From: SecuriTeam <support@securiteam.com.>
To: [email protected]
Date: 12 Apr 2005 16:18:36 +0200
Subject: [NEWS] Vulnerabilities in Cisco IOS Secure Shell Server
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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 Remote
Authentication Dial In User Service (RADIUS) is not affected by these
vulnerabilities.
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).
DETAILS
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>;
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/univercd/cc/td/doc/product/software/ios123/123newft/123t/123t_7/gt_ssh2.htm>; Secure Shell Version 2 Support.
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>sh 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 is affected.)
* IOS 12.1 (mainline - the "T" train is affected.)
* IOS 12.3 (mainline - the "T" train is affected.)
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 name> group tacacs+
local) and the account username includes a domain name. Please note that
the device is not affected if users are being authenticated against 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.
* 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 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#sh 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.
Impact:
Successful exploitation of the vulnerability described in Cisco bug ID
CSCed65778 (registered customers only) may result in a reload of the
device. Repeated exploitation could result in a sustained denial of
service condition.
Successful exploitation of the vulnerability described in Cisco bug ID
CSCed65285 (registered customers only) may result in resource depletion.
Repeated exploitation could cause a reload of the device, which in turn
could result in a sustained denial of service condition.
Software Versions and Fixes:
A table listing all the vulnerable software versions and their appropriate
fixes can be found at:
<http://www.cisco.com/warp/public/707/cisco-sa-20050406-ssh.shtml#software> Software Versions and Fixes
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://cisco.com/en/US/products/sw/iosswrel/ps1835/products_command_reference_chapter09186a00800873c8.html#wp1017389> IP Services Commands: access-class Through ip mask-reply.
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/warp/public/707/tacl.html>;
http://www.cisco.com/warp/public/707/tacl.html
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/warp/public/707/iacl.html>;
http://www.cisco.com/warp/public/707/iacl.html
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/warp/public/707/racl.html>;
http://www.cisco.com/warp/public/707/racl.html
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 any eq
22
access-list 140 deny tcp host <trusted host 2's IP address> any any eq
22
[...]
access-list 140 deny tcp host <trusted host N's IP address> any 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 "icmp-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/products/sw/iosswrel/ps1838/products_feature_guide09186a00801afad4.html>; Control Plane Policing
ADDITIONAL INFORMATION
The information has been provided by <mailto:psirt@cisco.com.> Cisco
Systems Product Security Incident Response Team.
The original article can be found at:
<http://www.cisco.com/warp/public/707/cisco-sa-20050406-ssh.shtml>;
http://www.cisco.com/warp/public/707/cisco-sa-20050406-ssh.shtml
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