Cisco Applied Mitigation Bulletin-
This Applied Mitigation Bulletin is a companion document to the PSIRT Security Advisory Multiple Vulnerabilities in Cisco Intrusion Prevention System Software and provides identification and mitigation techniques that administrators can deploy on Cisco network devices.
-
There are multiple vulnerabilities in the Cisco Intrusion Prevention System. The following subsections summarize these vulnerabilities:
Cisco IPS Software Malformed IP Packets Denial of Service Vulnerability: This vulnerability can be exploited remotely without authentication and without end-user interaction. Successful exploitation of this vulnerability could result 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 malformed IPv4 packets directed to the management interface. An attacker could exploit this vulnerability using spoofed packets.
This vulnerability has been assigned Common Vulnerabilities and Exposures (CVE) identifier CVE-2013-1243.
Cisco IPS NME Malformed IP Packets Denial Of Service Vulnerability: This vulnerability can be exploited remotely without authentication and without end-user interaction. Successful exploitation of this vulnerability could result 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 malformed IP packets sent to the management interface. An attacker could exploit this vulnerability using spoofed packets.
This vulnerability has been assigned the CVE identifier CVE-2013-3410.
Cisco IDSM-2 Malformed TCP Packets Denial Of Service Vulnerability: This vulnerability can be exploited remotely without authentication and without end-user interaction. Successful exploitation of this vulnerability could result 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 malformed TCP packets sent to the management interface. An attacker could exploit this vulnerability using spoofed packets.
This vulnerability has been assigned the CVE identifier CVE-2013-3411.
-
Information about vulnerable, unaffected, and fixed software is available in the Cisco Security Advisory, which is available at the following link:
https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20130717-ips
-
Cisco devices provide several countermeasures for these vulnerabilities. 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 the following methods:- Transit access control lists (tACLs)
- Unicast Reverse Path Forwarding (uRPF)
- IP source guard (IPSG)
The proper deployment and configuration of uRPF provides an effective means of protection against attacks that use packets with spoofed source IP addresses. uRPF should be deployed as close to all traffic sources as possible.
The proper deployment and configuration of IPSG provides an effective means of protection against spoofing attacks at the access layer.
Effective means of exploit prevention can also be provided by the Cisco ASA 5500 Series Adaptive Security Appliance, Cisco Catalyst 6500 Series ASA Services Module (ASASM), and the Firewall Services Module (FWSM) for Cisco Catalyst 6500 Series Switches and Cisco 7600 Series Routers using the following:- tACLs
- uRPF
Cisco IOS Software, Cisco ASA, Cisco ASASM, and Cisco FWSM firewalls, can provide visibility through syslog messages and counter values displayed in the output from show commands.
The Cisco Security Manager 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 these vulnerabilities. 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:- Cisco IOS Routers and Switches
- Cisco ASA, Cisco ASASM, and Cisco FWSM Firewalls
- Cisco Security Manager
Cisco IOS Routers and Switches
Mitigation: Transit Access Control Lists
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 these vulnerabilities when the attack originates from a trusted source address.
The tACL policy denies unauthorized IPv4 packets that are sent to affected devices. In the following example, 192.168.60.0/24 represents the IP address space that is used by the affected devices, and the host 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 management ports ! access-list 150 permit tcp host 192.168.100.1 192.168.60.0 0.0.0.255 eq 22 access-list 150 permit tcp host 192.168.100.1 192.168.60.0 0.0.0.255 eq 80 access-list 150 permit tcp host 192.168.100.1 192.168.60.0 0.0.0.255 eq 443 ! !-- 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 192.168.60.0 0.0.0.255 access-list 150 deny ip any any ! !-- Apply tACLs 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.Identification: Transit Access Control Lists
After the administrator applies the tACL to an interface, the show ip access-lists command will identify the number of IPv4 packets on that have been filtered. Administrators are advised to investigate filtered packets to determine whether they are attempts to exploit these vulnerabilities. Example output for show ip access-lists 150 follows:router#show ip access-lists 150 Extended IP access list 150 10 permit tcp host 192.168.100.1 192.168.60.0 0.0.0.255 eq 22 20 permit tcp host 192.168.100.1 192.168.60.0 0.0.0.255 eq 80 30 permit tcp host 192.168.100.1 192.168.60.0 0.0.0.255 eq 443 40 deny ip any 192.168.60.0 0.0.0.255 (19 matches) 50 deny ip any any router#In the preceding example, access list 150 has dropped 19 IPv4 packets for access control list entry (ACE) line 40.
For additional information about investigating incidents using ACE counters and syslog events, reference the Identifying Incidents Using Firewall and IOS Router Syslog Events Cisco Security Intelligence Operations white paper.
Administrators can use Embedded Event Manager to provide instrumentation when specific conditions are met, such as ACE counter hits. The Cisco Security Intelligence Operations white paper Embedded Event Manager in a Security Context provides additional details about how to use this feature.Identification: Access List Logging
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 IPv4 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 Cisco Security Intelligence Operations white paper.Mitigation: Spoofing Protection
Unicast Reverse Path Forwarding
The vulnerabilities that are described in this document can be exploited by spoofed IP packets. Administrators can deploy and configure Unicast Reverse Path Forwarding (uRPF) as a protection mechanism against spoofing.
uRPF is configured at the interface level and can detect and drop packets that lack a verifiable source IP address. Administrators should not rely on uRPF to provide complete spoofing protection because spoofed packets may enter the network through a uRPF-enabled interface if an appropriate return route to the source IP address exists. Administrators are advised to take care to ensure that the appropriate uRPF mode (loose or strict) is configured during the deployment of this feature because it can drop legitimate traffic that is transiting the network. In an enterprise environment, uRPF may be enabled at the Internet edge and the internal access layer on the user-supporting Layer 3 interfaces.
Additional information is in the Unicast Reverse Path Forwarding Loose Mode Feature Guide.
For additional information about the configuration and use of uRPF, reference the Understanding Unicast Reverse Path Forwarding Cisco Security Intelligence Operations white paper.IP Source Guard
IP source guard (IPSG) is a security feature that restricts IP traffic on nonrouted, Layer 2 interfaces by filtering packets based on the DHCP snooping binding database and manually configured IP source bindings. Administrators can use IPSG to prevent attacks from an attacker who attempts to spoof packets by forging the source IP address and/or the MAC address. When properly deployed and configured, IPSG coupled with strict mode uRPF provides the most effective means of spoofing protection for the vulnerabilities that are described in this document.
Additional information about the deployment and configuration of IPSG is in Configuring DHCP Features and IP Source Guard.Identification: Spoofing Protection Using Unicast Reverse Path Forwarding
With uRPF properly deployed and configured throughout the network infrastructure, administrators can use the show cef interface type slot/port internal, show ip interface, show cef drop, show ip cef switching statistics feature, and show ip traffic commands to identify the number of packets that uRPF has dropped.
Note: Beginning with Cisco IOS Software Release 12.4(20)T, the command show ip cef switching has been replaced by show ip cef switching statistics feature.
Note: The show command | begin regex and show command | include regex command modifiers are used in the following examples to minimize the amount of output that administrators will need to parse to view the desired information. Additional information about command modifiers is in the show command sections of the Cisco IOS Configuration Fundamentals Command Reference.router#show cef interface GigabitEthernet 0/0 internal | include drop
ip verify: via=rx (allow default), acl=0, drop=18, sdrop=0 router#
Note: show cef interface type slot/port internal is a hidden command that must be fully entered at the command-line interface. Command completion is not available for it.router#show cef drop CEF Drop Statistics Slot Encap_fail Unresolved Unsupported No_route No_adj ChkSum_Err RP 27 0 0 18 0 0 router#
router#show ip interface GigabitEthernet 0/0 | begin verify IP verify source reachable-via RX, allow default, allow self-ping 18 verification drops 0 suppressed verification drops router#
router#show ip cef switching statistics feature IPv4 CEF input features:
In the preceding show cef interface type slot/port internal, show cef drop, show ip interface type slot/port show ip cef switching statistics feature, and show ip traffic examples, uRPF has dropped 18 IP packets received globally on all interfaces with uRPF configured because of the inability to verify the source address of the IP packets within the forwarding information base of Cisco Express Forwarding.
Path Feature Drop Consume Punt Punt2Host Gave route
RP PAS uRPF 18 0 0 0 0 Total 18 0 0 0 0 -- CLI Output Truncated -- router# router#show ip traffic | include RPF 18 no route, 18 unicast RPF, 0 forced drop router#Cisco ASA, Cisco ASASM, and Cisco FWSM Firewalls
Mitigation: Transit Access Control Lists
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 these vulnerabilities when the attack originates from a trusted source address.
The tACL policy denies unauthorized IPv4 packets that are sent to affected devices. In the following example, 192.168.60.0/24 represents the IP address space that is used by the affected devices, and the hosts at 192.168.100.1 is considered trusted sources that require 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 protocols and ports ! access-list tACL-Policy extended permit tcp host 192.168.100.1 192.168.60.0 255.255.255.0 eq 22 access-list tACL-Policy extended permit tcp host 192.168.100.1 192.168.60.0 255.255.255.0 eq 80 access-list tACL-Policy extended permit tcp host 192.168.100.1 192.168.60.0 255.255.255.0 eq 443 ! !-- Permit or deny all other Layer 3 and Layer 4 traffic in accordance !-- with existing security policies and configurations ! access-list tACL-Policy extended deny ip any 192.168.60.0 255.255.255.0 ! !-- Explicit deny for all other IP traffic ! access-list tACL-Policy extended deny ip any any ! !-- Apply tACLs to interfaces in the ingress direction ! access-group tACL-Policy in interface outside
Identification: Transit Access Control Lists
After the tACL has been applied to an interface, administrators can use the show access-list command to identify the number of IPv4 packets that have been filtered. Administrators are advised to investigate filtered packets to determine whether they are attempts to exploit these vulnerabilities. Example output for show access-list tACL-Policy follows:firewall#show access-list tACL-Policy access-list tACL-Policy; 5 elements; name hash: 0x3452703d access-list tACL-Policy line 1 extended permit tcp host 192.168.100.1 192.168.60.0 255.255.255.0 eq ssh (hitcnt=31) access-list tACL-Policy line 2 extended permit tcp host 192.168.100.1 192.168.60.0 255.255.255.0 eq www (hitcnt=11) access-list tACL-Policy line 3 extended permit tcp host 192.168.100.1 192.168.60.0 255.255.255.0 eq https (hitcnt=61) access-list tACL-Policy line 4 extended deny ip any 192.168.60.0 255.255.255.0 (hitcnt=14) access-list tACL-Policy line 5 extended deny ip any any (hitcnt=8)In the preceding example, access list tACL-Policy has dropped 14 packets received from an untrusted host or network. In addition, syslog message 106023 can provide valuable information, which includes the source and destination IP address, the source and destination port numbers, and the IP protocol for the denied packet.Identification: Firewall Access List Syslog Messages
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 Cisco Catalyst 6500 Series ASA Services Module is in 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 vulnerabilities that are 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 10 2013 00:15:13: %ASA-4-106023: Deny tcp src outside:192.0.2.200/2945 dst inside:192.168.60.33/ssh by access-group "tACL-Policy" Jul 10 2013 00:15:13: %ASA-4-106023: Deny tcp src outside:192.0.2.99/2946 dst inside:192.168.60.240/www by access-group "tACL-Policy" Jul 10 2013 00:15:13: %ASA-4-106023: Deny tcp src outside:192.0.2.100/2947 dst inside:192.168.60.115/https by access-group "tACL-Policy" firewall#In the preceding example, the messages logged for the tACL tACL-Policy show potentially spoofed packets sent to the address block assigned to the infrastructure devices.
Additional information about syslog messages for Cisco ASA Series Adaptive Security Appliances is in Cisco ASA 5500 Series System Log Messages, 8.2. Additional information about syslog messages for Cisco Catalyst 6500 Series ASA Services Module is in the Analyzing Syslog Messages section of the Cisco ASASM CLI Configuration Guide. Additional information about syslog messages for the Cisco 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 Cisco Security Intelligence Operations white paper.
Mitigation: Spoofing Protection Using Unicast Reverse Path Forwarding
The vulnerabilities that are described in this document can be exploited by spoofed IP packets. Administrators can deploy and configure uRPF as a protection mechanism against spoofing.
uRPF is configured at the interface level and can detect and drop packets that lack a verifiable source IP address. Administrators should not rely on uRPF to provide complete spoofing protection because spoofed packets may enter the network through a uRPF-enabled interface if an appropriate return route to the source IP address exists. In an enterprise environment, uRPF may be enabled at the Internet edge and at the internal access layer on the user-supporting Layer 3 interfaces.
For additional information about the configuration and use of uRPF, reference the Cisco Security Appliance Command Reference for ip verify reverse-path and the Understanding Unicast Reverse Path Forwarding Cisco Security Intelligence Operations white paper.Identification: Spoofing Protection Using Unicast Reverse Path Forwarding
Firewall syslog message 106021 will be generated for packets denied by uRPF. Additional information about this syslog message is in Cisco ASA 5500 Series System Log Message, 8.2 - 106021.
Information about configuring syslog for the Cisco ASA 5500 Series Adaptive Security Appliance is in Monitoring - Configuring Logging. Information about configuring syslog for the Cisco Catalyst 6500 Series ASA Services Module is in 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 vulnerabilities that are 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 106021 Jul 10 2013 00:15:13: %ASA-1-106021: Deny TCP reverse path check from 192.168.60.1 to 192.168.60.100 on interface outside Jul 10 2013 00:15:13: %ASA-1-106021: Deny TCP reverse path check from 192.168.60.1 to 192.168.60.100 on interface outside Jul 10 2013 00:15:13: %ASA-1-106021: Deny UDP reverse path check from 192.168.60.1 to 192.168.60.100 on interface outsideThe show asp drop command can also identify the number of packets that the uRPF feature has dropped, as shown in the following example:firewall#show asp drop frame rpf-violated Reverse-path verify failed 11 firewall#
In the preceding example, uRPF has dropped 11 IP packets received on interfaces with uRPF configured. Absence of output indicates that the uRPF feature on the firewall has not dropped packets.
For additional information about debugging accelerated security path dropped packets or connections, reference the Cisco Security Appliance Command Reference for show asp drop.Cisco Security Manager
Identification: Cisco Security Manager
Beginning in software version 4.0, Cisco Security Manager can collect syslogs from Cisco firewalls and Cisco IPS devices and provides the Event Viewer, which can query for events that are related to the vulnerabilities that are described in this document.Using the following filters in the Firewall Denied Events predefined view in the Event Viewer provides all captured Cisco firewall access list deny syslog messages that could indicate potential attempts to exploit the vulnerabilities that are described in this document.- Use the Destination event filter to filter network objects that contain the IP address space that is used by the affected devices (for example, IPv4 address range 192.168.60.0/24)
- ASA-4-106021 (uRPF spoofing)
- ASA-4-106023 (ACL deny)
Identification: Event Management System Partner Events
Cisco works with industry-leading Security Information and Event Management (SIEM) companies through the Cisco Developer Network. This partnership helps Cisco deliver validated and tested SIEM systems that address business concerns such as long-term log archiving and forensics, heterogeneous event correlation, and advanced compliance reporting. Security Information and Event Management partner products can be leveraged to collect events from Cisco devices and then query the collected events for the incidents created by a Cisco IPS signature or deny syslog messages from firewalls that could indicate potential attempts to exploit the vulnerabilities that are described in this document. The queries can be made by Sig ID and Syslog ID as shown in the following list:- ASA-4-106021 (uRPF spoofing)
- ASA-4-106023 (ACL deny)
-
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.
-
Show LessVersion Description Section Date 1 Initial Release
2013-July-17 16:03 GMT
-
Complete information on reporting security vulnerabilities in Cisco products, obtaining assistance with security incidents, and registering to receive security information from Cisco, is available on Cisco's worldwide website at https://sec.cloudapps.cisco.com/security/center/resources/security_vulnerability_policy.html. This includes instructions for press inquiries regarding Cisco security notices. All Cisco security advisories are available at http://www.cisco.com/go/psirt.
-
The security vulnerability applies to the following combinations of products.
Primary Products Cisco Intrusion Prevention System (IPS) 5.0 ((1), (2), (3), (4), (5), (6)) | 5.1 ((1), (1a), (1b), (1c), (1d), (1e), (p1), (2), (5), (6)) | 6.0 (Base, (2), (3), (4)) | 6.1 (Base) | 7.0 (Base) | 7.1 (Base)
Associated Products
-
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 ALERTS AT ANY TIME.
A standalone copy or paraphrase of the text of this document that omits the distribution URL is an uncontrolled copy and may lack important information or contain factual errors. The information in this document is intended for end users of Cisco products