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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 https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20050406-ssh.
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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:
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IOS 12.0S (SSH version 1)
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IOS 12.1T (SSH version 1)
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IOS 12.2 (SSH version 1)
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IOS 12.2T (SSH version 1)
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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:
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1.5: only SSH protocol version 1 is enabled.
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1.99: SSH protocol version 2 with SSH protocol version 1
compatibility enabled.
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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.
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:
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All IOS versions prior to 12.0.
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IOS 12.0 (mainline - the "S" train supports SSH and is affected.)
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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.
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IOS 12.0S (SSH version 1)
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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:
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The device is configured to authenticate users against a TACACS+
server (via a command like aaa authentication login <group
name> 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.
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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.
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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.
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The device is configured to authenticate users against a TACACS+
server (via a command like aaa authentication login <group
name> 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.
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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 <trusted host 1's IP address> any eq 22 access-list 140 deny tcp host <trusted host 2's IP address> any eq 22 [...] access-list 140 deny tcp host <trusted host N's IP address> any eq 22 ! Trust an entire network if desired access-list 140 deny tcp <trusted network address> <trusted network mask> 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
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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
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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.
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To learn about Cisco security vulnerability disclosure policies and publications, see the Security Vulnerability Policy. This document also contains instructions for obtaining fixed software and receiving security vulnerability information from Cisco.
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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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