Cisco Security Advisory: Cisco IOS Secure Shell Denial of Service

21 May 2008

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Cisco Security Advisory: Cisco IOS Secure Shell Denial of Service
Vulnerabilities

Advisory ID: cisco-sa-20080521-ssh

http://www.cisco.com/warp/public/707/cisco-sa-20080521-ssh.shtml

Revision 1.0

For Public Release 2008 May 21 1600 UTC (GMT)

+--------------------------------------------------------------------

Summary
=======

The Secure Shell server (SSH) implementation in Cisco IOS contains
multiple vulnerabilities that allow unauthenticated users the ability
to generate a spurious memory access error or, in certain cases,
reload the device.

The IOS SSH server is an optional service that is disabled by
default, but its use is highly recommended as a security best
practice for management of Cisco IOS devices. SSH can be configured
as part of the AutoSecure feature in the initial configuration of IOS
devices, AutoSecure run after initial configuration, or manually.
Devices that are not configured to accept SSH connections are not
affected by these vulnerabilities.

Common Vulnerabilities and Exposures (CVE) identifier CVE-2008-1159
has been assigned to this vulnerability.

This advisory is posted at 
http://www.cisco.com/warp/public/707/cisco-sa-20080521-ssh.shtm

Affected Products
=================

Vulnerable Products
+------------------

Cisco devices running certain 12.4-based IOS releases and configured
to be managed via SSH may be affected by this issue.

The IOS secure shell server is disabled by default. To determine if
SSH is enabled, use the show ip ssh command.

    Router#show ip ssh
    SSH Enabled - version 2.0
    Authentication timeout: 120 secs; Authentication retries: 3

The previous output shows that SSH is enabled on this device and that
the SSH protocol major version that is being supported is 2.0. If the
text "SSH Disabled" is displayed, the device is not vulnerable.
Possible values for the SSH protocol version reported by IOS are:

  * 1.5: only SSH protocol version 1 is enabled
  * 1.99: SSH protocol version 2 with SSH protocol version 1
    compatibility enabled
  * 2.0: only SSH protocol version 2 is enabled

For more information about SSH versions in IOS, please check the
following URL: 
http://www.cisco.com/en/US/docs/ios/12_3t/12_3t4/feature/guide/gt_ssh2.html

The SSH server is not available in all IOS images. Devices that do
not support SSH are not vulnerable. Please consult the table of fixed
software in the Software Version and Fixes section for the specific
12.4-based IOS releases that are affected.

To determine the software running on a Cisco product, log in to the
device and issue the show version command to display the system
banner. Cisco IOS software will identify itself as "Internetwork
Operating System Software" or simply "IOS". The image name will be
displayed between parentheses on the next line of output followed by
"Version" and the IOS release name. Other Cisco devices will not have
the show version command or will give different output.

The following example identifies a Cisco product running IOS release
12.4(17):

    Cisco IOS Software, C2600 Software (C2600-ADVENTERPRISEK9-M), Version 12.4(17),
    RELEASE SOFTWARE (fc1)
    Technical Support: http://www.cisco.com/techsupport
    Copyright (c) 1986-2007 by Cisco Systems, Inc.
    Compiled Fri 07-Sep-07 16:05 by prod_rel_team

    ROM: System Bootstrap, Version 12.2(8r) [cmong 8r], RELEASE SOFTWARE (fc1)

    Router uptime is 1 week, 5 hours, 5 minutes
    System returned to ROM by power-on
    System image file is "flash:c2600-adventerprisek9-mz.124-17.bin"

Additional information about Cisco IOS release naming is available at
http://www.cisco.com/warp/public/620/1.html

Products Confirmed Not Vulnerable
+--------------------------------

Cisco devices that do not run IOS are not affected.

Cisco IOS devices that do not have the SSH server feature enabled are
not affected.

IOS-XR images are not affected.

The following IOS release trains are not affected:

  * 10-based releases
  * 11-based releases
  * 12.0-based releases
  * 12.1-based releases
  * 12.2-based releases
  * 12.3-based releases

IOS releases prior to 12.4(7), 12.4(13d)JA, and 12.4(9)T are not
affected by this vulnerability.

No other Cisco products are currently known to be affected by these
vulnerabilities.

Details
=======

Secure shell (SSH) was developed as a secure replacement for the
telnet, ftp, rlogin, rsh, and rcp protocols, which allow for the
remote access of devices. The main difference between SSH and older
protocols is that SSH provides strong authentication, guarantees
confidentiality, and uses encrypted transactions.

The server side of the SSH implementation in Cisco IOS contains
multiple vulnerabilities that allow an unauthenticated user to
generate a spurious memory access or, in certain cases, reload the
device. If the attacker is able to reload the device, these
vulnerabilities could be repeatedly exploited to cause an extended
Denial of Service (DoS) condition.

A device with the SSH server enabled is vulnerable.

These vulnerabilities are documented in Cisco Bug IDs:

  * CSCsk42419 ( registered customers only)
  * CSCsk60020 ( registered customers only)
  * CSCsh51293 ( registered customers only)

Vulnerability Scoring Details
=============================

Cisco is providing scores for the vulnerabilities in this advisory
based on the Common Vulnerability Scoring System (CVSS). The CVSS
scoring in this Security Advisory is done in accordance with CVSS
version 2.0.

Cisco will provide a base and temporal score. Customers can then
compute environmental scores to assist in determining the impact of
the vulnerability in individual networks.

CVSS is a standards-based scoring method that conveys vulnerability
severity and helps determine urgency and priority of response.

Cisco has provided an FAQ to answer additional questions regarding
CVSS at

http://www.cisco.com/web/about/security/intelligence/cvss-qandas.html

Cisco has also provided a CVSS calculator to help compute the
environmental impact for individual networks at

http://intellishield.cisco.com/security/alertmanager/cvss

* CSCsk42419 - SSHv2 spurious memory access

CVSS Base Score - 7.8
    Access Vector -             Network
    Access Complexity -         Low
    Authentication -            None
    Confidentiality Impact -    None
    Integrity Impact -          None
    Availability Impact -       Complete

CVSS Temporal Score - 6.4
    Exploitability -            Functional
    Remediation Level -         Official-Fix
    Report Confidence -         Confirmed

* CSCsk60020 - SSHv2 spurious memory access

CVSS Base Score - 7.8
    Access Vector -             Network
    Access Complexity -         Low
    Authentication -            None
    Confidentiality Impact -    None
    Integrity Impact -          None
    Availability Impact -       Complete

CVSS Temporal Score - 6.4
    Exploitability -            Functional
    Remediation Level -         Official-Fix
    Report Confidence -         Confirmed

* CSCsh51293 - Spurious memory access when SSH packets received

CVSS Base Score - 7.8
    Access Vector -             Network
    Access Complexity -         Low
    Authentication -            None
    Confidentiality Impact -    None
    Integrity Impact -          None
    Availability Impact -       Complete

CVSS Temporal Score - 6.4
    Exploitability -            Functional
    Remediation Level -         Official-Fix
    Report Confidence -         Confirmed

Impact
======

Successful exploitation of these vulnerabilities may result in a
spurious memory access or, in certain cases, reload the device
potentially resulting in a DoS condition.

Software Versions and Fixes
===========================

When considering software upgrades, also consult 
http://www.cisco.com/go/psirt and any subsequent advisories to 
determine exposure and a complete upgrade solution.

In all cases, customers should exercise caution to be certain the
devices to be upgraded contain sufficient memory and that current
hardware and software configurations will continue to be supported
properly by the new release. If the information is not clear, contact
the Cisco Technical Assistance Center ("TAC") or your contracted
maintenance provider for assistance.

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"
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).

For more information on the terms "Rebuild" and "Maintenance,"
consult the following URL: 
http://www.cisco.com/warp/public/620/1.html

IOS releases prior to 12.4(7), 12.4(13d)JA, and 12.4(9)T are not
affected by this vulnerability.

+----------------------------------------+
|   Major    | Availability of Repaired  |
|  Release   |         Releases          |
|------------+---------------------------|
|  Affected  | First Fixed | Recommended |
| 12.0-Based |   Release   |   Release   |
|  Releases  |             |             |
|----------------------------------------|
| There are no affected 12.0 based       |
| releases                               |
|----------------------------------------|
|  Affected  | First Fixed | Recommended |
| 12.1-Based |   Release   |   Release   |
|  Releases  |             |             |
|----------------------------------------|
| There are no affected 12.1 based       |
| releases                               |
|----------------------------------------|
|  Affected  | First Fixed | Recommended |
| 12.2-Based |   Release   |   Release   |
|  Releases  |             |             |
|----------------------------------------|
| There are no affected 12.2 based       |
| releases                               |
|----------------------------------------|
|  Affected  | First Fixed | Recommended |
| 12.3-Based |   Release   |   Release   |
|  Releases  |             |             |
|----------------------------------------|
| There are no affected 12.3 based       |
| releases                               |
|----------------------------------------|
|  Affected  | First Fixed | Recommended |
| 12.4-Based |   Release   |   Release   |
|  Releases  |             |             |
|------------+-------------+-------------|
|            | 12.4(13f)   |             |
|            |             |             |
|            | 12.4(16b)   |             |
| 12.4       |             | 12.4(18b)   |
|            | 12.4(17a)   |             |
|            |             |             |
|            | 12.4(18)    |             |
|------------+-------------+-------------|
|            | Only 12.4   |             |
|            | (13d)JA and |             |
|            | 12.4(13d)   |             |
|            | JA1 are     |             |
| 12.4JA     | vulnerable, | 12.4(16b)   |
|            | all other   | JA3         |
|            | 12.4JA      |             |
|            | releases    |             |
|            | are not     |             |
|            | affected.   |             |
|------------+-------------+-------------|
| 12.4JK     | Not         |             |
|            | Vulnerable  |             |
|------------+-------------+-------------|
| 12.4JMA    | Not         |             |
|            | Vulnerable  |             |
|------------+-------------+-------------|
| 12.4JMB    | Not         |             |
|            | Vulnerable  |             |
|------------+-------------+-------------|
| 12.4JMC    | Not         |             |
|            | Vulnerable  |             |
|------------+-------------+-------------|
| 12.4JX     | Not         |             |
|            | Vulnerable  |             |
|------------+-------------+-------------|
| 12.4MD     | Not         |             |
|            | Vulnerable  |             |
|------------+-------------+-------------|
| 12.4MR     | 12.4(16)MR2 | 12.4(16)MR  |
|------------+-------------+-------------|
| 12.4SW     | 12.4(15)SW1 | 12.4(15)SW1 |
|------------+-------------+-------------|
|            | 12.4(9)T6   |             |
|            |             |             |
|            | 12.4(11)T4  |             |
| 12.4T      |             | 12.4(15)T5  |
|            | 12.4(15)T2  |             |
|            |             |             |
|            | 12.4(20)T   |             |
|------------+-------------+-------------|
| 12.4XA     | Not         |             |
|            | Vulnerable  |             |
|------------+-------------+-------------|
| 12.4XB     | Not         |             |
|            | Vulnerable  |             |
|------------+-------------+-------------|
| 12.4XC     | Not         |             |
|            | Vulnerable  |             |
|------------+-------------+-------------|
| 12.4XD     | Not         |             |
|            | Vulnerable  |             |
|------------+-------------+-------------|
|            | Vulnerable; |             |
| 12.4XE     | first fixed | 12.4(15)T5  |
|            | in 12.4T    |             |
|------------+-------------+-------------|
|            | Vulnerable; |             |
| 12.4XF     | first fixed | 12.4(15)T5  |
|            | in 12.4T    |             |
|------------+-------------+-------------|
| 12.4XG     | Not         |             |
|            | Vulnerable  |             |
|------------+-------------+-------------|
|            | Vulnerable; |             |
| 12.4XJ     | first fixed | 12.4(15)T5  |
|            | in 12.4T    |             |
|------------+-------------+-------------|
|            | Vulnerable; |             |
| 12.4XK     | first fixed | 12.4(15)T5  |
|            | in 12.4T    |             |
|------------+-------------+-------------|
| 12.4XL     | Not         |             |
|            | Vulnerable  |             |
|------------+-------------+-------------|
| 12.4XM     | Not         |             |
|            | Vulnerable  |             |
|------------+-------------+-------------|
| 12.4XN     | Not         |             |
|            | Vulnerable  |             |
|------------+-------------+-------------|
| 12.4XQ     | Not         |             |
|            | Vulnerable  |             |
|------------+-------------+-------------|
| 12.4XT     | Not         |             |
|            | Vulnerable  |             |
|------------+-------------+-------------|
| 12.4XV     | Vulnerable; |             |
|            | contact TAC |             |
|------------+-------------+-------------|
| 12.4XW     | 12.4(11)XW6 | 12.4(11)XW6 |
|------------+-------------+-------------|
| 12.4XY     | Not         |             |
|            | Vulnerable  |             |
|------------+-------------+-------------|
| 12.4XZ     | Not         |             |
|            | Vulnerable  |             |
+----------------------------------------+

Workarounds
===========

If disabling the IOS SSH Server is not feasible, the following
workarounds may be useful to some customers in their environments.

Telnet
+-----

Telnet is not vulnerable to the issue described in this advisory and
may be used as an insecure alternative to SSH. Telnet does not
encrypt the authentication information or data; therefore, it should
only be enabled for trusted local networks.

VTY Access Class
+---------------

It is possible to limit the exposure of the Cisco device by applying
a VTY access class to allow only known, trusted hosts to connect to
the device via SSH.

For more information on restricting traffic to VTYs, please consult: 
http://cisco.com/en/US/products/sw/iosswrel/ps1835/products_command_referenc...

The following example permits access to VTYs from the 192.168.1.0/24
netblock and the single IP address 172.16.1.2 while denying access
from anywhere else:

    Router(config)# access-list 1 permit 192.168.1.0 0.0.0.255
    Router(config)# access-list 1 permit host 172.16.1.2
    Router(config)# line vty 0 4
    Router(config-line)# access-class 1 in

Different Cisco platforms support different numbers of terminal
lines. Check your device's configuration to determine the correct
number of terminal lines for your platform.

Infrastructure ACLs (iACL)
+-------------------------

Although it is often difficult to block traffic transiting your
network, it is possible to identify traffic that should never be
allowed to target your infrastructure devices and block that traffic
at the border of your network. Infrastructure ACLs are considered a
network security best practice and should be considered as a
long-term addition to good network security as well as a workaround
for this specific vulnerability. The ACL example shown below should
be included as part of the deployed infrastructure access-list, which
will protect all devices with IP addresses in the infrastructure IP
address range.

A sample access list for devices running Cisco IOS is below:

    !--- Permit SSH services from trusted hosts destined 
    !--- to infrastructure addresses.
    access-list 150 permit tcp TRUSTED_HOSTS MASK INFRASTRUCTURE_ADDRESSES MASK eq 22
    !--- Deny SSH packets from all other sources destined to infrastructure addresses.
    access-list 150 deny   tcp any INFRASTRUCTURE_ADDRESSES MASK eq 22
    !--- Permit all other traffic to transit the device.
    access-list 150 permit IP any any

    interface serial 2/0
      ip access-group 150 in

The white paper titled "Protecting Your Core: Infrastructure
Protection Access Control Lists" presents guidelines and recommended
deployment techniques for infrastructure protection access lists.
This white paper can be obtained here: 
http://www.cisco.com/en/US/tech/tk648/tk361/technologies_white_paper09186a00...

Control Plane Policing (CoPP)
+----------------------------

The Control Plane Policing (CoPP) feature may be used to mitigate
these vulnerabilities. In the following example, only SSH traffic
from trusted hosts and with 'receive' destination IP addresses is
permitted to reach the route processor (RP).

Note: Dropping traffic from unknown or untrusted IP addresses may
affect hosts with dynamically assigned IP addresses from connecting
to the Cisco IOS device.

    access-list 152 deny   tcp TRUSTED_ADDRESSES MASK any eq 22
        access-list 152 permit tcp any any eq 22
        !
        class-map match-all COPP-KNOWN-UNDESIRABLE
         match access-group 152
        !
        !
        policy-map COPP-INPUT-POLICY
         class COPP-KNOWN-UNDESIRABLE
          drop
        !
        control-plane
         service-policy input COPP-INPUT-POLICY

In the above CoPP example, the ACL entries that match the exploit
packets with the "permit" action result in these packets being
discarded by the policy-map "drop" function, while packets that match
the "deny" action are not affected by the policy-map drop function.

CoPP is available in Cisco IOS release trains 12.0S, 12.2SX, 12.2S,
12.3T, 12.4, and 12.4T.

Additional information on the configuration and use of the CoPP
feature can be found at the following URL: 
http://www.cisco.com/en/US/prod/collateral/iosswrel/ps6537/ps6586/ps6642/pro...

Obtaining Fixed Software
========================

Cisco has made free software available to address this vulnerability
for affected customers. Prior to deploying software, customers should
consult their maintenance provider or check the software for feature
set compatibility and known issues specific to their environment.

Customers may only install and expect support for the feature sets
they have purchased. By installing, downloading, accessing or
otherwise using such software upgrades, customers agree to be bound
by the terms of Cisco's software license terms found at 
http://www.cisco.com/public/sw-license-agreement.html, or as 
otherwise set forth at Cisco.com Downloads at 
http://www.cisco.com/public/sw-center/sw-usingswc.shtml

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

Customers with Service Contracts
+-------------------------------

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 http://www.cisco.com

Customers using Third-party Support Organizations
+------------------------------------------------

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 guidance and assistance
with the appropriate course of action in regards to this advisory.

The effectiveness of any workaround or fix is dependent on specific
customer situations such as product mix, network topology, traffic
behavior, and organizational mission. Due to the variety of affected
products and releases, customers should consult with their service
provider or support organization to ensure any applied workaround or
fix is the most appropriate for use in the intended network before it
is deployed.

Customers without Service Contracts
+----------------------------------

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)
  * e-mail: tac@cisco.com

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

Refer to 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.

Exploitation and Public Announcements
=====================================

The Cisco PSIRT is not aware of any public announcements or malicious
use of the vulnerability described in this advisory.

This vulnerability was discovered by Cisco internal testing and
customer service requests.

Status of This Notice: FINAL
============================

THIS DOCUMENT IS PROVIDED ON AN "AS IS" BASIS AND DOES NOT IMPLY ANY
KIND OF GUARANTEE OR WARRANTY, INCLUDING THE WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR USE. YOUR USE OF THE
INFORMATION ON THE DOCUMENT OR MATERIALS LINKED FROM THE DOCUMENT IS
AT YOUR OWN RISK. CISCO RESERVES THE RIGHT TO CHANGE OR UPDATE THIS
DOCUMENT AT ANY TIME.

A stand-alone copy or Paraphrase of the text of this document that
omits the distribution URL in the following section is an
uncontrolled copy, and may lack important information or contain
factual errors.

Distribution
============

This advisory is posted on Cisco's worldwide website at 
http://www.cisco.com/warp/public/707/cisco-sa-20080521-ssh.shtml

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
  * first-teams@first.org
  * bugtraq@securityfocus.com
  * vulnwatch@vulnwatch.org
  * cisco@spot.colorado.edu
  * cisco-nsp@puck.nether.net
  * full-disclosure@lists.grok.org.uk
  * comp.dcom.sys.cisco@newsgate.cisco.com

Future updates of this advisory, if any, will be placed on Cisco's
worldwide website, but may or may not be actively announced on
mailing lists or newsgroups. Users concerned about this problem are
encouraged to check the above URL for any updates.

Revision History
================

+---------------------------------------+
| Revision |             | Initial      |
| 1.0      | 2008-May-21 | public       |
|          |             | release.     |
+---------------------------------------+

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/en/US/products/products_security_vulnerability_policy.h...
 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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Cisco Security Advisory: Cisco IOS Secure Shell Denial of Service

Cisco Systems Product Security Incident Response Team