This Applied Mitigation Bulletin is a companion document to the PSIRT Security Advisory Cisco Content Services Gateway Denial of Service Vulnerability and provides identification and mitigation techniques that administrators can deploy on Cisco network devices.
The Cisco Content Services Gateway - Second Generation (CSG2) contains a vulnerability when processing a series of specially crafted ICMP packets. This vulnerability can be exploited remotely without authentication and without end-user interaction. Successful exploitation of this vulnerability may cause the affected device to reload, resulting in a denial of service (DoS) condition. Repeated attempts to exploit this vulnerability could result in a sustained DoS condition. The attack vector for exploitation is through a series of ICMP packets.
This vulnerability has been assigned CVE identifier CVE-2011-2064.
Information about vulnerable, unaffected, and fixed software is available in the PSIRT Security Advisory, which is available at the following link: https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20110706-csg.
Cisco devices provide several countermeasures for this vulnerability. Administrators are advised to consider these protection methods to be general security best practices for infrastructure devices and the traffic that transits the network. This section of the document provides an overview of these techniques.
Cisco IOS Software can provide effective means of exploit prevention using transit access control lists (tACLs).
This protection mechanism filters and drops packets that are attempting to exploit this vulnerability.
Effective exploit prevention can also be provided by the Cisco ASA 5500 Series Adaptive Security Appliance and the Firewall Services Module (FWSM) for Cisco Catalyst 6500 Series switches and Cisco 7600 Series routers using transit access control lists (tACLs).
This protection mechanism filters and drops packets that are attempting to exploit this vulnerability.
Effective use of Cisco Intrusion Prevention System (IPS) event actions provides visibility into and protection against attacks that attempt to exploit this vulnerability.
Cisco IOS NetFlow records can provide visibility into network-based exploitation attempts.
Cisco IOS Software, Cisco ASA, and FWSM firewalls can provide visibility through syslog messages and counter values displayed in the output from show commands.
The Cisco Security Monitoring, Analysis, and Response System (Cisco Security MARS) appliance can also provide visibility through incidents, queries, and event reporting.
Organizations are advised to follow their standard risk evaluation and mitigation processes to determine the potential impact of this vulnerability. Triage refers to sorting projects and prioritizing efforts that are most likely to be successful. Cisco has provided documents that can help organizations develop a risk-based triage capability for their information security teams. Risk Triage for Security Vulnerability Announcements and Risk Triage and Prototyping can help organizations develop repeatable security evaluation and response processes.
Caution: The effectiveness of any mitigation technique depends on specific customer situations such as product mix, network topology, traffic behavior, and organizational mission. As with any configuration change, evaluate the impact of this configuration prior to applying the change.
Specific information about mitigation and identification is available for these devices:
To protect the network from traffic that enters the network at ingress access points, which may include Internet connection points, partner and supplier connection points, or VPN connection points, administrators are advised to deploy transit access control lists (tACLs) to perform policy enforcement. Administrators can construct a tACL by explicitly permitting only authorized traffic to enter the network at ingress access points or permitting authorized traffic to transit the network in accordance with existing security policies and configurations. A tACL workaround cannot provide complete protection against this vulnerability when the attack originates from a trusted source address.
The tACL policy denies unauthorized ICMP packet types including echo request, echo-reply, host-unreachable, traceroute, packet-too-big, time-exceeded, and unreachable, that are sent to affected devices. In the following example, 192.168.60.0/24 is the IP address space that is used by the affected devices, and the host at 192.168.100.1 is considered a trusted source that requires access to the affected devices. Care should be taken to allow required traffic for routing and administrative access prior to denying all unauthorized traffic.
Additional information about tACLs is in Transit Access Control Lists: Filtering at Your Edge.
! !-- Include explicit permit statements for trusted sources !-- that require access on the vulnerable protocol ! access-list 150 permit icmp host 192.168.100.1 192.168.60.0 0.0.0.255 echo access-list 150 permit icmp host 192.168.100.1 192.168.60.0 0.0.0.255 echo-reply access-list 150 permit icmp host 192.168.100.1 192.168.60.0 0.0.0.255 host-unreachable access-list 150 permit icmp host 192.168.100.1 192.168.60.0 0.0.0.255 traceroute access-list 150 permit icmp host 192.168.100.1 192.168.60.0 0.0.0.255 packet-too-big access-list 150 permit icmp host 192.168.100.1 192.168.60.0 0.0.0.255 time-exceeded access-list 150 permit icmp host 192.168.100.1 192.168.60.0 0.0.0.255 unreachable ! !-- The following vulnerability-specific access control entries !-- (ACEs) can aid in identification of attacks ! access-list 150 deny icmp any 192.168.60.0 0.0.0.255 echo access-list 150 deny icmp any 192.168.60.0 0.0.0.255 echo-reply access-list 150 deny icmp any 192.168.60.0 0.0.0.255 host-unreachable access-list 150 deny icmp any 192.168.60.0 0.0.0.255 traceroute access-list 150 deny icmp any 192.168.60.0 0.0.0.255 packet-too-big access-list 150 deny icmp any 192.168.60.0 0.0.0.255 time-exceeded access-list 150 deny icmp any 192.168.60.0 0.0.0.255 unreachable ! !-- Permit or deny all other Layer 3 and Layer 4 traffic in accordance !-- with existing security policies and configurations ! !-- Explicit deny for all other IP traffic ! access-list 150 deny ip any any ! !-- Apply tACL to interfaces in the ingress direction ! interface GigabitEthernet0/0 ip access-group 150 in
Note that filtering with an interface access list will elicit the transmission of ICMP unreachable messages back to the source of the filtered traffic. Generating these messages could have the undesired effect of increasing CPU utilization on the device. In Cisco IOS Software, ICMP unreachable generation is limited to one packet every 500 milliseconds by default. ICMP unreachable message generation can be disabled using the interface configuration command no ip unreachables. ICMP unreachable rate limiting can be changed from the default using the global configuration command ip icmp rate-limit unreachable interval-in-ms.
After the administrator applies the tACL to an interface, the show ip access-lists command will identify the number of ICMP packet types including echo request, echo-reply, host-unreachable, traceroute, packet-too-big, time-exceeded, and unreachable, that have been filtered. Administrators are advised to investigate filtered packets to determine whether they are attempts to exploit this vulnerability. Example output for show ip access-lists 150 follows:
router#show ip access-lists 150
Extended IP access list 150
10 permit icmp host 192.168.100.1 192.168.60.0 0.0.0.255 echo
20 permit icmp host 192.168.100.1 192.168.60.0 0.0.0.255 echo-reply
30 permit icmp host 192.168.100.1 192.168.60.0 0.0.0.255 host-unreachable
40 permit icmp host 192.168.100.1 192.168.60.0 0.0.0.255 traceroute
50 permit icmp host 192.168.100.1 192.168.60.0 0.0.0.255 packet-too-big
60 permit icmp host 192.168.100.1 192.168.60.0 0.0.0.255 time-exceeded
70 permit icmp host 192.168.100.1 192.168.60.0 0.0.0.255 unreachable
80 deny icmp any 192.168.60.0 0.0.0.255 echo (12 matches)
90 deny icmp any 192.168.60.0 0.0.0.255 echo-reply (26 matches)
100 deny icmp any 192.168.60.0 0.0.0.255 host-unreachable (10 matches)
110 deny icmp any 192.168.60.0 0.0.0.255 traceroute (7 matches)
120 deny icmp any 192.168.60.0 0.0.0.255 packet-too-big (9 matches)
130 deny icmp any 192.168.60.0 0.0.0.255 time-exceeded (2 matches)
140 deny icmp any 192.168.60.0 0.0.0.255 unreachable (18 matches)
150 deny ip any any
router#
In the preceding example, access list 150 has dropped the following packets that are received from an untrusted host or network:
For additional information about investigating incidents using ACE counters and syslog events, reference the Identifying Incidents Using Firewall and IOS Router Syslog Events Applied Intelligence white paper.
Administrators can use Embedded Event Manager to provide instrumentation when specific conditions are met, such as ACE counter hits. The Applied Intelligence white paper Embedded Event Manager in a Security Context provides additional details about how to use this feature.
The log and log-input access control list (ACL) option will cause packets that match specific ACEs to be logged. The log-input option enables logging of the ingress interface in addition to the packet source and destination IP addresses and ports.
Caution: Access control list logging can be very CPU intensive and must be used with extreme caution. Factors that drive the CPU impact of ACL logging are log generation, log transmission, and process switching to forward packets that match log-enabled ACEs.
For Cisco IOS Software, the ip access-list logging interval interval-in-ms command can limit the effects of process switching induced by ACL logging. The logging rate-limit rate-per-second [except loglevel] command limits the impact of log generation and transmission.
The CPU impact from ACL logging can be addressed in hardware on the Cisco Catalyst 6500 Series switches and Cisco 7600 Series routers with Supervisor Engine 720 or Supervisor Engine 32 using optimized ACL logging.
For additional information about the configuration and use of ACL logging, reference the Understanding Access Control List Logging Applied Intelligence white paper.
Administrators can configure Cisco IOS NetFlow on Cisco IOS routers and switches to aid in the identification of traffic flows that may be attempts to exploit the vulnerability. Administrators are advised to investigate flows to determine whether they are attempts to exploit the vulnerability or whether they are legitimate traffic flows.
router#show ip cache flow
IP packet size distribution (90784136 total packets):
1-32 64 96 128 160 192 224 256 288 320 352 384 416 448 480
.000 .698 .011 .001 .004 .005 .000 .004 .000 .000 .003 .000 .000 .000 .000
512 544 576 1024 1536 2048 2560 3072 3584 4096 4608
.000 .001 .256 .000 .010 .000 .000 .000 .000 .000 .000
IP Flow Switching Cache, 4456704 bytes
1885 active, 63651 inactive, 59960004 added
129803821 ager polls, 0 flow alloc failures
Active flows timeout in 30 minutes
Inactive flows timeout in 15 seconds
IP Sub Flow Cache, 402056 bytes
0 active, 16384 inactive, 0 added, 0 added to flow
0 alloc failures, 0 force free
1 chunk, 1 chunk added
last clearing of statistics never
Protocol Total Flows Packets Bytes Packets Active(Sec) Idle(Sec)
-------- Flows /Sec /Flow /Pkt /Sec /Flow /Flow
TCP-Telnet 11393421 2.8 1 48 3.1 0.0 1.4
TCP-FTP 236 0.0 12 66 0.0 1.8 4.8
TCP-FTPD 21 0.0 13726 1294 0.0 18.4 4.1
TCP-WWW 22282 0.0 21 1020 0.1 4.1 7.3
TCP-X 719 0.0 1 40 0.0 0.0 1.3
TCP-BGP 1 0.0 1 40 0.0 0.0 15.0
TCP-Frag 70399 0.0 1 688 0.0 0.0 22.7
TCP-other 47861004 11.8 1 211 18.9 0.0 1.3
UDP-DNS 582 0.0 4 73 0.0 3.4 15.4
UDP-NTP 287252 0.0 1 76 0.0 0.0 15.5
UDP-other 310347 0.0 2 230 0.1 0.6 15.9
ICMP 11674 0.0 3 61 0.0 19.8 15.5
IPv6INIP 15 0.0 1 1132 0.0 0.0 15.4
GRE 4 0.0 1 48 0.0 0.0 15.3
Total: 59957957 14.8 1 196 22.5 0.0 1.5
SrcIf SrcIPaddress DstIf DstIPaddress Pr SrcP DstP Pkts
Gi0/0 192.168.10.201 Gi0/1 192.168.60.102 01 0984 0800 9
Gi0/0 192.168.11.54 Gi0/1 192.168.60.158 01 0911 0000 4
Gi0/1 192.168.150.60 Gi0/0 10.89.16.226 06 0016 12CA 1
Gi0/0 192.168.13.97 Gi0/1 192.168.60.28 01 0B3E 0301 5
Gi0/0 192.168.10.17 Gi0/1 192.168.60.97 01 0B89 0030 1
Gi0/0 10.88.226.1 Gi0/1 192.168.202.22 11 007B 007B 1
Gi0/0 192.168.12.185 Gi0/1 192.168.60.239 01 0BD7 0200 7
Gi0/0 192.168.15.130 Gi0/1 192.168.60.239 01 0BD7 1100 3
Gi0/0 192.168.23.220 Gi0/1 192.168.60.239 01 0BD7 0300 11
Gi0/0 10.89.16.226 Gi0/1 192.168.150.60 06 12CA 0016 1
router#
In the preceding example, there are multiple flows for the following ICMP packet types: ICMP echo request (hex value 0800), echo-reply (hex value 0000), host-unreachable (hex value 0301), traceroute (hex value 0030), packet-too-big (hex value 0200), time-exceeded (hex value 1100), and unreachable (hex value 0300).
To view only the traffic flows for the aforementioned ICMP packet types, the command show ip cache flow | include SrcIf| _01_.*(0800|0000|0301|0030|0200|1100|0300)_ will display the related ICMP NetFlow records as shown here:
ICMP Flows
router#show ip cache flow | include SrcIf|_01_.*(0800|0000|0301|0030|0200|1100|0300)_ SrcIf SrcIPaddress DstIf DstIPaddress Pr SrcP DstP Pkts Gi0/0 192.168.10.201 Gi0/1 192.168.60.102 01 0984 0800 9 Gi0/0 192.168.11.54 Gi0/1 192.168.60.158 01 0911 0000 4 Gi0/0 192.168.13.97 Gi0/1 192.168.60.28 01 0B3E 0301 5 Gi0/0 192.168.10.17 Gi0/1 192.168.60.97 01 0B89 0030 1 Gi0/0 192.168.12.185 Gi0/1 192.168.60.239 01 0BD7 0200 7 Gi0/0 192.168.15.130 Gi0/1 192.168.60.239 01 0BD7 1100 3 Gi0/0 192.168.23.220 Gi0/1 192.168.60.239 01 0BD7 0300 11 router#
To protect the network from traffic that enters the network at ingress access points, which may include Internet connection points, partner and supplier connection points, or VPN connection points, administrators are advised to deploy tACLs to perform policy enforcement. Administrators can construct a tACL by explicitly permitting only authorized traffic to enter the network at ingress access points or permitting authorized traffic to transit the network in accordance with existing security policies and configurations. A tACL workaround cannot provide complete protection against this vulnerability when the attack originates from a trusted source address.
The tACL policy denies unauthorized ICMP packet types including echo request, echo-reply, host-unreachable, traceroute, packet-too-big, time-exceeded, and unreachable, that are sent to affected devices. In the following example, 192.168.60.0/24 is the IP address space that is used by the affected devices, and the host at 192.168.100.1 is considered a trusted source that requires access to the affected devices. Care should be taken to allow required traffic for routing and administrative access prior to denying all unauthorized traffic.
Additional information about tACLs is in Transit Access Control Lists: Filtering at Your Edge.
!
!-- Include explicit permit statements for trusted sources
!-- that require access on the vulnerable protocol
!
access-list tACL-Policy extended permit icmp host 192.168.100.1
192.168.60.0 255.255.255.0 echo
access-list tACL-Policy extended permit icmp host 192.168.100.1
192.168.60.0 255.255.255.0 echo-reply
access-list tACL-Policy extended permit icmp host 192.168.100.1
192.168.60.0 255.255.255.0 traceroute
access-list tACL-Policy extended permit icmp host 192.168.100.1
192.168.60.0 255.255.255.0 2
access-list tACL-Policy extended permit icmp host 192.168.100.1
192.168.60.0 255.255.255.0 time-exceeded
access-list tACL-Policy extended permit icmp host 192.168.100.1
192.168.60.0 255.255.255.0 unreachable
!
!-- The following vulnerability-specific access control entries
!-- (ACEs) can aid in identification of attacks
!
access-list tACL-Policy extended deny icmp any 192.168.60.0 255.255.255.0 echo
access-list tACL-Policy extended deny icmp any 192.168.60.0 255.255.255.0 echo-reply
access-list tACL-Policy extended deny icmp any 192.168.60.0 255.255.255.0 traceroute
access-list tACL-Policy extended deny icmp any 192.168.60.0 255.255.255.0 2
access-list tACL-Policy extended deny icmp any 192.168.60.0 255.255.255.0 time-exceeded
access-list tACL-Policy extended deny icmp any 192.168.60.0 255.255.255.0 unreachable
!
!-- Permit or deny all other Layer 3 and Layer 4 traffic in accordance
!-- with existing security policies and configurations
!
!-- Explicit deny for all other IP traffic
!
access-list tACL-Policy extended deny ip any any
!
!-- Apply tACL to interface(s) in the ingress direction
!
access-group tACL-Policy in interface outside
After the tACL has been applied to an interface, administrators can use the show access-list command to identify the number of ICMP packet types including echo request, echo-reply, host-unreachable, traceroute, packet-too-big, time-exceeded, and unreachable, that have been filtered. Administrators are advised to investigate filtered packets to determine whether they are attempts to exploit this vulnerability. Example output for show access-list tACL-Policy follows:
firewall#show access-list tACL-Policy
access-list tACL-Policy; 13 elements
access-list tACL-Policy line 1 extended permit icmp host 192.168.100.1
192.168.60.0 255.255.255.0 echo
access-list tACL-Policy line 2 extended permit icmp host 192.168.100.1
192.168.60.0 255.255.255.0 echo-reply
access-list tACL-Policy line 3 extended permit icmp host 192.168.100.1
192.168.60.0 255.255.255.0 traceroute
access-list tACL-Policy line 4 extended permit icmp host 192.168.100.1
192.168.60.0 255.255.255.0 2
access-list tACL-Policy line 5 extended permit icmp host 192.168.100.1
192.168.60.0 255.255.255.0 time-exceeded
access-list tACL-Policy line 6 extended permit icmp host 192.168.100.1
192.168.60.0 255.255.255.0 unreachable
access-list tACL-Policy line 7 extended deny icmp any
192.168.60.0 255.255.255.0 echo (hitcnt=9)
access-list tACL-Policy line 8 extended deny icmp any
192.168.60.0 255.255.255.0 echo-reply (hitcnt=12)
access-list tACL-Policy line 9 extended deny icmp any
192.168.60.0 255.255.255.0 traceroute (hitcnt=7)
access-list tACL-Policy line 10 extended deny icmp any
192.168.60.0 255.255.255.0 2 (hitcnt=11)
access-list tACL-Policy line 11 extended deny icmp any
192.168.60.0 255.255.255.0 time-exceeded (hitcnt=5)
access-list tACL-Policy line 12 extended deny icmp any
192.168.60.0 255.255.255.0 unreachable (hitcnt=8)
access-list tACL-Policy line 13 extended deny ip any any (hitcnt=17)
fireall#
In the preceding example, access list tACL-Policy has dropped the following packets received from an untrusted host or network:
Firewall syslog message 106023 will be generated for packets denied by an access control entry (ACE) that does not have the log keyword present. Additional information about this syslog message is in Cisco ASA 5500 Series System Log Message, 8.2 - 106023.
Information about configuring syslog for the Cisco ASA 5500 Series Adaptive Security Appliance is in Monitoring - Configuring Logging. Information about configuring syslog on the FWSM for Cisco Catalyst 6500 Series switches and Cisco 7600 Series routers is in Monitoring the Firewall Services Module.
In the following example, the show logging | grep regex command extracts syslog messages from the logging buffer on the firewall. These messages provide additional information about denied packets that could indicate potential attempts to exploit the vulnerability that is described in this document. It is possible to use different regular expressions with the grep keyword to search for specific data in the logged messages.
Additional information about regular expression syntax is in Creating a Regular Expression.
firewall#show logging | grep 106023
Jul 6 2011 00:15:13: %ASA-4-106023: Deny icmp src outside:192.0.2.18/2944
dst inside:192.168.60.191/2048 by access-group "tACL-Policy"
Jul 6 2011 00:15:13: %ASA-4-106023: Deny icmp src outside:192.2.0.200/2945
dst inside:192.168.60.33/0 by access-group "tACL-Policy"
Jul 6 2011 00:15:13: %ASA-4-106023: Deny icmp src outside:192.0.2.99/2946
dst inside:192.168.60.240/48 by access-group "tACL-Policy"
Jul 6 2011 00:15:13: %ASA-4-106023: Deny icmp src outside:192.0.2.100/2947
dst inside:192.168.60.115/512 by access-group "tACL-Policy"
Jul 6 2011 00:15:13: %ASA-4-106023: Deny icmp src outside:192.0.2.88/2949
dst inside:192.168.60.38/4352 by access-group "tACL-Policy"
Jul 6 2011 00:15:13: %ASA-4-106023: Deny icmp src outside:192.0.2.175/2950
dst inside:192.168.60.250/768 by access-group "tACL-Policy"
firewall#
In the preceding example, the messages logged for the tACL tACL-Policy show ICMP packet types echo request, echo-reply, traceroute, packet-too-big, time-exceeded, and unreachable sent to the address block assigned to affected devices.
Additional information about syslog messages for ASA security appliances is in Cisco ASA 5500 Series System Log Messages, 8.2. Additional information about syslog messages for the FWSM is in Catalyst 6500 Series Switch and Cisco 7600 Series Router Firewall Services Module Logging System Log Messages.
For additional information about investigating incidents using syslog events, reference the Identifying Incidents Using Firewall and IOS Router Syslog Events Applied Intelligence white paper.
Administrators can use Cisco Intrusion Prevention System (IPS) appliances and services modules to provide threat detection and help prevent attempts to exploit the vulnerability that is described in this document. Beginning with signature update S580 for sensors running Cisco IPS version 6.x and greater, the vulnerability can be detected by signature 38247/0 (Signature Name: Cisco Content Services Gateway Denial of Service). Signature 38247/0 is enabled by default, triggers a Medium severity event, has a signature fidelity rating (SFR) of 90, and is configured with a default event action of produce-alert.
Signature 38247/0 fires when multiple packets sent using ICMP are detected. Firing of this signature may indicate a potential exploit of the vulnerability.
Administrators can configure Cisco IPS sensors to perform an event action when an attack is detected. The configured event action performs preventive or deterrent controls to help protect against an attack that is attempting to exploit the vulnerability that is described in this document.
Cisco IPS sensors are most effective when deployed in inline protection mode combined with the use of an event action. Automatic Threat Prevention for Cisco IPS 6.x and greater sensors that are deployed in inline protection mode provides threat prevention against an attack that is attempting to exploit the vulnerability that is described in this document. Threat prevention is achieved through a default override that performs an event action for triggered signatures with a riskRatingValue greater than 90.
For additional information about the risk rating and threat rating calculation, reference Risk Rating and Threat Rating: Simplify IPS Policy Management.
The Cisco Security Monitoring, Analysis, and Response System (Cisco Security MARS) appliance can create incidents regarding events that are related to the vulnerability that is described in this document using IPS signature 38247/0 (Signature Name: Cisco Content Services Gateway Denial of Service). After the S580 dynamic signature update has been downloaded, using keyword NR-38247/0 for IPS signature 38247/0 and a query type of All Matching Events on the Cisco Security MARS appliance will provide a report that lists the incidents created by the IPS signature.
Beginning with the 4.3.1 and 5.3.1 releases of Cisco Security MARS appliances, support for the Cisco IPS dynamic signature updates feature has been added. This feature downloads new signatures from Cisco.com or from a local web server, correctly processes and categorizes received events that match those signatures, and includes them in inspection rules and reports. These updates provide event normalization and event group mapping, and they also enable the MARS appliance to parse new signatures from the IPS devices.
Caution: If dynamic signature updates are not configured, events that match these new signatures appear as unknown event type in queries and reports. Because MARS will not include these events in inspection rules, incidents may not be created for potential threats or attacks that occur within the network.
By default, this feature is enabled but requires configuration. If it is not configured, the following Cisco Security MARS rule will be triggered:
System Rule: CS-MARS IPS Signature Update Failure
When this feature is enabled and configured, administrators can determine the current signature version downloaded by MARS by selecting Help > About and reviewing the IPS Signature Version value.
Additional information about dynamic signature updates and instructions for configuring dynamic signature updates are available for the Cisco Security MARS 4.3.1 and 5.3.1 releases.
THIS DOCUMENT IS PROVIDED ON AN "AS IS" BASIS AND DOES NOT IMPLY ANY KIND OF GUARANTEE OR WARRANTY, INCLUDING THE WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR USE. YOUR USE OF THE INFORMATION ON THE DOCUMENT OR MATERIALS LINKED FROM THE DOCUMENT IS AT YOUR OWN RISK. CISCO RESERVES THE RIGHT TO CHANGE OR UPDATE THIS DOCUMENT AT ANY TIME.
|
Revision 1.0 |
2011-July-06 |
Initial public release |
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