception of SSH clients that may be
configured with an effective user ID of root (setuid root). Attackers
could also crash a vulnerable SSH process, causing a denial of

III. Solution

Apply a patch or upgrade

Apply the appropriate patch or upgrade as specified by your vendor.
See Appendix A below and the Systems Affected section of VU#389665 for
specific information.

Restrict access

Limit access to SSH servers to trusted hosts and networks using
firewalls or other packet-filtering systems. Some SSH servers may have
the ability to restrict access based on IP addresses, or similar
effects may be achieved by using TCP wrappers or other related

SSH clients can reduce the risk of attacks by only connecting to
trusted servers by IP address.

While these workarounds will not prevent exploitation of these
vulnerabilities, they will make attacks somewhat more difficult, in
part by limiting the number of potential sources of attacks.

Appendix A. Vendor Information

This appendix contains information provided by vendors. When vendors
report new information, this section is updated and the changes are
noted in the revision history. If a vendor is not listed below, we
have not received their comments. The Systems Affected section of
VU#389665 contains additional vendor status information.

Cisco Systems, Inc.

The official statement regarding this is that we are not

Cray Inc.

Cray Inc. supports the OpenSSH product through their Cray Open
Software (COS) package. COS 3.3, available the end of December
2002, is not vulnerable. If a site is concerned, they can contact
their local Cray representive to obtain an early copy of the
OpenSSH contained in COS 3.3.


F-Secure SSH products are not exploitable via these attacks. While
F-Secure SSH versions 3.1.0 build 11 and earlier crash on these
malicious packets, we did not find ways to exploit this to gain
unauthorized access or to run arbitrary code. Furthermore, the
crash occurs in a forked process so the denial of service attacks
are not possible.


Fujitsu's UXP/V OS is not vulnerable because it does not support


IBM's AIX is not vulnerabible to the issues discussed in CERT
Vulnerability Note VU#389665.


I've now tried the testsuite with the latest stable release of lsh,
lsh-1.4.2. Both the client and the server seem NOT VULNERABLE.

NetScreen Technologies Inc.

Tested latest versions. Not Vulnerable.


From my testing it seems that the current version of OpenSSH (3.5)
is not vulnerable to these problems, and some limited testing shows
that no version of OpenSSH is vulnerable.

Pragma Systems, Inc.

December 16, 2002

Rapid 7 and CERT Coordination Center Vulnerability report VU#389665

Pragma Systems Inc. of Austin, Texas, USA, was notified regarding a
possible vulnerability with Version 2.0 of Pragma SecureShell.
Pragma Systems tested Pragma SecureShell 2.0 and the upcoming new
Version 3.0, and found that the attacks did cause a memory access
protection fault on Microsoft platforms.

After research, Pragma Systems corrected the problem. The
correction of the problem leads us to believe that any attack would
not cause a Denial of Service, or the ability of random code to run
on the server.

The problem is corrected in Pragma SecureShell Version 3.0. Any
customers with concerns regarding this vulnerability report should
contact Pragma Systems, Inc at support@pragmasys.com for
information on obtaining an upgrade free of charge. Pragma's web
site is located at www.pragmasys.com and the company can be reached
at 1-512-219-7270.


PuTTY 0.53b addresses vulnerabilities discovered by SSHredder.

SSH Communications Security

SSH Secure Shell products are not exploitable via these attacks.

Appendix B. References

* CERT/CC Vulnerability Note: VU#389665 -
* Rapid 7 Advisory: R7-0009 -
* Rapid 7 SSHredder test suite -
* IETF Draft: SSH Transport Layer Protocol -
* IETF Draft: SSH Protocol Architecture -
* Privilege Separated OpenSSH -

__________________________________________________ _______________

The CERT Coordination Center thanks Rapid7 for researching and
reporting these vulnerabilities.
__________________________________________________ _______________

Author: Art Manion.
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This document is available from:
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Revision History

December 16, 2002: Initial release

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