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. SSH is enabled any time RSA keys are generated such as when a http secure-server or trust points for digital certificates are configured. 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 https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20080521-ssh.

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 and IOS-XE 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)
    

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://www.cisco.com/en/US/docs/ios/12_2/ipaddr/command/reference/1rfip1.html#wp1017389.

  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_paper09186a00801a1a55.shtml.

  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/prod_white_paper0900aecd804fa16a.html.

Fixed Software

• 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 Release	Availability of Repaired Releases
  Affected 12.0-Based Releases	First Fixed Release	Recommended Release
  There are no affected 12.0 based releases
  Affected 12.1-Based Releases	First Fixed Release	Recommended Release
  There are no affected 12.1 based releases
  Affected 12.2-Based Releases	First Fixed Release	Recommended Release
  There are no affected 12.2 based releases
  Affected 12.3-Based Releases	First Fixed Release	Recommended Release
  There are no affected 12.3 based releases
  Affected 12.4-Based Releases	First Fixed Release	Recommended Release
  12.4	12.4(13f) 12.4(16b) 12.4(17a) 12.4(18)	12.4(18b)
  12.4JA	Only 12.4(13d)JA and 12.4(13d)JA1 are vulnerable, all other 12.4JA releases are not affected.	12.4(16b)JA3
  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
  12.4T	12.4(9)T6 12.4(11)T4 12.4(15)T2 12.4(20)T	12.4(15)T5
  12.4XA	Not Vulnerable
  12.4XB	Not Vulnerable
  12.4XC	Not Vulnerable
  12.4XD	Not Vulnerable
  12.4XE	Vulnerable; first fixed in 12.4T	12.4(15)T5
  12.4XF	Vulnerable; first fixed in 12.4T	12.4(15)T5
  12.4XG	Not Vulnerable
  12.4XJ	Vulnerable; first fixed in 12.4T	12.4(15)T5
  12.4XK	Vulnerable; first fixed in 12.4T	12.4(15)T5
  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

Exploitation and Public Announcements

• Cisco is releasing this IOS advisory out of our normal twice yearly cycle due to an increase in customer support cases linked to this bug. Cisco PSIRT has not received any reports of malicious exploitation.

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

URL

• https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20080521-ssh

Revision History

• Revision 1.1	2008-May-28	Content changes in these sections: Summary, Products Confirmed Not Vulnerable, and Exploitation and Public Announcements.
  Revision 1.0	2008-May-21	Initial public release.

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