AUSCERT External Security Bulletin Redistribution

                  ESB-2002.718 -- Cisco Security Advisory
                   SSH Malformed Packet Vulnerabilities
                             20 December 2002


        AusCERT Security Bulletin Summary

Product:                Catalyst Switches running Cisco CatOS
                        VPN3000 series concentrators
                        PIX Firewall
                        Secure Intrusion Detection System (NetRanger)
                        Secure Intrusion Detection System Catalyst Module
                        SN5400 Series Storage Routers
Vendor:                 Cisco Systems
Impact:                 Denial of Service
Access Required:        Remote

Ref:                    ESB-2002.705

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Hash: SHA1

Cisco Security Advisory: SSH Malformed Packet Vulnerabilities

Revision 1.0: INTERIM

For Public Release 2002 December 19th 23:00 GMT

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Please provide your feedback on this document.

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Affected Products
Software Versions and Fixes
Obtaining Fixed Software
Exploitation and Public Announcements
Status of This Notice
Revision History
Cisco Security Procedures

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Certain Cisco products containing support for the Secure Shell (SSH) server are
vulnerable to a Denial of Service (DoS) if the SSH server is enabled on the
device. A malformed SSH packet directed at the affected device can cause a
reload of the device. No authentication is necessary for the packet to be
received by the affected device. The SSH server in Cisco IOS is disabled by

Cisco will be making free software available to correct the problem as soon as

The malformed packets can be generated using the SSHredder test suite from
Rapid7, Inc. Workarounds are available. The Cisco PSIRT is not aware of any
malicious exploitation of this vulnerability.

This advisory is available at 

Affected Products

Multiple Cisco products which contain support for an SSH server are vulnerable
if the SSH server is enabled. Cisco routers and Catalyst switches running the
affected versions of IOS shown in the Software Version and Fixes section below
have been confirmed to be vulnerable.

Cisco products which contain SSH server functionality that are confirmed not to
be vulnerable include:

  * Cisco Catalyst Switches running Cisco CatOS
  * Cisco VPN3000 series concentrators
  * Cisco PIX Firewall
  * Cisco Secure Intrusion Detection System (NetRanger) appliance
  * Cisco Secure Intrusion Detection System Catalyst Module
  * Cisco SN5400 Series Storage Routers


A suite of crafted packets has been developed to test implementations of the
Secure Shell (SSH) protocol. If the SSH server has been enabled, several of the
test cases cause a forced reload of the device before the authentication
process is called. Each time an SSH connection attempt is made to a Cisco
device running IOS with one of the crafted packets, and the SSH server is
enabled on the device, the device reboots.

The SSH server feature is available in the following Cisco IOS release trains:
12.0S, 12.0ST, 12.1T, 12.1E, 12.2, 12.2T, 12.2S. All releases which have the
SSH server feature are vulnerable when the SSH server is enabled by issuing the
command "crypto key generate rsa" in configuration mode.

All products running vulnerable versions of Cisco IOS except the Cisco 3550
will automatically reload and resume service following the crash. The Cisco
3550 will not reload, and will require manual intervention to resume normal

This Cisco IOS defect is documented in DDTS CSCdz60229.


The vulnerability can be exploited to make an affected product unavailable for
several minutes while the device reloads. Once it has resumed normal
processing, the device is still vulnerable and can be forced to reload

Software Versions and Fixes

The SSH server feature is available beginning in the following Cisco IOS
releases: 12.0(5)S, 12.0(16)ST, 12.1(1)T, 12.1(5a)E, 12.2(1), 12.2(1)T,
12.2(1)S. All of these versions are vulnerable if the SSH feature is enabled.

Each row of the Cisco IOS software table (below) describes a release train and
the platforms or products for which it is intended. If a given release train is
vulnerable, then the earliest possible releases that contain the fix (the
"First Fixed Release") and the anticipated date of availability for each are
listed in the "Rebuild," "Interim," and "Maintenance" columns. A device running
a release in the given train that is earlier than the release in a specific
column (less than the First Fixed Release) is known to be vulnerable. The
release should be upgraded at least to the indicated release or a later version
(greater than or equal to the First Fixed Release label). When selecting a
release, keep in mind the following definitions:

        Most heavily tested, stable, and highly recommended release of a
        release train in any given row of the table.
        Constructed from the previous maintenance or major release in the same
        train, it contains the fix for a specific defect. Although it receives
        less testing, it contains only the minimal changes necessary to repair
        the vulnerability.
        Built at regular intervals between maintenance releases and receives
        less testing. Interims should be selected only if there is no other
        suitable release that addresses the vulnerability. Interim images
        should be upgraded to the next available maintenance release as soon as
        possible. Interim releases are not available through manufacturing, and
        usually they are not available for customer download from CCO without
        prior arrangement with the Cisco TAC.
In all cases, customers should exercise caution to confirm that the devices to
be upgraded contain sufficient memory and that current hardware and software
configurations will continue to be supported properly by the new software
release. If the information is not clear, contact the Cisco TAC for assistance
as shown in the "Obtaining Fixed Software" section below.

More information on Cisco IOS software release names and abbreviations is
available at http://www.cisco.com/warp/public/620/1.html.

The fixes will be available at the Software Center located at 

For software installation and upgrade procedures, see 

For a current view of all posted and repaired images for IOS, please check the
listing available to registered CCO users at:

|  Train  |                             |     Availability of First Fixed     |
|   or    |   Description or Platform   |              Releases*              |
| Release |                             |                                     |
|             12.0 Releases             |   Rebuild   | Interim | Maintenance |
|                                       |             |   **    |             |
|         |                             | 12.0(21)S6  |         |             |
|         |                             |-------------|         |             |
|         |                             | 2003-Jan-27 |         |             |
|         |                             |-------------+---------+-------------|
|         | Core/ISP support: GSR, RSP, | 12.0(22)S4  |         |             |
|12.0S    |c7200                        |-------------|         |             |
|         |                             | 2003-Mar    |         |             |
|         |                             |-------------+---------+-------------|
|         |                             | 12.0(23)S2  |         |             |
|         |                             |-------------|         |             |
|         |                             | 2003-Feb-17 |         |             |
|         |                             | 12.0(20)ST7 |         |             |
|         |                             |-------------|         |             |
|         |                             | 2003-Jan-27 |         |             |
|12.0ST   |Early Deployment release     |-------------+---------+-------------|
|         |                             | 12.0(21)ST6 |         |             |
|         |                             |-------------|         |             |
|         |                             | 2003-Feb-03 |         |             |
|             12.1 Releases             |   Rebuild   | Interim | Maintenance |
|                                       |             |   **    |             |
|         |                             | 12.1(13)E3  |         |             |
|         |                             |-------------|         |             |
|         |                             | 2003-Jan-06 |         |             |
|12.1E    |Early Deployment release     |-------------+---------+-------------|
|         |                             | 12.1(14)E1  |         |             |
|         |                             |-------------|         |             |
|         |                             | 2003-Feb    |         |             |
|         |                             |             Vulnerable              |
|12.1EA   |Early Deployment release     |-------------------------------------|
|         |                             |          Schedule unknown           |
|         | Early deployment release    |             Vulnerable              |
|12.1T    |all major platforms          |-------------------------------------|
|         |                             |             Not Planned             |
|             12.2 Releases             |   Rebuild   | Interim | Maintenance |
|                                       |             |   **    |             |
|         |                             | 12.2(12b)   |         |             |
|         |                             |-------------|         |             |
|         | Major release for all       | 2003-Jan-06 |         |             |
|12.2     |platforms                    |-------------+---------+-------------|
|         |                             | 12.2(13a)   |         |             |
|         |                             |-------------|         |             |
|         |                             | 2003-Feb-07 |         |             |
|         |                             |             |         | 12.2(14)S   |
|12.2S    |Core ISP support             |             |         |-------------|
|         |                             |             |         | 2003-Jan-27 |
|         |                             | 12.2(11)T3  |         |             |
|         |                             |-------------|         |             |
|         | Early deployment release    | 2003-Jan-06 |         |             |
|12.2T    |all major platforms          |-------------+---------+-------------|
|         |                             | 12.2(13)T1  |         |             |
|         |                             |-------------|         |             |
|         |                             | 2003-Feb-03 |         |             |
| NOTES:                                                                      |
| * All dates are tentative and subject to change.                            |
| ** Interim releases receive the less testing than Maintenance or Rebuild    |
| releases. Interim release labels are provided to identify vulnerable        |
| pre-existing Interim releases. A first fixed Interim release should be used |
| only when no other suitable release is available.                           |

Obtaining Fixed Software

Customers with contracts should obtain upgraded software through their regular
update channels. For most customers, this means that upgrades should be
obtained through the Software Center on Cisco's worldwide website at 

Customers whose Cisco products are provided or maintained through prior or
existing agreement with third-party support organizations such as Cisco
Partners, authorized resellers, or service providers should contact that
support organization for assistance with obtaining the free software upgrade

Customers who purchase direct from Cisco but who do not hold a Cisco service
contract and customers who purchase through third-party vendors but are
unsuccessful at obtaining fixed software through their point of sale should get
their upgrades by contacting the Cisco Technical Assistance Center (TAC). TAC
contacts are as follows:

  * +1 800 553 2447 (toll-free from within North America)
  * +1 408 526 7209 (toll call from anywhere in the world)
  * email: tac@cisco.com .

See http://www.cisco.com/warp/public/687/Directory/DirTAC.shtml for additional
TAC contact information, including special localized telephone numbers and
instructions and e-mail addresses for use in various languages.

Please have your product serial number available and give the URL of this
advisory as evidence of your entitlement to a free upgrade. Free upgrades for
non-contract customers must be requested through the TAC.

Please do not contact either "psirt@cisco.com" or "security-alert@cisco.com"
for software upgrades.


Workarounds consist of disabling the SSH server, removing SSH as a remote
access method, permitting only trusted hosts to connect to the server, and
blocking SSH traffic to the device completely via external mechanisms.

Caution: The following workaround will have undesirable side effects for IPSEC
sessions that terminate on the device that use RSA key pairs for device
authentication, or that use certificates based on those RSA key pairs. IPSEC
sessions using other authentication methods will not be affected.

    For Cisco IOS the SSH server can be disabled by applying the command 
    "crypto key zeroize rsa" while in configuration mode. The SSH server is
    enabled automatically upon generating an RSA key pair. Zeroing the RSA keys
    is the only way to completely disable the SSH server.

Access to the SSH server on Cisco IOS may also be disabled via removing SSH as
a valid transport protocol. This can be done by reapplying the "transport
input" command with 'ssh' removed from the list of permitted transports on VTY
lines while in configuration mode. For example:

line vty 0 4                                                                    
  transport input telnet                                                        

If SSH server functionality is desired, access to the server can be restricted
to specific source IP addresses or blocked entirely through the use of Access
Control Lists (ACLs) on the VTY lines as shown in the following URL:

More information on configuring ACLs can be found on Cisco's public website:

An example of a VTY access-list can be found here:

access-list 2 permit
access-list 2 deny any

line vty 0 4
access-class 2 in

You may also block inbound SSH connections for your device with an external
packet filtering device such as a firewall or a router that blocks traffic to
TCP port 22.

Exploitation and Public Announcements

Cisco PSIRT is not aware of any malicious exploitation of this vulnerability.
This suite of crafted packets from Rapid7, Inc. has been publically announced
via CERT/CC advisory CA-2002-36, and is available from the researcher's
website. Cisco was initially listed in the CERT/CC advisory as not vulnerable
based on initial testing of the suite, however upon continued internal testing
it was determined that some products were vulnerable.

Status of This Notice: INTERIM

This is an interim Security Advisory notice. Cisco anticipates issuing
updated versions of this notice at weekly intervals as there are material
changes in the facts, and will continue to update this notice as necessary.

The reader is warned that this notice may contain inaccurate or incomplete
information. Although Cisco cannot guarantee the accuracy of all statements
in this notice, all of the facts have been checked to the best of our
ability. Cisco anticipates updates of this notice until it reaches final


This notice will be posted on Cisco's worldwide website at 
In addition to worldwide web posting, a text version of this notice is
clear-signed with the Cisco PSIRT PGP key and is posted to the following
e-mail and Usenet news recipients: 

  * cust-security-announce@cisco.com
  * bugtraq@securityfocus.com
  * full-disclosure@lists.netsys.com
  * first-teams@first.org (includes CERT/CC)
  * cisco@spot.colorado.edu
  * cisco-nsp@puck.nether.net
  * comp.dcom.sys.cisco
  * Various internal Cisco mailing lists

Future updates of this notice, if any, will be placed on Cisco's worldwide web
Users concerned about this problem are encouraged to check the URL given above
for any updates.

Revision History

|Revision   |Thu Dec 19 23:00: |Initial Public Release                    |
|Number 1.0 |00 GMT 2002       |                                          |

Cisco Security Procedures

Complete information on reporting security vulnerabilities in Cisco products,
obtaining assistance with security incidents, and registering to receive
security information from Cisco, is available on Cisco's worldwide website at 
http://www.cisco.com/warp/public/707/sec_incident_response.shtml. This includes
instructions for press inquiries regarding Cisco security notices. All Cisco
Security Advisories are available at http://www.cisco.com/go/psirt/.

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This notice is Copyright 2002 by Cisco Systems, Inc. This notice may be
redistributed freely after the release date given at the top of the text,
provided that redistributed copies are complete and unmodified, and include all
date and version information.

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All contents are Copyright © 1992--2002 Cisco Systems, Inc. All rights
reserved. Important Notices and Privacy Statement.

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