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CA-2001-35 Recent Activity Against Secure Shell Daemons

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CERT Advisory CA-2001-35 Recent Activity Against Secure Shell Daemons

Original release date: December 13, 2001
Last revised: --
Source: CERT/CC

A complete revision history can be found at the end of this file.

Systems Affected

Systems running implementations of the Secure Shell (SSH) protocol

Overview

There are multiple vulnerabilities in several implementations of the
Secure Shell (SSH) protocol. The SSH protocol enables a secure
communications channel from a client to a server. We are seeing a high
amount of scanning for SSH daemons, and we are receiving reports of
exploitation. System administrators should review their configurations
to ensure that they have applied all relevant patches prior to the
holiday break.

I. Description

There are multiple vulnerabilities in several implementations of the
Secure Shell (SSH) protocol. While these problems have been previously
disclosed, we believe many system and network administrators may have
overlooked one or more of these vulnerabilities. We are issuing this
document primarily to encourage system and network administrators to
check their systems, prior to the holiday break, for exposure to each
of these vulnerabilities. The CERT/CC is still seeing active scanning
and exploitation of vulnerabilities related to SSH.

We also believe that it is important for system administrators to
realize that several implementations of SSH version 2 will use their
implementation of SSH version 1 if it is present and requested by the
client. Therefore, upgrading to SSH version 2 is not necessarily a
sufficient means to patch vulnerabilities that are present in the SSH
version 1 implementation.

The following vulnerability note and incident note describe activity
regarding the SSH CRC32 attack detection code integer overflow
vulnerability.

VU#945216 - SSH CRC32 attack detection code contains remote integer
overflow

There is a remote integer overflow vulnerability in several
implementations of the SSH1 protocol. This vulnerability is located in
a segment of code that was introduced to defend against exploitation
of CRC32 weaknesses in the SSH1 protocol (see VU#13877). The attack
detection function (detect_attack, located in deattack.c) makes use of
a dynamically allocated hash table to store connection information
that is then examined to detect and respond to CRC32 attacks. By
sending a crafted SSH1 packet to an affected host, an attacker can
cause the SSH daemon to create a hash table with a size of zero. When
the detection function then attempts to hash values into the
null-sized hash table, these values can be used to modify the return
address of the function call, thus causing the program to execute
arbitrary code with the privileges of the SSH daemon, typically root.

IN-2001-12 - Exploitation of vulnerability in SSH1 CRC-32 compensation
attack detector

In reports received by the CERT/CC, systems compromised via this
vulnerablity have exhibited the following pattern in system log
messages:

hostname sshd[xxx]: Disconnecting: Corrupted check bytes on input.
hostname sshd[xxx]: Disconnecting: crc32 compensation attack: network attack detected
hostname sshd[xxx]: Disconnecting: crc32 compensation attack: network attack detected
..

Some exploits for this vulnerability appear to use a brute force
method, so many messages of this type may be logged before a system is
successfully compromised.

The following artifacts have been discovered on systems that were
successfully compromised:

* Installation of rootkits that modify standard system utilities to
hide the intruder's actions
* Installation of Trojan horse versions of the SSH software,
compiled from the latest OpenSSH source code plus
intruder-supplied modifications
* Installation of tools to scan large network blocks for other
systems that are vulnerable to compromise. Log files left behind
from these tools indicate that they operate by looking for the
banner displayed upon connection to the sshd service.

For a list of vulnerability notes related to SSH vulnerabilities,
please see the References section.

II. Impact

The CRC32 attack detection code integer overflow vulnerability, as
well as some of the vulnerabilities listed in the References section,
can be exploited remotely. In some cases, they allow an intruder to
execute arbitrary code with the privileges of the SSH application
daemon, usually root. In some cases, an intruder must be an authorized
user of the system.

For specific information about the impacts of each of these
vulnerabilities, please consult the CERT Vulnerability Notes Database
(http://www.kb.cert.org/vuls).

III. Solution

Update to the latest version

If possible, update your implementation of SSH to the latest release.
If you are unable to update to the latest version, apply all relevant
patches to your current version. It is also recommended that you look
at the security or support section on each vendor's site.

Note that it is important for system administrators to realize that
several implementations of SSH version 2 will use their implementation
of SSH version 1 if it is present and requested by the client.
Therefore, upgrading to SSH version 2 is not necessarily a sufficient
means to patch vulnerabilities that are present in the SSH version 1
implementation.

Current versions for Data Fellows (F-Secure) can be found at
http://www.f-secure.com/products/ssh/.

Current versions for SSH Communications Security can be found at
http://www.ssh.com/products/ssh/download.cfm.

Current versions for OpenSSH can be found at http://www.openssh.com.

Please visit your vendor's web site for the latest version.

Apply a patch from your vendor

Appendix A contains information provided by vendors for this advisory.
As vendors report new information to the CERT/CC, we will update this
section and note the changes in our revision history. If a particular
vendor is not listed below, we have not received their comments for
the advisory. Please review the CERT Vulnerability Notes Database
(http://www.kb.cert.org/vuls) or contact your vendor directly.

Restrict access to the SSH service

As a general practice, we recommend disabling all services that are
not explicitly required. You may wish to disable the SSH access if
there is not a patch available from your vendor.

If you cannot disable the service, you can limit your exposure to
these vulnerabilities by using a router or firewall to restrict access
to port 22/TCP (SSH). Use tcp wrappers or a program that provides
similar functionality, or use the key-based IP restriction offered by
your implementation. Note that this does not protect you against
attackers from within your network.

Appendix A. - Vendor Information

This appendix contains information provided by vendors for this
advisory. As vendors report new information to the CERT/CC, we will
update this section and note the changes in our revision history. If a
particular vendor is not listed below, we have not received their
comments for the advisory. Please review the CERT Vulnerability Notes
Database (http://www.kb.cert.org/vuls) or contact your vendor
directly.

Berkeley Software Design, Inc. (BSDI)

The current 3.0.2p1 version of OpenSSH is available for BSD/OS version
4.2 in patch M420-018 and for BSD/OS 4.3 in patch M430-001. Patches
are available via ftp from ftp://ftp.bsdi.com/bsdi/patches or via our
web site at http://www.bsdi.com/support.

Fujitsu

Fujitsu's UXP/V operating system is not affected by the SSH security
vulnerabilities because it does not support the SSH package.

Hewlett-Packard Company

This issue does not apply to HP-UX. HP does not ship SSH.

IBM Corporation

IBM's AIX operating system does not ship with OpenSSH; however,
OpenSSH isavailable for installation on AIX via the Linux Affinity
Toolkit. The version included on the CD containing the Toolkit is
vulnerable to the latest discovered vulnerability discussed here,
VU#157447, as was the version of OpenSSH available for downloading
from the IBM Linux Affinity website. We have updated this version on
the website to one that is not vulnerable to this security exposure.
This version also fixes the other vulnerabilities described in this
advisory. Customers can download this version by going to:

http://www6.software.ibm.com/dl/aixtbx/aixtbx-p

This site contains Linux Affinity applications containing
cryptographic algorithms, and new users of this site are asked to
register first.

NetBSD

The CRC32 attack vulnerability was patched in NetBSD-current on
October 30, 2000. NetBSD 1.5 and later already include the patch.
Users maintaining earlier revisions of NetBSD should update their
systems using the security/openssh package from NetBSD pkgsrc if they
have not already done so.

Up to date NetBSD security information on SSH, and other
vulnerabilities is available from http://www.netbsd.org/Security/

Sun Microsystems

Sun does not ship the Secure Shell (SSH), thus Solaris is not affected
by this issue.
_________________________________________________________________

The CERT Coordination Center thanks Markus Friedl of OpenSSH for the
technical assistance he provided.
_________________________________________________________________

Feedback on this document can be directed to the authors,
Jason A. Rafail and Chad Dougherty
_________________________________________________________________

References

ID Date Public Name

VU#19124 01/20/98 SSH authentication agent follows symlinks via a UNIX domain socket

VU#13877 06/11/98 Weak CRC allows packet injection into SSH sessions encrypted with block ciphers

VU#40327 06/09/2000 OpenSSH UseLogin option allows remote execution of commands as root

VU#363181 12/07/2000 OpenSSH disregards client configuration and allows server access to ssh-agent and/or X11 after session negotiation

VU#850440 01/16/2001 SSH1 may generate weak passphrase when using Secure RPC

VU#684820 01/18/2001 SSH-1 allows client authentication to be forwarded by a malicious server to another server

VU#565052 01/18/2001 Passwords sent via SSH encrypted with RC4 can be easily cracked

VU#786900 01/18/2001 SSH host key authentication can be bypassed when DNS is used to resolve localhost

VU#25309 01/18/2001 Weak CRC allows RC4 encrypted SSH1 packets to be modified without notice

VU#118892 01/18/2001 Older SSH clients do not allow users to disable X11 forwarding

VU#665372 01/18/2001 SSH connections using RC4 and password authentication can be replayed

VU#315308 01/18/2001 Weak CRC allows last block of IDEA-encrypted SSH packet to be changed without notice

VU#945216 02/08/2001 SSH CRC32 attack detection code contains remote integer overflow

VU#596827 03/19/2001 Weaknesses in the SSH protocol simplify brute-force attacks against passwords typed in an existing SSH session

VU#655259 06/12/2001 OpenSSH allows arbitrary file deletion via symlink redirection of temporary file

VU#737451 07/20/2001 SSH Secure Shell sshd2 does not adequately authenticate logins to accounts with encrypted password fields containing two or fewer characters

VU#279763 11/19/2001 RhinoSoft Serv-U remote administration client transmits password in plaintext

VU#157447 12/04/2001 OpenSSH UseLogin directive permits privilege escalation

______________________________________________________________________

This document is available from:
http://www.cert.org/advisories/CA-2001-35.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/CC personnel answer the hotline 08:00-17:00 EST(GMT-5) /
EDT(GMT-4) Monday through Friday; they are on call for emergencies
during other hours, on U.S. holidays, and on weekends.

Using encryption

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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
Mellon University makes no warranties of any kind, either expressed or
implied as to any matter including, but not limited to, warranty of
fitness for a particular purpose or merchantability, exclusivity or
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_________________________________________________________________

Conditions for use, disclaimers, and sponsorship information

Copyright 2001 Carnegie Mellon University.

Revision History
December 13, 2001: Initial release

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