Vulnerabilities in Cisco IOS Secure Shell Server

Summary

• Certain release trains of Cisco Internetwork Operating System (IOS)^®, when configured to use the IOS Secure Shell (SSH) server in combination with Terminal Access Controller Access Control System Plus (TACACS+) as a means to perform remote management tasks on IOS devices, may contain two vulnerabilities that can potentially cause IOS devices to exhaust resources and reload. Repeated exploitation of these vulnerabilities can result in a Denial of Service (DoS) condition. Use of SSH with other authentication methods like Remote Authentication Dial In User Service (RADIUS) and the local user database may also be affected.

  Cisco has made free software available to address these vulnerabilities for all affected customers. There are workarounds available to mitigate the effects of the vulnerability (see the Workarounds section.)

  This advisory will be posted at http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20050406-ssh.

Affected Products

• This section provides details on affected products.

  Vulnerable Products

  These issues affect any Cisco device running an unfixed version of Cisco IOS that supports, and is configured to use, the SSH server functionality.

  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 shortly after this identification (possibly in the next line), 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 device running IOS release 12.2(15)T14 (release train label "12.2T") with an installed image name of C806-K9OSY6-M:

  Router1> show version
  Cisco Internetwork Operating System Software
  IOS (tm) C806 Software (C806-K9OSY6-M), Version 12.2(15)T14, RELEASE SOFTWARE (fc4)
  [...]

  The next example shows a device running IOS release 12.3(10) (release train label "12.3 mainline") with an image name of C2600-IK9OS3-M:

  Router2> show version
  Cisco Internetwork Operating System Software
  IOS (tm) C2600 Software (C2600-IK9O3S3-M), Version 12.3(10), RELEASE SOFTWARE (fc3)
  [...]

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

  SSH protocol was introduced in the following IOS release trains:

  • IOS 12.0S (SSH version 1)
    
  • IOS 12.1T (SSH version 1)
    
  • IOS 12.2 (SSH version 1)
    
  • IOS 12.2T (SSH version 1)
    
  • IOS 12.3T (SSH version 2)
    

  To determine if the IOS image that your IOS device is running supports the server side of the SSH protocol, whether it is enabled (if supported), and the SSH protocol version being used (if SSH is supported and enabled), use the show ip ssh command in global mode:

  Router> show ip ssh
  SSH Enabled - version 1.5
  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 1. 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_ssh4.html.

  Note: SSH protocols versions 1 and 2 cannot interoperate, but usually a SSH server knows how to handle connections from clients using either version of the protocol, but in most cases the server has to be explicitly configured to do this. The latest revision of protocol version 1 is "1.5", which is documented in a now expired Internet Engineering Task Force (IETF) draft.

  The show ip ssh command was introduced in IOS release 12.1(1)T. If this command is not available then the IOS image in use does not have SSH server support and therefore it is not vulnerable to the issues discussed in this advisory.

  As you will see in the Details section, the behavior of the vulnerabilities described in this document can depend on the version of the SSH protocol that the IOS device is using. Therefore, it is important to use the show ip ssh command as shown above to obtain this information.

  When the show ip ssh command is executed on an image that does not support SSH the following output will be generated:

  Router> show ip ssh
                  ^
  % Invalid input detected at '^' marker.
  
  Router>

  Finally, even if the release and image running on an IOS device support SSH, the SSH server may not be enabled. The following example shows the output from the show ip ssh command on a device that supports SSH but that does not have the SSH server enabled (note the "SSH Disabled" message):

  Router> show ip ssh
  SSH Disabled - version 1.5
  %Please create RSA keys to enable SSH.
  Authentication timeout: 120 secs; Authentication retries: 3
  Router>

  Products Confirmed Not Vulnerable

  Devices not running IOS, running an IOS train without the SSH server functionality, or running an IOS version supporting SSH but without the SSH server enabled are not affected.

  See the Affected Products section for a detailed list of IOS release trains that implement the SSH functionality. In particular, the following IOS release train do not contain any SSH code:

  • All IOS versions prior to 12.0.
    
  • IOS 12.0 (mainline - the "S" train supports SSH and is affected.)
    
  • IOS 12.1 (mainline - the "T" train supports SSH and is affected.)
    

  Additionally, while 12.3 mainline does support the SSH server functionality it is not vulnerable to the issues discussed in this document. For all other releases and trains, please check the Software Versions and Fixes section.

  Cisco IOS XR is not affected.

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

Details

• Secure Shell (SSH) is a protocol that provides a secure, remote connection to a network device. There are currently two versions of the SSH protocol, SSH Version 1 and SSH Version 2, both of which are supported by Cisco IOS. The SSH server component of IOS identifies itself as version "1.5" if running only version 1.0 of the protocol, as version "2.0" if running only version 2 of the protocol, and as version "1.99" if running protocol version 2 with fall-back to protocol version 1.

  The SSH server feature of IOS enables a SSH client to make a secure, encrypted connection to a Cisco IOS device. This connection provides functionality that is similar to a telnet connection with the difference that all traffic between the server and the client, including authentication information, travels encrypted through the wires.

  TACACS provides a way to centrally validate users attempting to gain access to servers, workstations, routers, switches, access servers, and other network devices.

  The two vulnerabilities described in this document can cause denial of service (DoS) conditions that affect IOS devices configured to use the IOS SSH server feature for remote management.

  The first vulnerability may cause a device to reload when the IOS device is configured to act as a SSH version 2 server and any of the following events occurs:

  • The device is configured to authenticate users against a TACACS+ server (via a command like aaa authentication login group tacacs+ local) and the account username includes a domain name. This condition does not apply to devices configured to authenticate via different methods, like a RADIUS server or the local user database.
    
  • A new SSH session is in the authentication phase (the server is waiting for a username or password) and another, already logged-in user uses the send command. This condition applies to any authentication method, including TACACS+, RADIUS, and the local user database.
    
  • Logging of messages is being directed to a SSH session that is already established (through the terminal monitor command) and the SSH session to the IOS device terminates while the SSH server is still sending data to the client. This condition applies to any authentication method, including TACACS+, RADIUS, and the local user database.
    

  This vulnerability is documented in the Cisco bug ID CSCed65778 ( registered customers only) -- Crash in SSHv2 due to TACACS+ username containing domain name.

  Note: this vulnerability affects SSH protocol version 2. SSH protocol version 1 is not affected.

  The second vulnerability consists of a memory leak that happens when an IOS device is configured to authenticate SSH users against a TACACS+ server and the login fails due to an invalid username or password. This affects both SSH version 1 and version 2 connections. In the case of SSH version 2 connections, the memory leak occurs even after a successful login. Please note that the device is not affected if users are being authenticated against a RADIUS server or the local user database.

  The memory leak can be detected by running the command show tcp brief, like in the following example:

  Router# show tcp brief
  TCB       Local Address           Foreign Address        (state)
  637202B8  10.0.0.19.13294       172.16.112.29.49       ESTAB
  6371C978  10.0.0.19.13233       172.16.112.29.49       ESTAB
  636CB228  10.0.0.19.13041       172.16.112.29.49       CLOSEWAIT
  636B6900  10.0.0.19.12912       172.16.112.29.49       CLOSEWAIT
  63697548  10.0.0.19.12848       172.16.112.29.49       CLOSEWAIT
  63687930  10.0.0.19.12784       172.16.112.29.49       CLOSEWAIT
  635F4A80  10.0.0.19.12659       172.16.112.29.49       CLOSEWAIT

  In the output above, those Transmission Control Blocks (TCBs) in the state CLOSEWAIT will not go away and represent memory leaks. Please note that only TCP connections with a foreign TCP port of 49 (the well-known port for TACACS) are relevant.

  This vulnerability is documented in the Cisco bug ID CSCed65285 ( registered customers only) -- SSH leaks memory and buffers.

Workarounds

• The effectiveness of any workaround 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 is the most appropriate for use in the intended network before it is deployed.

  Mitigation Strategies

  Not all of the mitigation strategies listed will work for all customers. Some of the workarounds listed are dependent on which versions and feature-sets of IOS you have in your network.

  Configuring a VTY Access Class

  It is possible to limit the exposure of the Cisco device by applying a VTY access class to permit 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.

  Configuring Access Lists (ACLs)

  In addition to configuring a VTY Access Class, it may be desirable to block all SSH traffic destined to your network infrastructure.

  Telnet and reverse telnet should be blocked as part of a Transit ACL controlling all access to the trusted network. Transit 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 white paper entitled "Transit Access Control Lists: Filtering at Your Edge" presents guidelines and recommended deployment techniques for transit ACLs:

  http://www.cisco.com/en/US/tech/tk648/tk361/technologies_white_paper09186a00801afc76.shtml

  Configuring Infrastructure Access Lists (iACLs)

  Although it is often difficult to block traffic transiting your network, it is possible to identify traffic which 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 white paper entitled "Protecting Your Core: Infrastructure Protection Access Control Lists" presents guidelines and recommended deployment techniques for infrastructure protection ACLs:

  http://www.cisco.com/en/US/tech/tk648/tk361/technologies_white_paper09186a00801a1a55.shtml

  Configuring Receive Access Lists (rACLs)

  For distributed platforms, rACLs may be an option starting in Cisco IOS Software Versions 12.0(21)S2 for the 12000 series GSR and 12.0(24)S for the 7500 series. The receive access lists protect the device from harmful traffic before the traffic can impact the route processor. Receive path 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 CPU load is distributed to the line card processors and helps mitigate load on the main route processor. The white paper entitled "GSR: Receive Access Control Lists" will help identify and allow legitimate traffic to your device and deny all unwanted packets:

  http://www.cisco.com/en/US/tech/tk648/tk361/technologies_white_paper09186a00801a0a5e.shtml

  Control Plane Policing

  The Control Plane Policy (CoPP) feature may be used to mitigate this vulnerability, as in the following example:

  ! Do not police SSH traffic from trusted hosts
  access-list 140 deny   tcp host  any eq 22
  access-list 140 deny   tcp host  any eq 22
  [...]
  access-list 140 deny   tcp host  any eq 22
  ! Trust an entire network if desired
  access-list 140 deny   tcp   any eq 22
  ! Police SSH traffic from untrusted hosts
  access-list 140 permit tcp any any eq 22
  ! Do not police any other type of traffic going to the router
  access-list 140 deny   ip  any any
  !
  class-map match-all ssh-class
    match access-group 140
  !
  policy-map control-plane-policy
    ! Drop all traffic that matches the class "ssh-class"
    class ssh-class
       drop
  !
  control-plane
    service-policy input control-plane-policy

  Note: CoPP is available only in IOS release trains 12.0S, 12.2S and 12.3T. 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

• 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 further information on the terms "Rebuild" and "Maintenance" please consult the following URL:

  http://www.cisco.com/warp/public/620/1.html

  When considering software upgrades, please also consult http://www.cisco.com/en/US/products/products_security_advisories_listing.html 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") for assistance.

  Major Release	Availability of Repaired Releases
  Affected 12.0-Based Release	Rebuild	Maintenance
  12.0S	12.0(26)S5
  	12.0(27)S4
  	12.0(28)S2
  		12.0(30)S
  12.0SC	Vulnerable; contact TAC
  12.0SL	Migrate to 12.0(26)S5 or later
  12.0SP	Migrate to 12.0(26)S5 or later
  12.0ST	Migrate to 12.0(26)S5 or later
  12.0SX		12.0(30)SX
  12.0SY	Migrate to 12.0(26)S5 or later
  12.0SZ	Migrate to 12.0(26)S5 or later
  12.0XK	Migrate to 12.2(26) or later
  Affected 12.1-Based Release	Rebuild	Maintenance
  12.1AX	Migrate to 12.2(25)EY or later
  12.1AY	12.1(4)AY4
  12.1AZ	Migrate to 12.1(22)EA1 or later
  12.1DB	Migrate to 12.3(4)T11 or later
  12.1DC	Vulnerable; contact TAC
  12.1E		12.1(23)E
  12.1EA	12.1(22)EA1
  12.1EB		12.1(23)EB
  12.1EC	Migrate to 12.3BC latest
  12.1EU	Migrate to 12.2(20)EU or later
  12.1EV	Migrate to 12.2(24)SV or later
  12.1EW	Migrate to 12.2(18)EW2 or later
  12.1EX	Migrate to 12.1(23)E or later
  12.1EY	Migrate to 12.1(23)E or later
  12.1EZ	Vulnerable; contact TAC
  12.1T	Migrate to 12.2(26) or later
  12.1XA	Migrate to 12.2(26) or later
  12.1XB	Migrate to 12.2(26) or later
  12.1XC	Migrate to 12.2(26) or later
  12.1XD	Migrate to 12.2(26) or later
  12.1XE	Migrate to 12.1(23)E or later
  12.1XF	Migrate to 12.2(26) or later
  12.1XG	Migrate to 12.3(8)T or later
  12.1XH	Migrate to 12.2(26) or later
  12.1XI	Migrate to 12.2(26) or later
  12.1XJ	Migrate to 12.3 or later
  12.1XL	Migrate to 12.3(8)T or later
  12.1XM	Migrate to 12.3(8)T or later
  12.1XP	Migrate to 12.2(26) or later
  12.1XQ	Migrate to 12.3(8)T or later
  12.1XR	Migrate to 12.3(8)T or later
  12.1XT	Migrate to 12.3(8)T or later
  12.1XU	Migrate to 12.3(8)T or later
  12.1XV	Migrate to 12.3(8)T or later
  12.1YA	Migrate to 12.3(8)T or later
  12.1YB	Migrate to 12.3(8)T or later
  12.1YC	Migrate to 12.3(8)T or later
  12.1YD	Migrate to 12.3(8)T or later
  12.1YE	Migrate to 12.3(8)T or later
  12.1YF	Migrate to 12.3(8)T or later
  12.1YH	Migrate to 12.3(8)T or later
  12.1YI	Migrate to 12.3(8)T or later
  12.1YJ	Migrate to 12.1(22)EA1 or later
  Affected 12.2-Based Release	Rebuild	Maintenance
  12.2		12.2(26)
  12.2B	Migrate to 12.3(8)T or later
  12.2BC	Migrate to 12.3BC latest
  12.2BW	Migrate to 12.3 or later
  12.2BY	Migrate to 12.3(8)T or later
  12.2BZ	Migrate to 12.3(7)XI3 or later
  12.2CX	Migrate to 12.3BC latest
  12.2CY	Migrate to 12.3BC latest
  12.2DD	Migrate to 12.3(8)T or later
  12.2DX	Migrate to 12.3(8)T or later
  12.2EU		12.2(20)EU
  12.2EW	12.2(18)EW2
  		12.2(25)EW
  12.2EWA		12.2(20)EWA
  12.2EX	Migrate to 12.2(25)SEA
  12.2EY		12.2(25)EY
  12.2EZ		12.2(25)EZ
  12.2MB	Migrate to 12.2SW latest
  12.2MC	Migrate to 12.3(8)T or later
  12.2S	12.2(14)S13
  	12.2(18)S7
  	12.2(20)S7
  		12.2(25)S
  12.2SE	12.2(20)SE4
  		12.2(25)SE
  12.2SEA		12.2(25)SEA
  12.2SEB		12.2(25)SEB
  12.2SU	Migrate to 12.3(8)T or later
  12.2SV		12.2(24)SV
  12.2SX	Migrate to 12.2(17d)SXB1
  12.2SXA	Migrate to 12.2(17d)SXB1
  12.2SXB	12.2(17d)SXB1
  12.2SXD		12.2(18)SXD
  12.2SY	Migrate to 12.2(17d)SXB1
  12.2SZ	Migrate to 12.2(20)S7 or later
  12.2T	Migrate to 12.3(8)T or later
  12.2XA	Migrate to 12.3(8)T or later
  12.2XB	Migrate to 12.3(8)T or later
  12.2XC	Migrate to 12.3(8)T or later
  12.2XD	Migrate to 12.3(8)T or later
  12.2XE	Migrate to 12.3(8)T or later
  12.2XF	Migrate to 12.3BC latest
  12.2XG	Migrate to 12.3(8)T or later
  12.2XH	Migrate to 12.3(8)T or later
  12.2XI	Migrate to 12.3(8)T or later
  12.2XJ	Migrate to 12.3(8)T or later
  12.2XK	Migrate to 12.3(8)T or later
  12.2XL	Migrate to 12.3 or later
  12.2XM	Migrate to 12.3 or later
  12.2XN	Migrate to 12.3(8)T or later
  12.2XQ	Migrate to 12.3(8)T or later
  12.2XR	Vulnerable; contact TAC
  12.2XS	Migrate to 12.3(8)T or later
  12.2XT	Migrate to 12.3(8)T or later
  12.2XU	Migrate to 12.3(14)T
  12.2XW	Migrate to 12.3 or later
  12.2YA	Migrate to 12.3 or later
  12.2YB	Migrate to 12.3 or later
  12.2YC	Migrate to 12.3(8)T or later
  12.2YD	Migrate to 12.3(8)T or later
  12.2YE	Migrate to 12.2S or later
  12.2YF	Migrate to 12.3 or later
  12.2YG	Migrate to 12.3 or later
  12.2YH	Migrate to 12.3 or later
  12.2YJ	Migrate to 12.3 or later
  12.2YK	Migrate to 12.3(8)T or later
  12.2YL	Migrate to 12.3(8)T or later
  12.2YM	Migrate to 12.3(8)T or later
  12.2YN	Migrate to 12.3(8)T or later
  12.2YO	Migrate to 12.2(17d)SXB1
  12.2YP	Migrate to 12.3 or later
  12.2YQ	Migrate to 12.3(8)T or later
  12.2YR	Migrate to 12.3(8)T or later
  12.2YW	Migrate to 12.3(8)T or later
  12.2YX	Migrate to 12.3(11)T3 or later
  12.2YY	Migrate to 12.3(8)T or later
  12.2YZ	Migrate to 12.2(20)S7 or later
  12.2ZA	Migrate to 12.2(17d)SXB1 or later
  12.2ZB	Migrate to 12.3(8)T or later
  12.2ZC	Migrate to 12.3(8)T or later
  Affected 12.3-Based Release	Rebuild	Maintenance
  12.3JA		12.3(4)JA
  12.3T	12.3(4)T11
  	12.3(7)T7
  		12.3(8)T
  12.3XD	Migrate to 12.3(8)T or later
  12.3XE	Migrate to 12.3(8)T or later
  12.3XF	Migrate to 12.3(11)T or later
  12.3XG	Migrate to 12.3(11)T or later
  12.3XH	Migrate to 12.3(11)T or later
  12.3XI	12.3(7)XI3
  12.3XJ	Migrate to 12.3(8)XW
  12.3XK	Migrate to 12.3(14)T
  12.3XL		12.3(11)XL
  12.3XM		12.3(7)XM
  12.3XQ	12.3(4)XQ1
  12.3XR		12.3(7)XR
  12.3XS		12.3(7)XS
  12.3XU		12.3(8)XU
  12.3XW		12.3(8)XW
  12.3XX		12.3(8)XX
  12.3XY		12.3(8)XY
  12.3YA		12.3(8)YA
  12.3YD		12.3(8)YD
  12.3YF		12.3(11)YF
  12.3YG		12.3(8)YG
  12.3YH		12.3(8)YH
  12.3YJ		12.3(11)YJ
  12.3YK		12.3(11)YK

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 during internal testing.

URL

• http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20050406-ssh

Revision History

• Revision 1.1	2005-May-03	- Clarify that 12.3 does support SSH, but it is not vulnerable. - Updated table of fixed software. - 12.1AZ migrates to 12.1(22)EA1, not 12.2(22)EA1. - Correct the Access Control List entries of the Control Plane Policing example. - Clarify that the SSH version 2 vulnerability (CSCed65778 ( registered customers only) ) affects devices using authentication methods other than TACACS+ (RADIUS and local user database, for example.)
  Revision 1.0	2005-April-06	Initial Public Release

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