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Cisco Security

Cisco Applied Mitigation Bulletin: Identifying and Mitigating Exploitation of the Cisco IOS Next Hop Resolution Protocol Vulnerability

Identifying and Mitigating Exploitation of the Cisco IOS Next Hop Resolution Protocol Vulnerability

Advisory ID: cisco-amb-20070808-nhrp

http://tools.cisco.com/security/center/content/CiscoAppliedMitigationBulletin/cisco-amb-20070808-nhrp

Revision 1.2

For Public Release 2007 August 8 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 Cisco IOS Next Hop Resolution Protocol Vulnerability and provides identification and mitigation techniques that administrators can deploy on Cisco network devices.

Vulnerability Characteristics

There is a vulnerability in Cisco IOS when processing a specially crafted Next Hop Resolution Protocol (NHRP, RFC 2332 leavingcisco.com) packet. This vulnerability can be exploited remotely without authentication and without end-user interaction. Successful exploitation of this vulnerability may allow arbitrary code execution or cause the affected device to crash. Repeated attempts to exploit this vulnerability could result in a sustained denial of service (DoS) condition. The attack vectors for exploitation are through a packet or frame using the following protocols:

  • NHRP (protocol number 54)
  • Generic Routing Encapsulation (GRE, protocol number 47)
  • IEEE 802.2 Subnetwork Access Protocol (SNAP)

Note: NHRP is a primary component of the Dynamic Multipoint Virtual Private Network (DMVPN) feature, was extended for use with DMVPN, and can be carried in GRE and multipoint GRE (mGRE) tunnels. When the DMVPN feature is in use, NHRP is encapsulated within GRE. DMVPN is most often deployed with IPsec for encryption of the traffic sent between DMVPN-enabled devices. Thus, NHRP is encapsulated within GRE, and GRE is encapsulated by IPsec. To successfully exploit this vulnerability via IPsec when DMVPN is used with GRE or mGRE tunnels, an attacker must encapsulate the specially crafted NHRP packet in GRE and then encapsulate GRE within IPsec. The affected device would need to process the IPsec packet to the inner GRE packet and then process the GRE packet to the specially crafted NHRP packet.

This vulnerability is susceptible to exploitation through spoofed packets. This vulnerability has been assigned CVE name CVE-2007-4286 leavingcisco.com.

Information about vulnerable, unaffected, and fixed software is available in the PSIRT Security Advisory: http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20070808-nhrp.

Mitigation Technique Overview

Cisco devices provide several countermeasures for the Cisco IOS NHRP vulnerability. Administrators are advised to consider these protection methods as 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 attempting to exploit the vulnerability described in this document.

The proper deployment and configuration of Unicast RPF provides the most effective means of protection against attacks using 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 (ASA), 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 methods:

  • Transit access control lists (tACLs)
  • Unicast RPF

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

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

Effective use of Cisco Intrusion Prevention System event actions provides visibility into and protection against attacks that attempt to exploit this vulnerability.

Cisco IOS NetFlow can provide visibility into 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 queries and event reporting.

Risk Management

Organizations should follow their standard risk evaluation and mitigation process 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 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 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

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 unauthorized packets for NHRP (protocol number 54) and GRE (protocol number 47) 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 requiring access on the vulnerable protocols
  !

  permit 54 host 192.168.100.1 192.168.1.0 0.0.0.255
  permit 47 host 192.168.100.1 192.168.1.0 0.0.0.255
  
  !
  !-- The following vulnerability-specific access control entries
  !-- (ACEs) can aid identification of attacks
  !

  deny 54 any 192.168.1.0 0.0.0.255
  deny 47 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 traffic in accordance with existing
  !-- security policies and configurations
  !


!-- Apply iACL to interface(s) in the ingress direction

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

!

Unicast Reverse Path Forwarding

The Cisco IOS NHRP vulnerability can be exploited by spoofed IP packets. Protection mechanisms for spoofing exist through the proper deployment and configuration of Unicast Reverse Path Forwarding (Unicast RPF). 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 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/or 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.

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 NHRP (protocol number 54) and GRE (protocol number 47) packets that have been filtered on interfaces where the iACL is applied. Administrators should investigate filtered packets to determine whether they are attempts to exploit this vulnerability. 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 54 host 192.168.100.1 192.168.1.0 0.0.0.255
    20 permit gre host 192.168.100.1 192.168.1.0 0.0.0.255 (67 matches)
    30 deny 54 any 192.168.1.0 0.0.0.255 (13 matches)
    40 deny gre any 192.168.1.0 0.0.0.255 (79 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 13 NHRP (protocol number 54) packets on ACE sequence ID 30, and 79 GRE (protocol number 47) packets on ACE sequence ID 40.

Identification: Access List Logging

The log or log-input 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: 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 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 will 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
  2989 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        2989           0           0
router#

router#show cef interface GigabitEthernet 0/0 internal | include drop
      --      CLI Output Truncated       --
  ip verify: via=rx, acl=0, drop=2989, sdrop=0
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
         13 no route, 2989 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 a total of 2989 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 (FIB).

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 potential attempts to exploit the vulnerability described in this document. Administrators should investigate flows to determine whether they are attempts to exploit this vulnerability or whether they are legitimate traffic flows.

router#show ip cache flow
IP packet size distribution (126524557 total packets):
   1-32   64   96  128  160  192  224  256  288  320  352  384  416  448  480
   .000 .988 .002 .000 .000 .000 .000 .000 .000 .005 .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, 4456704 bytes
  4955 active, 60581 inactive, 1243041 added
  21163686 ager polls, 0 flow alloc failures
  Active flows timeout in 1 minutes
  Inactive flows timeout in 15 seconds
IP Sub Flow Cache, 795272 bytes
  9910 active, 22858 inactive, 2486084 added, 1243041 added to flow
  0 alloc failures, 0 force free
  2 chunks, 4 chunks 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         735      0.0        35    41      0.0       9.2      10.0
TCP-FTP              9      0.0        10    69      0.0       0.0       4.4
TCP-WWW        1073230      1.7       116    53    208.8      11.4       4.0
TCP-NNTP             1      0.0         6    54      0.0       1.2       1.8
TCP-other        62674      0.1        12   268      1.3       2.3       9.0
UDP-DNS            512      0.0         2    72      0.0       0.8      15.4
UDP-NTP          34303      0.0         1    76      0.0       0.0      15.5
UDP-other        42604      0.0         1   168      0.1       0.1      15.4
ICMP             13731      0.0         7    56      0.1      37.5       7.9
IPINIP               4      0.0         1    88      0.0       0.0      15.4
GRE               2521      0.0         1    89      0.0       0.0      18.2
IP-other          7762      0.0         2    87      0.0       0.0      17.6
Total:         1238086      2.0       102    54    210.6      10.4       5.1

SrcIf         SrcIPaddress    DstIf         DstIPaddress    Pr SrcP DstP  Pkts
Gi0/0         10.89.16.9      Gi0/1         192.168.150.60  06 0C47 0016     3 
Gi0/0         10.88.226.1     Gi0/1         192.168.206.5   11 007B 007B     1 
Gi0/0         192.168.208.63  Gi0/0         10.89.16.9      06 0016 0D5D    57 
Gi0/0         192.168.208.63  Gi0/0         10.89.16.9      06 0016 0D61  1248 
Gi0/0         192.168.208.63  Gi0/0         10.89.16.9      06 0016 0D63   306 
Gi0/0         192.168.208.63  Local         192.168.208.20  06 B4F1 0017  1193 
Gi0/0         10.88.226.1     Gi0/1         192.168.128.21  11 007B 007B     1 
Gi0/0         10.89.254.17    Gi0/1         192.168.150.65  01 0000 030D     1 
Gi0/0         10.87.98.218    Gi0/1         192.168.130.41  06 C21A 01BB   118 
Gi0/0         192.168.208.64  Null          192.168.208.255 11 0089 0089     3 
Gi0/0         192.168.3.16    Gi0/0         192.168.1.135   36 0000 0000     8 
Gi0/0         192.168.1.144   Null          192.168.1.92    2F 0000 0000     1 
Gi0/0         192.168.4.146   Gi0/0         192.168.1.6     36 0000 0000     2 
Gi0/0         192.168.7.208   Gi0/0         192.168.1.205   36 0000 0000     3 
Gi0/0         192.168.6.30    Gi0/0         192.168.1.208   2F 0000 0000    13 
Gi0/0         192.168.4.43    Gi0/0         192.168.1.195   2F 0000 0000     2 
Gi0/0         192.168.1.61    Null          192.168.1.178   2F 0000 0000     1 
Gi0/0         192.168.2.66    Gi0/0         192.168.1.92    36 0000 0000    11 
Gi0/0         192.168.3.155   Gi0/0         192.168.1.2     36 0000 0000    23 
Gi0/0         192.168.3.144   Gi0/0         192.168.1.65    2F 0000 0000    29 
Gi0/0         192.168.1.179   Null          192.168.1.235   36 0000 0000     2 
Gi0/0         192.168.1.71    Null          192.168.1.140   2F 0000 0000     4 
Gi0/0         192.168.3.148   Gi0/0         192.168.1.84    2F 0000 0000     6 
Gi0/0         192.168.3.113   Gi0/0         192.168.1.18    36 0000 0000     7 
Gi0/0         192.168.4.217   Gi0/0         192.168.1.171   36 0000 0000     1 
Gi0/0         192.168.2.175   Gi0/0         192.168.1.36    2F 0000 0000     7 
Gi0/0         192.168.2.12    Gi0/0         192.168.1.115   36 0000 0000     9 
Gi0/0         192.168.4.63    Gi0/0         192.168.1.67    36 0000 0000    17 
Gi0/0         192.168.7.74    Gi0/0         192.168.1.163   2F 0000 0000     4 
Gi0/0         192.168.3.14    Gi0/0         192.168.1.195   2F 0000 0000     9 
router#

In the preceding example, there are multiple flows for NHRP (protocol number 54, hex value 36) and GRE (protocol number 47, hex value 2F). Some of this traffic is sourced from and sent to addresses within the 192.168.1.0/24 address block, which is used for infrastructure devices. The packets in these flows may be spoofed and may indicate an attempt to exploit the vulnerability described in this document. Administrators should compare these flows to baseline utilization for NHRP and GRE traffic and also investigate the flows to determine whether they are sourced from untrusted hosts or networks.

To view only the traffic flows for NHRP (protocol number 54, hex value 36) and GRE (protocol number 47, hex value 2F), the command show ip cache flow | include SrcIf|_(36|2F)_0000_0000 will display the related NetFlow records as shown here:

Note: The values for SrcP (source port) and DstP (destination port) in these NetFlow records are hex value 0000 because NHRP and GRE do not have port fields in their headers.

router#show ip cache flow | include SrcIf|_(36|2F)_0000_0000
SrcIf         SrcIPaddress    DstIf         DstIPaddress    Pr SrcP DstP  Pkts
Gi0/0         192.168.3.16    Gi0/0         192.168.1.135   36 0000 0000     8 
Gi0/0         192.168.1.144   Null          192.168.1.92    2F 0000 0000     1 
Gi0/0         192.168.4.146   Gi0/0         192.168.1.6     36 0000 0000     2 
Gi0/0         192.168.7.208   Gi0/0         192.168.1.205   36 0000 0000     3 
Gi0/0         192.168.6.30    Gi0/0         192.168.1.208   2F 0000 0000    13 
Gi0/0         192.168.4.43    Gi0/0         192.168.1.195   2F 0000 0000     2 
Gi0/0         192.168.1.61    Null          192.168.1.178   2F 0000 0000     1 
Gi0/0         192.168.2.66    Gi0/0         192.168.1.92    36 0000 0000    11 
Gi0/0         192.168.3.155   Gi0/0         192.168.1.2     36 0000 0000    23 
Gi0/0         192.168.3.144   Gi0/0         192.168.1.65    2F 0000 0000    29 
Gi0/0         192.168.1.179   Null          192.168.1.235   36 0000 0000     2 
Gi0/0         192.168.1.71    Null          192.168.1.140   2F 0000 0000     4 
Gi0/0         192.168.3.148   Gi0/0         192.168.1.84    2F 0000 0000     6 
Gi0/0         192.168.3.113   Gi0/0         192.168.1.18    36 0000 0000     7 
Gi0/0         192.168.4.217   Gi0/0         192.168.1.171   36 0000 0000     1 
Gi0/0         192.168.2.175   Gi0/0         192.168.1.36    2F 0000 0000     7 
Gi0/0         192.168.2.12    Gi0/0         192.168.1.115   36 0000 0000     9 
Gi0/0         192.168.4.63    Gi0/0         192.168.1.67    36 0000 0000    17 
Gi0/0         192.168.7.74    Gi0/0         192.168.1.163   2F 0000 0000     4 
Gi0/0         192.168.3.14    Gi0/0         192.168.1.195   2F 0000 0000     9 
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 packets for NHRP (protocol number 54) and GRE (protocol number 47) 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
!-- requiring access on the vulnerable protocols
!

access-list Transit-ACL-Policy extended permit 54 host 192.168.100.1
   192.168.1.0 255.255.255.0
access-list Transit-ACL-Policy extended permit 47 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 54 any 192.168.1.0 255.255.255.0
access-list Transit-ACL-Policy extended deny 47 any 192.168.1.0 255.255.255.0

!
!-- Permit/deny all other Layer 3 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 interface(s) in the ingress direction
!

access-group Transit-ACL-Policy in interface outside

!


Mitigation: Spoofing Protection Using Unicast Reverse Path Forwarding

Attackers can exploit the Cisco IOS NHRP vulnerability using 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 IP source 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 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.

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 NHRP (protocol number 54) and GRE (protocol number 47) packets that have been filtered. Administrators should investigate filtered packets to determine whether they are attempts to exploit this vulnerability. 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 54 host 192.168.100.1
   192.168.1.0 255.255.255.0 (hitcnt=0)
access-list Transit-ACL-Policy line 2 extended permit gre host 192.168.100.1
   192.168.1.0 255.255.255.0 (hitcnt=67)
access-list Transit-ACL-Policy line 3 extended deny 54 any 192.168.1.0
   255.255.255.0 (hitcnt=13)
access-list Transit-ACL-Policy line 4 extended deny gre any 192.168.1.0
   255.255.255.0 (hitcnt=79)
access-list Transit-ACL-Policy line 5 extended deny ip any any (hitcnt=666)
firewall#

In the preceding example, the access list Transit-ACL-Policy has dropped 13 NHRP (protocol number 54) packets and 79 GRE (protocol number 47) 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 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 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 regexp 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 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 106023
Jul 04 2007 06:06:06: %ASA-4-106023: Deny protocol 54 src outside:192.168.2.77
   dst inside:192.168.1.138 by access-group "Transit-ACL-Policy"
Jul 04 2007 06:06:06: %ASA-4-106023: Deny protocol 54 src outside:192.168.3.134
   dst inside:192.168.1.17 by access-group "Transit-ACL-Policy"
Jul 04 2007 06:06:06: %ASA-4-106023: Deny protocol 47 src outside:192.168.4.218
   dst inside:192.168.1.98 by access-group "Transit-ACL-Policy"
Jul 04 2007 06:06:06: %ASA-4-106023: Deny protocol 54 src outside:192.168.5.39
   dst inside:192.168.1.66 by access-group "Transit-ACL-Policy"
Jul 04 2007 06:06:06: %ASA-4-106023: Deny protocol 47 src outside:192.168.6.17
   dst inside:192.168.1.234 by access-group "Transit-ACL-Policy"
firewall#

In the preceding example, the messages logged for the tACL Transit-ACL-Policy show potentially spoofed packets for NHRP (protocol number 54) and GRE (protocol number 47) 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 regexp 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 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.

Note: The reason NARP is present in the following syslog messages is that NBMA Address Resolution Protocol (NARP, RFC 1735 leavingcisco.com) was originally assigned protocol number 54 by IANA leavingcisco.com, and was later superseded by NHRP. IANA has not changed the keyword assignment from NARP to NHRP in the Assigned Internet Protocol Numbers leavingcisco.com.

firewall#show logging | grep 106021
Jul 04 2007 06:06:06: %ASA-1-106021: Deny NARP reverse path check from
   192.168.2.77 to 192.168.1.138 on interface outside
Jul 04 2007 06:06:06: %ASA-1-106021: Deny NARP reverse path check from
   192.168.3.134 to 192.168.1.17 on interface outside
Jul 04 2007 06:06:06: %ASA-1-106021: Deny GRE reverse path check from
   192.168.4.218 to 192.168.1.98 on interface outside
Jul 04 2007 06:06:06: %ASA-1-106021: Deny NARP reverse path check from
   192.168.5.39 to 192.168.1.66 on interface outside
Jul 04 2007 06:06:06: %ASA-1-106021: Deny GRE reverse path check from
   192.168.6.17 to 192.168.1.234 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                               2989
  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 2989 IP packets received on interfaces with Unicast RPF configured.

For additional information about the configuration and use of Unicast RPF, 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. This vulnerability may be detected by the following signatures:

  • 5884/0 - IOS NHRP Buffer Overflow (S295)
  • 5884/1 - IOS NHRP Buffer Overflow (S296)

5884/0 - IOS NHRP Buffer Overflow

Starting with signature update S295 for sensors running Cisco IPS version 6.x or 5.x, the vulnerability described in this document can be detected by signature 5884/0 (Signature Name: IOS NHRP Buffer Overflow). Signature 5884/0 is enabled by default, triggers a High severity event, has a signature fidelity rating (SFR) of 90, and is configured with a default event action of produce-alert. Signature 5884/0 fires when a single packet sent using NHRP (protocol number 54) is detected. Firing of this signature may indicate a potential exploit of the vulnerability described in this document.

5884/1 - IOS NHRP Buffer Overflow

Starting with signature update S296 for sensors running Cisco IPS version 6.x or 5.x, the vulnerability described in this document can be detected by signature 5884/1 (Signature Name: IOS NHRP Buffer Overflow). Signature 5884/1 is enabled by default, triggers a High severity event, has a signature fidelity rating (SFR) of 90, and is configured with a default event action of produce-alert. Signature 5884/1 fires when a single packet sent using GRE (protocol number 47) 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.

Because NHRP-based (protocol number 54) and GRE-based (protocol number 47) exploits can easily be spoofed, an attack that contains spoofed addresses may cause a configured event action to inadvertently deny traffic from trusted sources. Event actions that perform blocking through ACLs or the shun command are usually configured on sensors deployed in promiscuous mode.

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 this vulnerability. Threat prevention is achieved through a default override that performs an event action of deny-packet-inline for triggered signatures with a riskRatingValue greater than 90. Additional information about the risk rating and the calculation of its value is available in Cisco IPS Risk Rating Explained.

Cisco IPS 5.x sensors deployed in inline protection mode will need to have 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 the deny-packet-inline event action on sensors deployed in inline protection mode provides the most effective exploit prevention.

Identification: IPS Signature Events

Signature: 5884/0 - IOS NHRP Buffer Overflow

sensor6x# show events alert | include id=5884

evIdsAlert: eventId=1184072489279145943 severity=high vendor=Cisco 
  originator: 
    hostId: sensor6x
    appName: sensorApp
    appInstanceId: 395
  time: 2007/08/09 20:44:58 2007/08/09 15:44:58 CDT
  signature: description=IOS NHRP Buffer Overflow id=5884 version=S295 
    subsigId: 0
    sigDetails: IOS NHRP Buffer Overflow
    marsCategory: Penetrate/BufferOverflow/Misc
  interfaceGroup: vs1
  vlan: 0
  participants: 
    attacker: 
      addr: locality=OUT 192.168.208.63
    target: 
      addr: locality=IN 192.168.134.21
      os: idSource=unknown relevance=unknown type=unknown 
  actions: 
    denyPacketRequestedNotPerformed: true
  triggerPacket: 

--      CLI Output Truncated       --

  riskRatingValue: targetValueRating=medium watchlist=25 100
  threatRatingValue: 100
  interface: ge0_1
  protocol: IP protocol 54

sensor6x#

Signature: 5884/1 - IOS NHRP Buffer Overflow

sensor6x# show events alert | include id=5884

evIdsAlert: eventId=1184072489279145948 severity=high vendor=Cisco 
  originator: 
    hostId: sensor6x
    appName: sensorApp
    appInstanceId: 395
  time: 2007/08/09 20:45:07 2007/08/09 15:45:07 CDT
  signature: description=IOS NHRP Buffer Overflow id=5884 version=S296 
    subsigId: 1
    sigDetails: IOS NHRP Buffer Overflow
    marsCategory: Penetrate/BufferOverflow/Misc
  interfaceGroup: vs1
  vlan: 0
  participants: 
    attacker: 
      addr: locality=OUT 192.168.208.63
    target: 
      addr: locality=IN 192.168.134.21
      os: idSource=unknown relevance=unknown type=unknown 
  actions: 
    denyPacketRequestedNotPerformed: true
  triggerPacket: 

--      CLI Output Truncated       --

  riskRatingValue: targetValueRating=medium watchlist=25 100
  threatRatingValue: 100
  interface: ge0_1
  protocol: IP protocol 47

sensor6x#

Cisco Security Monitoring, Analysis, and Response System

Identification: Cisco Security Monitoring, Analysis, and Response System Query Type and Keyword

The Cisco Security Monitoring, Analysis, and Response System (Cisco Security MARS) appliance can query on events for the Cisco IOS NHRP vulnerability using a query type and keyword. Using a keyword of NR-5884/0 and NR-5884/1 for IPS signatures 5884/0 and 5884/1, which were created for this vulnerability, and a query type of All Matching Event Raw Messages on the Cisco Security MARS appliance will provide a report that lists the events created by IPS signatures 5884/0 and 5884/1.

The following screen shot shows the values used to query for events created by IPS signatures 5884/0 (Signature Name: IOS NHRP Buffer Overflow) and 5884/1 (Signature Name: IOS NHRP Buffer Overflow).

cisco-amb-20070808-nhrp_query-01.jpg

The following screen shot shows the query results for NR-5884/0 and NR-5884/1 created by the Cisco Security MARS appliance using a query type and keyword regex query.

cisco-amb-20070808-nhrp_result-01.jpg

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 1.2

2007-August-16

Updated with CVE name (CVE-2007-4286)

Revision 1.1

2007-August-10

Updated with IPS and MARS sections

Revision 1.0

2007-August-08

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.

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