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LDAP and VPN Vulnerabilities in PIX and ASA Appliances
------------------------------------------------------------------------
SUMMARY
Multiple vulnerabilities exist in the Cisco Adaptive Security Appliance
(ASA) and PIX security appliances. These vulnerabilities include two
Lightweight Directory Access Protocol (LDAP) authentication bypass
vulnerabilities and two denial of service (DoS) vulnerabilities.
DETAILS
The Lightweight Directory Access Protocol (LDAP) authentication bypass
vulnerabilities are caused by a specific processing path followed when the
device is setup to use a Lightweight Directory Access Protocol (LDAP)
authentication server. These vulnerabilities may allow unauthenticated
users to access either the internal network or the device itself.
The two DoS vulnerabilities may be triggered when devices are terminating
Virtual Private Networks (VPN). These denial of service vulnerabilities
may allow an attacker to disconnect VPN users, prevent new connections, or
prevent the device from transmitting traffic.
These vulnerabilities are distributed in the authentication, IPSec VPN,
and SSL VPN code. They are categorized in this advisory by their Cisco bug
descriptions:
* LDAP Authentication Bypass
* Denial of Service in VPNs with Password Expiry
* Denial of Service in SSL VPNs
Cisco has made free software available to address these vulnerabilities
for affected customers.
LDAP Authentication Bypass Vulnerability
Two configuration scenarios exist where Cisco PIX or ASA devices are
vulnerable:
* Layer 2 Tunneling Protocol (L2TP)
Devices configured to use a LDAP authentication server and use an
authentication protocol other than PAP may be vulnerable. The LDAP server
is specified in the configuration via the aaa-server ldap server host <ip
address> command line interface (CLI) configuration command. The
authentication protocol is specified via the authentication <protocol>
command within the tunnel-group <tunnel-group> ppp-attributes section of
the configuration.
Relevant configuration segments of a vulnerable device are shown below. In
the following example configuration, the authentication server is
specified as LDAP and the authentication protocol is specified as
ms-chap-v2:
aaa-server ldap_server protocol ldap
aaa-server ldap_server host 192.168.1.100
timeout 5
ldap-scope onelevel
tunnel-group example_l2tp_group general-attributes
address-pool inside_addresses
authentication-server-group ldap_server
tunnel-group example_l2tp_group ppp-attributes
authentication ms-chap-v2
* Remote Management Access
Devices configured to allow remote management access (telnet, SSH, HTTP)
and use LDAP authentication, authorization, accounting (AAA) server for
credential validation may be vulnerable.
In the configuration file, the server_group is defined as a LDAP server
with the command aaa-server <server_group> protocol ldap. The LDAP
authentication server for remote management access is defined via the
command, aaa authentication {telnet | ssh | http | serial} console
server_group.
Relevant configuration segments of a vulnerable device are shown below.
The authentication server is specified as LDAP, and remote management
access for SSH is permitted and credentials checked by the defined LDAP
AAA server:
ssh 192.168.1.2 255.255.255.255 inside
aaa-server ldap_server protocol ldap
aaa-server ldap_server host 192.168.1.100
timeout 5
ldap-scope onelevel
aaa authentication ssh console ldap_server
Denial of Service in VPNs with Password Expiry
A device may be affected by this vulnerability if the password-management
command is present in the tunnel-group section, as shown in the following
examples:
tunnel-group example_group general-attributes
address-pool inside_addresses
default-group-policy example_group
password-management
tunnel-group example_group general-attributes
address-pool inside_addresses
default-group-policy example_group
password-management password-expire-in-days 30
Denial of Service in SSL VPNs
Clientless SSL VPNs must be enabled on an interface in order for the
device to be affected by this vulnerability.
Devices with clientless SSL VPNs enabled have a webvpn section in the
running configuration. This will be similar to the following entry:
webvpn
enable outside
url-list ServerList "WSHAWLAP" cifs://10.2.2.2 1
url-list ServerList "FOCUS_SRV_1" https://10.2.2.3 2
url-list ServerList "FOCUS_SRV_2" http://10.2.2.4 3
Details
The PIX is a firewall appliance that delivers user and application policy
enforcement, multi-vector attack protection, and secure connectivity
services.
The Adaptive Security Appliance (ASA) is a modular platform that provides
security and VPN services. The ASA offers firewall, intrusion prevention
(IPS), anti-X, and VPN services.
LDAP Authentication Bypass
Cisco ASA and PIX devices leveraging LDAP AAA servers for authentication
of terminating L2TP IPSec tunnels or remote management sessions may be
vulnerable to an authentication bypass attack. See the following bullets
for more details:
* Layer 2 Tunneling Protocol (L2TP)
Devices terminating L2TP IPSec tunnels must be configured to use LDAP in
conjunction with CHAP, MS-CHAPv1, or MS-CHAPv2 authentication protocols to
be vulnerable. If LDAP authentication is used in conjunction with PAP, the
device is not vulnerable to the LDAP L2TP authentication bypass.
* Remote Management Access
Cisco ASA and PIX devices leveraging LDAP AAA servers for authentication
of management sessions (telnet, SSH and HTTP) may be vulnerable to an
authentication bypass attack. Access for management sessions must be
explicitly enabled and is limited to the defined source IP address within
the device configuration.
This vulnerability is documented as bug ID
<http://www.cisco.com/pcgi-bin/Support/Bugtool/onebug.pl?bugid=CSCsh42793>
CSCsh42793 ( registered customers only) .
Denial of Service in VPNs with Password Expiry
Cisco ASA and PIX devices terminating remote access VPN connections may be
vulnerable to a DoS attack if the tunnel group is configured with password
expiry. To exploit this vulnerability for IPSec VPN connections, an
attacker would need to know the group name and group password. An attacker
would not need this information for SSL VPN connections. A successful
attack may result in a reload of the device.
This vulnerability is documented as software bug
<http://www.cisco.com/pcgi-bin/Support/Bugtool/onebug.pl?bugid=CSCsh81111>
CSCsh81111 ( registered customers only) .
Denial of Service in SSL VPNs
Cisco ASAs using clientless SSL VPNs are vulnerable to a denial of service
attack via the SSL VPN HTTP server. A successful attack must exploit a
race condition in the processing non-standard SSL sessions and may result
in a reload of the device.
More details are available in bug
<http://www.cisco.com/pcgi-bin/Support/Bugtool/onebug.pl?bugid=CSCsi16248>
CSCsi16248 ( registered customers only).
Impact
Successful exploitation of the LDAP Authentication bypass vulnerability
may allow unauthorized users to access the device or internal resources.
The DoS vulnerability in VPN password expiry and the DoS vulnerability in
clientless SSL VPNs could be repeatedly exploited to cause an extended DoS
condition.
Workarounds
This section of the advisory describes workarounds that may be useful in
some environments. Additional mitigations that can be deployed on Cisco
devices within the network are available in the Cisco Applied Intelligence
companion document for this advisory at the following link:
<http://www.cisco.com/warp/public/707/cisco-air-20070502-asa.shtml>
http://www.cisco.com/warp/public/707/cisco-air-20070502-asa.shtml
* L2TP
For Cisco ASA or PIX devices configured to use a LDAP authentication
server for L2TP over IPSec connections, configuring the device to use PAP
as an authentication protocol may mitigate this vulnerability. It is
important to note that PAP transmits passwords in clear-text. PAP
authentication is encrypted via IPSec when it is used for the L2TP
connection. Communications between the security appliance and the LDAP
server are not encrypted by default and can be secured with SSL using the
ldap-over-ssl command. Configuration of PAP authentication can be done
using the following example as a guide or by referring to the security
appliance configuration guides listed:
ciscoasa#configure terminal
ciscoasa(config)#tunnel-group l2tp_group ppp-attributes
ciscoasa(config-ppp)#authentication pap
ciscoasa(config-ppp)#no authentication ms-chap-v1
ciscoasa(config-ppp)#no authentication ms-chap-v2
ciscoasa(config-ppp)#no authentication chap
Information on configuring L2TP over IPSEC using the CLI is available at
the following link:
<http://www.cisco.com/en/US/partner/products/ps6120/products_configuration_guide_chapter09186a008066ebb6.html>
http://www.cisco.com/en/US/partner/products/ps6120/products_configuration_guide_chapter09186a008066ebb6.html
Information on configuring L2TP over IPSEC using the ADSM can be found at:
<http://www.cisco.com/en/US/partner/products/ps6121/products_configuration_guide_chapter09186a00806a81bc.html>
http://www.cisco.com/en/US/partner/products/ps6121/products_configuration_guide_chapter09186a00806a81bc.html
* Remote Management
Cisco ASA or PIX devices that authenticate remote management sessions with
either the local database or an AAA server other than a LDAP server are
not affected by this vulnerability. More information on changing the AAA
server protocol used with remote management sessions is available at the
following link:
<http://www.cisco.com/univercd/cc/td/doc/product/multisec/asa_sw/v_7_2/conf_gd/sysadmin/mgaccess.htm>
http://www.cisco.com/univercd/cc/td/doc/product/multisec/asa_sw/v_7_2/conf_gd/sysadmin/mgaccess.htm.
Remote management sessions must be explicitly enabled before the Cisco ASA
or PIX will accept sessions. The source IP addresses are defined within
the command that enables remote management access. Below are examples of
enabling remote management sessions (Note that other commands are
required, but these commands control the source IP address of the device
that is allowed access to the Cisco ASA or PIX device):
For remote telnet, ssh and http access:
ciscoasa#configure terminal
ciscoasa(config)#telnet source_IP_address mask source_interface
ciscoasa(config)#ssh source_IP_address mask source_interface
ciscoasa(config)#http source_IP_address mask source_interface
Denial of Service in VPNs with Password Expiry
Disabling password expiry for remote access users until a device can be
updated with non-vulnerable code can prevent the exposure of this
vulnerability. This can be accomplished by removing the password
management entry in the general attributes of the tunnel group, as shown
in the following example:
ciscoasa#configure terminal
ciscoasa(config)#tunnel-group remote_access_group general-attributes
ciscoasa(config-tunnel-general)#no password-management
Implementing this workaround will disable the password expiry feature, and
users will not be forced to change their passwords.
More information on the password-management command is available in the
Security Appliance Command reference at the following link:
<http://www.cisco.com/en/US/products/ps6120/products_command_reference_chapter09186a008063f0f8.html#wp1725278>
http://www.cisco.com/en/US/products/ps6120/products_command_reference_chapter09186a008063f0f8.html#wp1725278
Denial of Service in SSL VPNs
If clientless SSL VPNs are used, there is no workaround for the SSL VPN
vulnerability. Client-based VPNs are not affected, and may be used as an
alternative to the clientless VPN connections.
More information on configuring clientless SSL VPNs on the ASA is
available in the configuration example at the following link:
<http://www.cisco.com/en/US/products/ps6120/products_configuration_example09186a00806ea271.shtml>
http://www.cisco.com/en/US/products/ps6120/products_configuration_example09186a00806ea271.shtml
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-20070502-asa.shtml>
http://www.cisco.com/warp/public/707/cisco-sa-20070502-asa.shtml
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