Products & Services
Support

Product Categories


Popular Downloads


Manage Software

How to Buy

For Home

Linksys Products Store
Linksys is now part of Belkin
Products for everyone

All Ordering Options

Training & Events Partners
Guest

Cisco Applied Mitigation Bulletin

Identifying and Mitigating Exploitation of the Multiple DLSw Denial of Service Vulnerabilities in Cisco IOS

Advisory ID: cisco-amb-20080326-dlsw

http://tools.cisco.com/security/center/content/CiscoAppliedMitigationBulletin/cisco-amb-20080326-dlsw

Revision 1.1

For Public Release 2008 March 26 16:00  UTC (GMT)

Related Resources:

View related Security Advisory


Contents

Cisco Response
Device-Specific Mitigation and Identification
Additional Information
Revision History
Cisco Security Procedures
Related Information

Cisco Response

This Applied Mitigation Bulletin is a companion document to the PSIRT Security Advisory Multiple DLSw Denial of Service Vulnerabilities in Cisco IOS and provides identification and mitigation techniques that administrators can deploy on Cisco network devices.

Vulnerability Characteristics

The Cisco IOS Software contains a vulnerability processing specially crafted Data-Link Switching (DLSw) packets. This vulnerability can be exploited remotely without authentication and without end-user interaction. Successful exploitation of this vulnerability may result in a denial of service (DoS) condition. Repeated attempts to exploit this vulnerability could result in a sustained DoS condition.

The attack vectors for exploitation are through packets using the following port and protocol:

  • DLSw using UDP port 2067
  • DLSw using IP protocol 91

An attacker could exploit this vulnerability using spoofed packets.

This vulnerability has been assigned CVE identifier CVE-2008-1152.

Information about vulnerable, unaffected, and fixed software is available in the PSIRT Security Advisory, which is available at the following link: http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20080326-dlsw.

Mitigation Technique Overview

Cisco devices provide several countermeasures for the DLSw 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.

Cisco IOS Software can provide effective means of exploit prevention using the following methods:

  • Infrastructure access control lists (iACLs)
  • Unicast Reverse Path Forwarding (Unicast RPF)
  • IP source guard (IPSG)

These protection mechanisms filter and drop, as well as verify the source IP address of, packets that are attempting to exploit the vulnerabilities described in this document.

The proper deployment and configuration of Unicast RPF provides the most effective means of protection against attacks that use packets with spoofed source IP addresses. Unicast RPF should be deployed as close to all traffic sources as possible.

The proper deployment and configuration of IPSG provides the most effective means of protection against attacks with spoofed source MAC addresses.

Effective means of exploit prevention can also be provided by the Cisco ASA 5500 Series Adaptive Security Appliance, the Cisco PIX 500 Series Security Appliance, and the Firewall Services Module (FWSM) for Cisco Catalyst 6500 Series switches and Cisco 7600 Series routers using the following:

  • Transit Access Control Lists (tACLs)
  • Unicast RPF

These protection mechanisms filter and drop, as well as verify the source IP address of, packets that are attempting to exploit the vulnerabilities described in this document.

Effective use of Cisco Intrusion Prevention System (IPS) event actions provides visibility into and protection against attacks that attempt to exploit this vulnerability as discussed later in this document.

The proper deployment and configuration of Unicast RPF provides the most effective means of protection against attacks that use packets with spoofed source IP addresses. Unicast RPF should be deployed as close to all traffic sources as possible.

Cisco IOS NetFlow can provide visibility into these exploitation attempts using flow records.

Cisco IOS Software, Cisco ASA, Cisco PIX security appliances, and FWSM firewalls can provide visibility through syslog messages and the 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.

Risk Management

Organizations should 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 in Information Security Engagements can help organizations develop repeatable security evaluation and response processes.

Device-Specific Mitigation and Identification

caution Caution: The effectiveness of any mitigation technique is dependent 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.

Cisco IOS Routers and Switches

Mitigation: Infrastructure Access Control Lists

In an effort to protect infrastructure devices and minimize the risk, impact, and effectiveness of direct infrastructure attacks, administrators should deploy infrastructure access control lists (iACLs) to perform policy enforcement of traffic sent to infrastructure equipment. Administrators can construct an iACL by explicitly permitting only authorized traffic sent to infrastructure devices in accordance with existing security policies and configurations. For the maximum protection of infrastructure devices, deployed iACLs should be applied in the ingress direction on all interfaces to which an IP address has been configured.

In the following example, the address block 192.168.1.0/24 is the infrastructure address space and the host at 192.168.100.1 is considered a trusted endpoint. The iACL policy denies untrusted DLSw packets on UDP port 2067 and IP Protocol 91 sent to addresses that are part of the infrastructure address space. Care should be taken to allow required traffic for routing and administrative access prior to denying all traffic sent directly to infrastructure devices. Whenever possible, infrastructure address space should be distinct from the address space used for user and services segments. Using this addressing methodology will assist with the construction and deployment of iACLs.

Additional information about iACLs is available in Protecting Your Core: Infrastructure Protection Access Control Lists.

ip access-list extended Infrastructure-ACL-Policy
  
  !
  !--- When applicable, include explicit permit statements for trusted
  !--- sources that require access on the vulnerable port and protocol
  !

  permit udp host 192.168.100.1 192.168.1.0 0.0.0.255 eq 2067
  permit 91 host 192.168.100.1 192.168.1.0 0.0.0.255
  
  !
  !--- The following vulnerability-specific access control entries
  !--- (ACEs) can aid in identification of attacks
  !

  deny udp any 192.168.1.0 0.0.0.255 eq 2067
  deny 91 any 192.168.1.0 0.0.0.255
  
  !
  !--- Explicit deny ACE for traffic sent to addresses configured within
  !--- the infrastructure address space
  !

  deny ip any 192.168.1.0 0.0.0.255
  
  !
  !--- Permit/deny all other Layer 3 and Layer 4 traffic in accordance
  !--- with existing security policies and configurations
  !
!--- Apply iACL to interfaces in the ingress direction

interface GigabitEthernet0/0
 ip access-group Infrastructure-ACL-Policy in

!

Mitigation: Spoofing Protection

Unicast Reverse Path Forwarding

All vulnerabilities described in this document can be exploited by spoofed IP packets. Administrators can deploy and configure Unicast RPF as a protection mechanism against spoofing.

Unicast RPF is configured at the interface level and can detect and drop packets that lack a verifiable source IP address. Administrators should not rely on Unicast RPF to provide 100 percent spoofing protection because spoofed packets may enter the network through a Unicast RPF-enabled interface if an appropriate return route to the source IP address exists. Administrators should take care to ensure that the appropriate Unicast RPF 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, Unicast RPF might be enabled at the Internet edge and the internal access layer on the user-supporting Layer 3 interfaces.

Additional information is available in the Unicast Reverse Path Forwarding Loose Mode Feature Guide.

For additional information about the configuration and use of Unicast RPF, reference the Understanding Unicast Reverse Path Forwarding Applied Intelligence 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 Unicast RPF provides the most effective means of spoofing protection for the vulnerabilities described in this document.

Additional information about the deployment and configuration of IPSG is available in Configuring DHCP Features and IP Source Guard.

Identification: Infrastructure Access Control Lists

After the administrator applies the iACL to an interface, the show ip access-lists command will identify the number of DLSw packets on UDP port 2067 and IP protocol 91 that have been filtered on interfaces on which the iACL is applied. Administrators should investigate filtered packets to determine whether they are attempts to exploit these vulnerabilities. Example output for show ip access-lists Infrastructure-ACL-Policy follows:

router#show ip access-lists Infrastructure-ACL-Policy
Extended IP access list Infrastructure-ACL-Policy
    10 permit udp host 192.168.100.1 192.168.1.0 0.0.0.255 eq 2067
    20 permit 91 host 192.168.100.1 192.168.1.0 0.0.0.255
    30 deny udp any 192.168.1.0 0.0.0.255 eq 2067 (31 matches)
    40 deny 91 any 192.168.1.0 0.0.0.255 (3 matches)
    50 deny ip any 192.168.1.0 0.0.0.255
router#

In the preceding example, the access list Infrastructure-ACL-Policy has dropped 31 DLSw packets on UDP port 2067 for access control entry (ACE) sequence ID 30 and 3 DLSw packets on IP protocol 91 for ACE sequence ID 40.

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.

Identification: Access List Logging

The log or 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 Caution: 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.

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.

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.

For additional information about the configuration and use of ACL logging, reference the Understanding Access Control List Logging Applied Intelligence white paper.

Identification: Spoofing Protection Using Unicast Reverse Path Forwarding

With Unicast RPF properly deployed and configured throughout the network infrastructure, administrators can use the show ip interface, show cef drop, show cef interface type slot/port internal, and show ip traffic commands to identify the number of packets that Unicast RPF has dropped.

NOTE: The show command | begin regexp and show command | include regexp command modifiers are used in the following examples to minimize the amount of output that administrators need to parse to view the desired information. Additional information about command modifiers is available in the "show command" sections of the Cisco IOS Configuration Fundamentals Command Reference.

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 ip interface GigabitEthernet 0/0 | begin verify
    --      CLI Output Truncated       --  
        IP verify source reachable-via RX, allow default, allow self-ping
  18 verification drops
  0 suppressed verification drops
router#


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
IPv6 CEF Drop Statistics
Slot  Encap_fail  Unresolved Unsupported    No_route      No_adj
RP             0           0           0           3           0
router#


router#show cef interface GigabitEthernet 0/0 internal | include drop
      --      CLI Output Truncated       --
  ip verify: via=rx (allow default), acl=0, drop=18, sdrop=0, allow self-ping
router#


router#show ip traffic
IP statistics:
  Rcvd:  68051015 total, 2397325 local destination
         43999 format errors, 0 checksum errors, 33 bad hop count
         2 unknown protocol, 929 not a gateway
         21 security failures, 190123 bad options, 542768 with options
  Opts:  352227 end, 452 nop, 36 basic security, 1 loose source route
         45 timestamp, 59 extended security, 41 record route
         53 stream ID, 3 strict source route, 40 alert, 45 cipso, 0 ump
         361634 other
  Frags: 0 reassembled, 10008 timeouts, 56866 couldn't reassemble
         0 fragmented, 0 fragments, 0 couldn't fragment
  Bcast: 64666 received, 0 sent
  Mcast: 1589885 received, 2405454 sent
  Sent:  3001564 generated, 65359134 forwarded
  Drop:  4256 encapsulation failed, 0 unresolved, 0 no adjacency
         18 no route, 18 unicast RPF, 0 forced drop
         0 options denied
  Drop:  0 packets with source IP address zero
  Drop:  0 packets with internal loop back IP address 
    --      CLI Output Truncated       --
router#

In the preceding examples, Unicast RPF has dropped 18 IP packets received globally on all interfaces with Unicast RPF configured because of the inability to verify the source address of the IP packets within the Cisco Express Forwarding Forwarding Information Base.

Cisco IOS NetFlow

Identification: Traffic Flow Identification Using NetFlow Records

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 vulnerabilities described in this document. Administrators should investigate flows to determine whether they are attempts to exploit these vulnerabilities or whether they are legitimate traffic flows.

router#sh ip cache flow
IP packet size distribution (505 total packets):
   1-32   64   96  128  160  192  224  256  288  320  352  384  416  448  480
   .667 .332 .000 .000 .000 .000 .000 .000 .000 .000 .000 .000 .000 .000 .000

    512  544  576 1024 1536 2048 2560 3072 3584 4096 4608
   .000 .000 .000 .000 .000 .000 .000 .000 .000 .000 .000

IP Flow Switching Cache, 278544 bytes
  70 active, 4026 inactive, 70 added
  197 ager polls, 0 flow alloc failures
  Active flows timeout in 30 minutes
  Inactive flows timeout in 15 seconds
IP Sub Flow Cache, 21640 bytes
  0 active, 1024 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

SrcIf         SrcIPaddress    DstIf         DstIPaddress    Pr SrcP DstP  Pkts
Et0/0         192.168.152.37  Et0/1         192.168.251.82  11 343E B392     4 
Et0/0         192.168.247.249 Et0/1         192.168.1.242   11 9D61 0813     3
Et0/0         192.168.77.108  Et0/1         192.168.1.125   5B 0000 0000    18 
Et0/0         192.168.113.177 Et0/1         192.168.161.228 06 F100 B1D8     3 
Et0/0         192.168.105.207 Et0/1         192.168.142.166 06 74C3 7E5B     5 
Et0/0         192.168.77.74   Et0/1         192.168.1.180   11 D2D5 0813     1 
Et0/0         192.168.194.57  Et0/1         192.168.130.208 06 14DE 0B23     1 
Et0/0         192.168.238.224 Et0/1         192.168.1.56    5B 0000 0000     7 
Et0/0         192.168.158.234 Et0/1         192.168.224.187 11 9685 0FD3    12 
Et0/0         192.168.222.183 Et0/1         192.168.116.253 06 5D8A 27B6     9 
Et0/0         192.168.250.25  Et0/1         192.168.14.45   11 DE99 50B9     5 
Et0/0         192.168.112.88  Et0/1         192.168.1.173   11 ED18 0813     3 
Et0/0         192.168.105.65  Et0/1         192.168.113.241 11 1237 4E9E    11 
Et0/0         192.168.191.120 Et0/1         192.168.7.220   06 82FD B289     4 
Et0/0         192.168.224.144 Et0/1         192.168.141.8   11 E2AA 8792     1 
Et0/0         192.168.158.190 Et0/1         192.168.87.107  06 38FC 0893     9 
Et0/0         192.168.214.41  Et0/1         192.168.141.250 06 9982 9B7D     3 
Et0/0         192.168.221.146 Et0/1         192.168.1.117   11 E533 0813    43 
Et0/0         192.168.147.62  Et0/1         192.168.15.36   06 7E8A 995E    11 
Et0/0         192.168.232.56  Et0/1         192.168.180.29  11 4DDB DB96     1 
Et0/0         192.168.136.210 Et0/1         192.168.237.139 11 6BCA C7F5     8 
Et0/0         192.168.121.156 Et0/1         192.168.7.5     06 B85E 9C8F    11 
Et0/0         192.168.15.44   Et0/1         192.168.1.215   5B 0000 0000    10 
Et0/0         192.168.50.19   Et0/1         192.168.132.67  06 257A 0FDB     7 
router# 

In the preceding example, there are multiple flows for DLSw on UDP port 2067 (hex value 0813) and IP protocol 91 (hex value 5B). Administrators should compare these flows to baseline utilization for DLSw traffic sent on UDP port 2067 and IP protocol 91 and also investigate the flows to determine whether they are sourced from untrusted hosts or networks.

The command show ip cache flow | include SrcIf|_11_.*0813 will display only the traffic flows for packets using UDP port 2067 (hex value 0813), as shown here:

router#sh ip cache flow | include SrcIf|_11_.*0813
SrcIf         SrcIPaddress    DstIf         DstIPaddress    Pr SrcP DstP  Pkts
Et0/0         192.168.224.74  Et0/1         192.168.1.234   11 EC58 0813     5 
Et0/0         192.168.154.168 Et0/1         192.168.1.65    11 AAEB 0813    20 
Et0/0         192.168.56.187  Et0/1         192.168.1.218   11 821B 0813    16 
Et0/0         192.168.247.204 Et0/1         192.168.1.5     11 99A7 0813     2 
Et0/0         192.168.40.228  Et0/1         192.168.1.153   11 90D2 0813    21 
Et0/0         192.168.57.65   Et0/1         192.168.1.241   11 31B9 0813     1 
Et0/0         192.168.197.164 Et0/1         192.168.1.112   11 B2ED 0813     3 
Et0/0         192.168.204.123 Et0/1         192.168.1.10    11 68BF 0813    14 
Et0/0         192.168.140.244 Et0/1         192.168.1.106   11 9926 0813    10 
Et0/0         192.168.52.40   Et0/1         192.168.1.11    11 A47F 0813    12 
Et0/0         192.168.124.93  Et0/1         192.168.1.40    11 2E94 0813     1 
router#

The command show ip cache flow | include SrcIf|_5B_ will display only the traffic flows for packets using IP protocol 91 (hex value 5B), as shown here:

router#sh ip cache flow | include SrcIf|_5B_
SrcIf         SrcIPaddress    DstIf         DstIPaddress    Pr SrcP DstP  Pkts
Et0/0         192.168.29.18   Et0/1         192.168.1.134   5B 0000 0000     3 
Et0/0         192.168.163.226 Et0/1         192.168.1.218   5B 0000 0000     3 
Et0/0         192.168.248.187 Et0/1         192.168.1.83    5B 0000 0000     8 
Et0/0         192.168.58.79   Et0/1         192.168.1.66    5B 0000 0000    25 
Et0/0         192.168.68.125  Et0/1         192.168.1.115   5B 0000 0000    19 
Et0/0         192.168.81.227  Et0/1         192.168.1.151   5B 0000 0000    16 
Et0/0         192.168.38.68   Et0/1         192.168.1.178   5B 0000 0000     1 
Et0/0         192.168.215.242 Et0/1         192.168.1.127   5B 0000 0000    16 
Et0/0         192.168.0.63    Et0/1         192.168.1.93    5B 0000 0000     1 
router#

Cisco ASA, PIX, and FWSM Firewalls

Mitigation: Transit Access Control Lists

In an effort 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 should 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.

The tACL policy denies unauthorized DLSw packets on UDP port 2067 and IP protocol 91 sent to affected devices. In the following example, 192.168.1.0/24 is the network IP address space 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 available in Transit Access Control Lists: Filtering at Your Edge.


!
!--- Include any explicit permit statements for trusted sources
!--- that require access on the vulnerable port and protocol
!

access-list Transit-ACL-Policy extended permit udp host 192.168.100.1 192.168.1.0 255.255.255.0 eq 2067
access-list Transit-ACL-Policy extended permit 91 host 192.168.100.1 192.168.1.0 255.255.255.0

!
!--- The following vulnerability-specific access control entries
!--- (ACEs) can aid in identification of attacks
!

access-list Transit-ACL-Policy extended deny udp any 192.168.1.0 255.255.255.0 eq 2067
access-list Transit-ACL-Policy extended deny 91 any 192.168.1.0 255.255.255.0

!
!--- Permit/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 Transit-ACL-Policy extended deny ip any any

!
!--- Apply tACL to interfaces in the ingress direction
!

access-group Transit-ACL-Policy in interface outside

!

Mitigation: Spoofing Protection Using Unicast Reverse Path Forwarding

All vulnerabilities described in this document can be exploited by spoofed IP packets. Administrators can deploy and configure Unicast RPF as a protection mechanism against spoofing.

Unicast RPF is configured at the interface level and can detect and drop packets that lack a verifiable source IP address. Administrators should not rely on Unicast RPF to provide 100 percent spoofing protection because spoofed packets may enter the network through a Unicast RPF-enabled interface if an appropriate return route to the source IP address exists. In an enterprise environment, Unicast RPF might 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 Unicast RPF, reference the Cisco Security Appliance Command Reference for ip verify reverse-path and the Understanding Unicast Reverse Path Forwarding Applied Intelligence white paper.

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 DLSw packets on UDP port 2067 and IP protocol 91 that have been filtered. Administrators should investigate filtered packets to determine whether they are attempts to exploit these vulnerabilities. Example output for show access-list Transit-ACL-Policy follows:

firewall#show access-list Transit-ACL-Policy
access-list Transit-ACL-Policy; 5 elements
access-list Transit-ACL-Policy line 1 extended permit udp host 192.168.100.1 192.168.1.0 255.255.255.0 eq 2067 (hitcnt=321)
access-list Transit-ACL-Policy line 2 extended permit 91 host 192.168.100.1 192.168.1.0 255.255.255.0 (hitcnt=34)
access-list Transit-ACL-Policy line 3 extended deny udp any 192.168.1.0 255.255.255.0 eq 2067 (hitcnt=19)
access-list Transit-ACL-Policy line 4 extended deny 91 any 192.168.1.0 255.255.255.0 (hitcnt=199)
access-list Transit-ACL-Policy line 5 extended deny ip any any (hitcnt=25)
firewall#

In the preceding example, the access list Transit-ACL-Policy has dropped 19 DLSw packets on UDP port 2067 and 199 DLSw packets using IP protocol 91 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 available in Cisco Security Appliance System Log Message - 106023.

Information about configuring syslog for the Cisco ASA 5500 Series Adaptive Security Appliance or the Cisco PIX 500 Series Security Appliance is available in Configuring Logging on the Cisco Security Appliance. Information about configuring syslog on the FWSM for Cisco Catalyst 6500 Series switches and Cisco 7600 Series routers is available in Configuring Monitoring and Logging on the Cisco FWSM.

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 attempts to exploit the vulnerabilities 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 available in Using the Command Line Interface.

For additional information about investigating incidents using syslog events, reference the Identifying Incidents Using Firewall and IOS Router Syslog Events Applied Intelligence white paper.

firewall#show logging | grep 106023
Oct 10 2007 20:36:58: %ASA-4-106023: Deny udp src outside:192.168.143.24/5951 dst
   inside:192.168.1.156/2067 by access-group "Transit-ACL-Policy"
Oct 10 2007 22:31:39: %ASA-4-106023: Deny protocol 91 src outside:192.168.208.183 dst
   inside:192.168.1.153 by access-group "Transit-ACL-Policy"
Oct 11 2007 22:39:12: %ASA-4-106023: Deny protocol 91 src outside:192.168.5.87 dst
   inside:192.168.1.155 by access-group "Transit-ACL-Policy"
Oct 12 2007 08:33:35: %ASA-4-106023: Deny protocol 91 src outside:192.168.80.224 dst
   inside:192.168.1.204 by access-group "Transit-ACL-Policy"
Oct 12 2007 14:05:53: %ASA-4-106023: Deny udp src outside:192.168.85.153/6841 dst
   inside:192.168.1.4/2067 by access-group "Transit-ACL-Policy"
Oct 13 2007 17:49:49: %ASA-4-106023: Deny udp src outside:192.168.81.231/13324 dst
   inside:192.168.1.25/2067 by access-group "Transit-ACL-Policy"
firewall#

In the preceding example, the messages logged for the tACL Transit-ACL-Policy show potentially spoofed DLSw packets on UDP port 2067 and IP protocol 91 sent to the address block assigned to the network infrastructure.

Additional information about syslog messages for ASA and PIX security appliances is available in Cisco Security Appliance System Log Messages. Additional information about syslog messages for the FWSM is available in Catalyst 6500 Series Switch and Cisco 7600 Series Router Firewall Services Module Logging Configuration and System Log Messages.

Identification: Spoofing Protection Using Unicast Reverse Path Forwarding

Firewall syslog message 106021 will be generated for packets denied by Unicast RPF. Additional information about this syslog message is available in Cisco Security Appliance System Log Message - 106021.

Information about configuring syslog for the Cisco ASA 5500 Series Adaptive Security Appliance or the Cisco PIX 500 Series Security Appliance is available in Configuring Logging on the Cisco Security Appliance. Information about configuring syslog on the FWSM for Cisco Catalyst 6500 Series switches and Cisco 7600 Series routers is available in Configuring Monitoring and Logging on the Cisco FWSM.

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 attempts to exploit the vulnerabilities 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 available in Using the Command Line Interface.

firewall#show logging | grep 106021
Feb 21 2007 00:15:13: %ASA-1-106021: Deny UDP reverse path check from
   192.168.0.1 to 192.168.0.100 on interface outside
Feb 21 2007 00:15:13: %ASA-1-106021: Deny UDP reverse path check from
   192.168.0.1 to 192.168.0.100 on interface outside
Feb 21 2007 00:15:13: %ASA-1-106021: Deny TCP reverse path check from
   192.168.0.1 to 192.168.0.100 on interface outside
firewall#

The show asp drop command can also identify the number of packets that Unicast RPF has dropped, as shown in the following example:

firewall#show asp drop

Frame drop:
  Reverse-path verify failed                                 11
  Flow is denied by configured rule                         855
  Expired flow                                                1
  Interface is down                                           2

Flow drop:

firewall#

In the preceding example, Unicast RPF has dropped 11 IP packets received on interfaces with Unicast RPF configured.

For additional information about debugging accelerated security path dropped packets or connections, reference the Cisco Security Appliance Command Reference for show asp drop.

Cisco Intrusion Prevention System

Mitigation: Cisco IPS Signature Event Actions

Administrators can use the Cisco Intrusion Prevention System (IPS) appliances and services modules to provide threat detection and help prevent attempts to exploit the vulnerability described in this document. Starting with signature update S324 for sensors running Cisco IPS version 6.x or 5.x, the vulnerability described in this document can be detected by signature 6926/0 (Signature Name: Cisco IOS DLSw DoS). Signature 6926/0 is enabled by default, triggers a Medium severity event, has a signature fidelity rating (SFR) of 85, and is configured with a default event action of produce-alert. Signature 6926/0 fires when a single packet sent using UDP port 2067 is detected. Firing of this signature may indicate a potential exploit of the vulnerability described in this document.

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 described in this document.

Exploits that are easily spoofed may cause a configured event action to inadvertently deny traffic from trusted sources.

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 sensors deployed in inline protection mode provides threat prevention against an attack that is attempting to exploit the vulnerability 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.

Cisco IPS 5.x sensors that are deployed in inline protection mode require an event action configured on a per-signature basis. Alternatively, administrators can configure an override that can perform an event action for any signatures that are triggered and are calculated as a high-risk threat. Using an event action on sensors deployed in inline protection mode provides the most effective exploit prevention.

For additional information about the risk rating and threat rating calculation, reference Risk Rating and Threat Rating: Simplify IPS Policy Management.

Identification: IPS Signature Events

Signature: 6926/0 - Cisco IOS DLSw DoS

ips#show events alert | include id=6926

evIdsAlert: eventId=1184140689302163325 severity=medium vendor=Cisco 
  originator: 
    hostId: ips
    appName: sensorApp
    appInstanceId: 10952
  time: 2008/03/26 18:45:30 2008/03/26 13:45:30 CDT
  signature: description=Cisco IOS DLSw DoS id=6926 version=S324
    subsigId: 0
    sigDetails: Malformed DLSw UDP packet
    marsCategory: DoS/NetworkDevice
    marsCategory: Info/Misc
  interfaceGroup: vs0
  vlan: 0
  participants: 
    attacker: 
      addr: locality=OUT 192.168.6.66
      port: 34188
    target: 
      addr: locality=OUT 192.168.60.1
      port: 2067
      os: idSource=unknown relevance=unknown type=unknown 
  triggerPacket: 

  
!--- "triggerPacket" Output Truncated


  riskRatingValue: targetValueRating=medium 53
  threatRatingValue: 53
  interface: ge0_0
  protocol: udp

ips#

Cisco Security Monitoring, Analysis, and Response System

Identification: Cisco Security Monitoring, Analysis, and Response System Incidents

The Cisco Security Monitoring, Analysis, and Response System (Cisco Security MARS) appliance can create incidents on events for the multiple DLSw DoS vulnerabilities in Cisco IOS using IPS signature 6926/0 (Signature Name: Cisco IOS DLSw DoS). After the S324 dynamic signature update has been downloaded, using keyword NR-6926/0 for IPS signature 6926/0 and a query type of All Matching Event Raw Messages on the Cisco Security MARS appliance will provide a report that lists the incidents created by the IPS signature.

The following screen shot shows the value used to query for event(s) created by the IPS signature related to this vulnerability:

cisco-amb-20080326-dlsw-query.gif

The following screen shot shows the query results for this vulnerability created by the Cisco Security MARS appliance:

cisco-amb-20080326-dlsw-result.gif

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 Caution: If dynamic signature updates are not configured, events that match these new signatures appear as unknown event type in queries and reports. MARS will not include these events in inspection rules, thus 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 and instructions for configuring dynamic signature updates are available for the Cisco Security MARS 4.3.1 and 5.3.1 releases.

Additional Information

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 History

Revision 1.1

2008-August-19

Corrected link in Identification: Spoofing Protection Using Unicast Reverse Path Forwarding section

Revision 1.0

2008-March-26

Initial Public Release

Cisco Security Procedures

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 http://www.cisco.com/web/about/security/psirt/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.

Related Information


Download this document (PDF)
View Printable Version