Cisco Applied Mitigation Bulletin

Identifying and Mitigating Exploitation of Multiple Vulnerabilities in Cisco NX-OS-Based Products

 
Threat Type:IntelliShield: Applied Mitigation Bulletin
IntelliShield ID:28737
Version:1
First Published:2013 April 24 16:14 GMT
Last Published:2013 April 24 16:14 GMT
Port: 161
CVE:CVE-2013-1178 , CVE-2013-1179 , CVE-2013-1180 , CVE-2013-1181
Urgency:Unlikely Use
Credibility:Confirmed
Severity:Moderate Damage
 
Version Summary:Cisco Applied Mitigation Bulletin initial public release.
 

Cisco Response

This Applied Mitigation Bulletin is a companion document to the PSIRT Security Advisory Multiple Vulnerabilities in Cisco NX-OS-Based Products and provides identification and mitigation techniques that administrators can deploy on Cisco network devices. It applies only to the SNMP-based vulnerabilities.

Vulnerability Characteristics

Cisco NX-OS Software SNMP and License Manager Buffer Overflow Vulnerability: This vulnerability can be exploited remotely with authentication and without end-user interaction. Successful exploitation of this vulnerability could allow arbitrary code execution. The attack vector for exploitation is through SNMP IPv4 and IPv6 packets using UDP port 161. An attacker could exploit this vulnerability using spoofed packets.

This vulnerability has has been assigned Common Vulnerabilities and Exposures (CVE) identifier CVE-2013-1179.

Cisco NX-OS Software SNMP Buffer Overflow Vulnerability: This vulnerability can be exploited remotely with authentication and without end-user interaction. Successful exploitation of this vulnerability could allow arbitrary code execution. The attack vector for exploitation is through SNMP IPv4 and IPv6 packets using UDP port 161. An attacker could exploit this vulnerability using spoofed packets.

This vulnerability has has been assigned Common Vulnerabilities and Exposures (CVE) identifier CVE-2013-1180.

Mitigation Technique Overview

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

  • Infrastructure access control lists (iACLs)
  • Unicast Reverse Path Forwarding (uRPF)
  • 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 this vulnerability.

The proper deployment and configuration of uRPF provides an 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 an effective means of protection against spoofed packets at the access layer.

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

  • tACLs
  • uRPF

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

Cisco IOS NetFlow records can provide visibility into network-based exploitation attempts.

Cisco IOS Software, Cisco ASA, 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.

Risk Management

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.

Device-Specific Mitigation and Identification

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

To protect infrastructure devices and minimize the risk, impact, and effectiveness of direct infrastructure attacks, administrators are advised to 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. An iACL workaround cannot provide complete protection against these vulnerabilities when the attack originates from a trusted source address.

The iACL policy denies unauthorized UDP IPv4 and IPv6 packets on UDP port 161 that are sent to affected devices. In the following example, 192.168.60.0/24 and 2001:DB8:1:60::/64 represent the IP address space that is used by the affected devices, and the hosts at 192.168.100.1 and 2001:DB8::100:1 are 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.

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 in Protecting Your Core: Infrastructure Protection Access Control Lists.

ip access-list extended Infrastructure-ACL-Policy
!
!-- Include explicit permit statements for trusted sources
!-- that require access on the vulnerable port
!
permit udp host 192.168.100.1 192.168.60.0 0.0.0.255 eq 161
!
!-- The following vulnerability-specific access control entries
!-- (ACEs) can aid in identification of attacks
!
deny udp any 192.168.60.0 0.0.0.255 eq 161
!
!-- Explicit deny ACE for traffic sent to addresses configured within
!-- the infrastructure address space
!
deny ip any 192.168.60.0 0.0.0.255  
!
!-- Permit or deny all other Layer 3 and Layer 4 traffic in accordance
!-- with existing security policies and configurations
!
!
!-- Create the corresponding IPv6 iACL 
!
ipv6  access-list IPv6-Infrastructure-ACL-Policy
!
!-- Include explicit permit statements for trusted sources
!-- that require access on the vulnerable protocols and ports
!
permit udp host 2001:DB8::100:1 2001:DB8:1:60::/64 eq 161
!
!-- The following vulnerability-specific access control entries
!-- (ACEs) can aid in identification of attacks to global and
!-- link-local addresses
!
deny udp any 2001:DB8:1:60::/64 eq 161
!
!-- Permit other required traffic to the infrastructure address
!-- range and allow IPv6 neighbor discovery packets, which
!-- include neighbor solicitation packets and neighbor
!-- advertisement packets
!
permit icmp any any nd-ns
permit icmp any any nd-na
!
!--  Explicit deny for all other IPv6 traffic to the global
!--  infrastructure address range
!
deny ipv6 any 2001:DB8:1:60::/64 ! !-- Permit or deny all other Layer 3 and Layer 4 traffic !-- in accordance with existing security policies and configurations ! ! !-- Apply iACLs to interfaces in the ingress direction ! interface GigabitEthernet0/0 ip access-group Infrastructure-ACL-Policy in ipv6 traffic-filter IPv6-Infrastructure-ACL-Policy 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 and no ipv6 unreachables. ICMP unreachable rate limiting can be changed from the default using the global configuration command ip icmp rate-limit unreachable interval-in-ms and ipv6 icmp error-interval interval-in-ms.

Identification: Infrastructure Access Control Lists

After the administrator applies the iACL to an interface, the show ip access-lists and show ipv6 access-lists commands will identify the number of SNMP packets on UDP port 161 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 this vulnerability. Example output for show ip access-lists Infrastructure-ACL-Policy and show ipv6 access-list IPv6-Infrastructure-ACL-Policy follows:

    router#show ip access-lists Infrastructure-ACL-Policy
    Extended IP access list Infrastructure-ACL-Policy
    10 permit udp host 192.168.100.1 192.168.60.0 0.0.0.255 eq 161
    20 deny udp any 192.168.60.0 0.0.0.255 eq 161 (49 matches)
    30 deny ip any 192.168.60.0 0.0.0.255
    router#

In the preceding example, access list Infrastructure-ACL-Policy has dropped 49 SNMP packets on UDP port 161 for access control list entry (ACE) line 20.

    router#show ipv6 access-list IPv6-Infrastructure-ACL-Policy 
    IPv6 access list IPv6-Infrastructure-ACL-Policy
    permit udp host 2001:DB8::100:1 2001:DB8:1:60::/64 eq 161 (512 matches) sequence 10
    deny udp any 2001:DB8:1:60::/64 eq 161 (216 matches) sequence 20
    permit icmp any any nd-ns (80 matches) sequence 30
    permit icmp any any nd-na (80 matches) sequence 40
    deny ipv6 any 2001:DB8:1:60::/64 (5 matches) sequence 50

In the preceding example, access list IPv6-Infrastructure-ACL-Policy has dropped 216 SNMP packets on UDP port 161 for access control list entry (ACE) line 20.

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 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

The vulnerability that is 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.

Mitigation: Spoofing Protection Using 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 | includeregex 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
  IPv6 unicast RPF: via=rx acl=None, drop=10, 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 ipv6 interface GigabitEthernet 0/0 | section IPv6 verify
IPv6 verify source reachable-via rx
0 verification drop(s) (process), 10 (CEF)
0 suppressed verification drop(s) (process), 0 (CEF)
  --      CLI Output Truncated       --  
router#
router#show ip cef switching statistics feature
IPv4 CEF input features:
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 ipv6 cef switching statistics feature
IPv6 CEF input features:
Feature                       Drop   Consume   Punt   Punt2Host   Gave route
RP LES Verify Unicast R         10         0      0           0            0
Total                           10         0      0           0            0
    --      CLI Output Truncated       --  
router#
router#show ip traffic | include RPF
18 no route, 18 unicast RPF, 0 forced drop
router#
router#show ipv6 traffic | include RPF
10 RPF drops, 0 RPF suppressed, 0 forced drop
router#

In the preceding show cef interface type slot/port internal, show cef drop, show ip interface type slot/port and show ipv6 interface type slot/port, show ip cef switching statistics feature andshow ipv6 cef switching statistics feature, and show ip traffic and show ipv6 traffic examples, uRPF has dropped the following 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.

Cisco IOS NetFlow and Cisco IOS Flexible NetFlow

Identification: IPv4 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 vulnerability. Administrators are advised to investigate flows to determine whether they are attempts to exploit the vulnerability or whether they are legitimate traffic flows.

router#show ip cache flow
IP packet size distribution (90784136 total packets):
1-32 64 96 128 160 192 224 256 288 320 352 384 416 448 480 .000 .698 .011 .001 .004 .005 .000 .004 .000 .000 .003 .000 .000 .000 .000
512 544 576 1024 1536 2048 2560 3072 3584 4096 4608 .000 .001 .256 .000 .010 .000 .000 .000 .000 .000 .000
IP Flow Switching Cache, 4456704 bytes 1885 active, 63651 inactive, 59960004 added 129803821 ager polls, 0 flow alloc failures Active flows timeout in 30 minutes Inactive flows timeout in 15 seconds IP Sub Flow Cache, 402056 bytes 0 active, 16384 inactive, 0 added, 0 added to flow 0 alloc failures, 0 force free 1 chunk, 1 chunk added last clearing of statistics never Protocol Total Flows Packets Bytes Packets Active(Sec) Idle(Sec) -------- Flows /Sec /Flow /Pkt /Sec /Flow /Flow TCP-Telnet 11393421 2.8 1 48 3.1 0.0 1.4 TCP-FTP 236 0.0 12 66 0.0 1.8 4.8 TCP-FTPD 21 0.0 13726 1294 0.0 18.4 4.1 TCP-WWW 22282 0.0 21 1020 0.1 4.1 7.3 TCP-X 719 0.0 1 40 0.0 0.0 1.3 TCP-BGP 1 0.0 1 40 0.0 0.0 15.0 TCP-Frag 70399 0.0 1 688 0.0 0.0 22.7 TCP-other 47861004 11.8 1 211 18.9 0.0 1.3 UDP-DNS 582 0.0 4 73 0.0 3.4 15.4 UDP-NTP 287252 0.0 1 76 0.0 0.0 15.5 UDP-other 310347 0.0 2 230 0.1 0.6 15.9 ICMP 1614 0.0 3 61 0.0 19.8 15.5 IPv6INIP 15 0.0 1 1132 0.0 0.0 15.4 GRE 4 0.0 1 48 0.0 0.0 15.3 Total: 59957957 14.8 1 196 22.5 0.0 1.5
SrcIf SrcIPaddress DstIf DstIPaddress Pr SrcP DstP Pkts Gi0/0 192.168.10.201 Gi0/1 192.168.60.102 11 0984 00A1 1 Gi0/0 192.168.11.54 Gi0/1 192.168.60.158 11 0911 00A1 3 Gi0/1 192.168.150.60 Gi0/0 10.89.16.226 06 0016 12CA 1 Gi0/0 192.168.13.97 Gi0/1 192.168.60.28 11 0B3E 00A1 5 Gi0/0 192.168.10.17 Gi0/1 192.168.60.97 11 0B89 00A1 1 Gi0/0 10.88.226.1 Gi0/1 192.168.202.22 06 007B 007B 1 Gi0/0 192.168.12.185 Gi0/1 192.168.60.239 11 0BD7 00A1 1 Gi0/0 10.89.16.226 Gi0/1 192.168.150.60 06 12CA 0016 1 router#

In the preceding example, there are multiple flows for SNMP on UDP port 161 (hex value 00A1).

This traffic is sourced from and sent to addresses within the 192.168.60.0/24 address block, which is used for infrastructure devices. The packets in these flows may be spoofed and may indicate an attempt to exploit this vulnerability. Administrators are advised to compare these flows to baseline utilization for SNMP traffic sent on UDP port 161 and also investigate the flows to determine whether they are sourced from untrusted hosts or networks.

To view only the traffic flows for SNMP packets on UDP port 161 (hex value 00A1), the command show ip cache flow | include SrcIf|_11_.*00A1 will display the related UDP NetFlow records as shown here:

UDP Flows

router#show ip cache flow | include SrcIf|_11_.*00A1
SrcIf         SrcIPaddress     DstIf         DstIPaddress    Pr SrcP DstP  Pkts
Gi0/0         192.168.10.201   Gi0/1         192.168.60.102  11 0984 00A1     1
Gi0/0         192.168.11.54    Gi0/1         192.168.60.158  11 0911 00A1     3
Gi0/0         192.168.13.97    Gi0/1         192.168.60.28   11 0B3E 00A1     5
Gi0/0         192.168.10.17    Gi0/1         192.168.60.97   11 0B89 00A1     1
Gi0/0         192.168.12.185   Gi0/1         192.168.60.239  11 0BD7 00A1     1
router#

Identification: IPv6 Traffic Flow Identification Using Cisco IOS NetFlow

Administrators can configure Cisco IOS NetFlow on Cisco IOS routers and switches to aid in the identification of IPv6 traffic flows that may be attempts to exploit the vulnerabilities that are described in this document. Administrators are advised to investigate flows to determine whether they are attempts to exploit these vulnerabilities or whether they are legitimate traffic flows.

The following output is from a Cisco IOS device running Cisco IOS Software 12.4 mainline train. The command syntax will vary for different Cisco IOS Software trains.

router#show ipv6 flow cache
IP packet size distribution (50078919 total packets):
   1-32  64   96  128  160  192  224  256  288  320  352  384  416  448  480
   .000 .990 .001 .008 .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, 475168 bytes
  8 active, 4088 inactive, 6160 added
  1092984 ager polls, 0 flow alloc failures
  Active flows timeout in 30 minutes
  Inactive flows timeout in 15 seconds
IP Sub Flow Cache, 33928 bytes
  16 active, 1008 inactive, 12320 added, 6160 added to flow
  0 alloc failures, 0 force free
  1 chunk, 1 chunk added
SrcAddress        InpIf    DstAddress       OutIf    Prot SrcPrt DstPrt Packets
2001:DB...6A:5BA6 Gi0/0    2001:DB...28::21 Gi0/1    3A   0000   8000   1191
2001:DB...6A:5BA6 Gi0/0    2001:DB...134::3 Gi0/1    3A   0000   8000   1191
2001:DB...6A:5BA6 Gi0/0    2001:DB...128::4 Gi0/1    3A   0000   8000   1192    
2001:DB...6A:5BA6 Gi0/0    2001:DB...128::2 Gi0/1    11   160A   00A1   1992
2001:DB...06::201 Gi0/0    2001:DB...128::3 Gi0/1    11   1619   00A1   1900  
2001:DB...06::201 Gi0/0    2001:DB...128::4 Gi0/1    11   1714   00A1   1194  
2001:DB...6A:5BA6 Gi0/0    2001:DB...128::3 Gi0/1    3A   0000   8000   1155
2001:DB...6A:5BA6 Gi0/0    2001:DB...146::3 Gi0/1    3A   0000   8000   1092
2001:DB...6A:5BA6 Gi0/0    2001:DB...144::4 Gi0/1    3A   0000   8000   1193 

To permit display of the full 128-bit IPv6 address, use the terminal width 132 exec mode command.

In the preceding example, there are multiple IPv6 flows for SNMP on UDP port 161 (hex value 00A1).

The SNMP packets on UDP port 161 are sourced from and sent to addresses within the 2001:DB8:1:60::/64 address block that is used by affected devices. The packets in the UDP flows may be spoofed and could indicate an attempt to exploit these vulnerabilities. Administrators are advised to compare these flows to baseline utilization for SNMP traffic on UDP port 161 and also investigate the flows to determine whether they are sourced from untrusted hosts or networks.

As shown in the following example, to view only the SNMP packets on UDP port 161 (hex value 00A1), use the show ip cache flow | include SrcIf|_11_.*00A1 command to display the related Cisco NetFlow records:

UDP Flows

router#show ip cache flow | include SrcIf|_11_.*00A1
SrcAddress        InpIf    DstAddress       OutIf    Prot SrcPrt DstPrt Packets
2001:DB...06::201 Gi0/0    2001:DB...28::20 Local    0x11 0x16C1 0x00A1 1368 
2001:DB...06::201 Gi0/0    2001:DB...128::3 Gi0/1    0x11 0x1629 0x00A1 1201 
2001:DB...06::201 Gi0/0    2001:DB...128::4 Gi0/1    0x11 0x1644 0x00A1 1993 
router#

Identification: IPv4 Traffic Flow Identification Using Cisco Flexible NetFlow

Introduced in Cisco IOS Software Releases 12.2(31)SB2 and 12.4(9)T, Cisco IOS Flexible NetFlow improves original Cisco NetFlow by adding the capability to customize the traffic analysis parameters for the administrator's specific requirements. Original Cisco NetFlow uses a fixed seven tuples of IP information to identify a flow, whereas Cisco IOS Flexible NetFlow allows the flow to be user defined. It facilitates the creation of more complex configurations for traffic analysis and data export by using reusable configuration components.

The following example output is from a Cisco IOS device that is running a version of Cisco IOS Software in the 15.1T train. Although the syntax will be almost identical for the 12.4T and 15.0 trains, it may vary slightly depending on the actual Cisco IOS release being used. In the following configuration, Cisco IOS Flexible NetFlow will collect information on interface GigabitEthernet0/0 for incoming IPv4 flows based on source IPv4 address, as defined by the match ipv4 source address key field statement. Cisco IOS Flexible NetFlow will also include nonkey field information about source and destination IPv4 addresses, protocol, ports (if present), ingress and egress interfaces, and packets per flow.

!
!-- Configure key and nonkey fields
!-- in the user-defined flow record
!
flow record FLOW-RECORD-ipv4
 match ipv4 source address
 collect ipv4 protocol
 collect ipv4 destination address
 collect transport source-port
 collect transport destination-port
 collect interface input
 collect interface output
 collect counter packets
!
!-- Configure the flow monitor to
!-- reference the user-defined flow 
!-- record
!
flow monitor FLOW-MONITOR-ipv4
 record FLOW-RECORD-ipv4
!
!-- Apply the flow monitor to the interface
!-- in the ingress direction
!
interface GigabitEthernet0/0
 ip flow monitor FLOW-MONITOR-ipv4 input

The Cisco IOS Flexible NetFlow flow output is as follows:

router#show flow monitor FLOW-MONITOR-ipv4 cache format table
  Cache type:                               Normal
  Cache size:                                 4096
  Current entries:                               6
  High Watermark:                                1

  Flows added:                                   9181
  Flows aged:                                    9175
    - Active timeout      (  1800 secs)          9000
    - Inactive timeout    (    15 secs)           175
    - Event aged                                    0
    - Watermark aged                                0
    - Emergency aged                                0

IPV4 SRC ADDR   ipv4 dst addr  trns src port trns dst port intf input intf output pkts  ip prot
==============  ============== ============= ============= ========== =========== ===== =======
192.168.10.201  192.168.60.102          1456            80      Gi0/0       Gi0/1  1128       6
192.168.11.54   192.168.60.158          1216           161      Gi0/0       Gi0/1  1912      17
192.168.150.60  10.89.16.226            2567           443      Gi0/0       Gi0/1    13       6
192.168.13.97   192.168.60.28           3451            80      Gi0/0       Gi0/1     1       6
192.168.10.17   192.168.60.97           9439           161      Gi0/0       Gi0/1   546      17
10.88.226.1     192.168.202.22          2678           443      Gi0/0       Gi0/1 10567       6
10.89.16.226    192.168.150.60          3562            80      Gi0/0       Gi0/1 30012       6

To view only the SNMP packets on UDP port 161, use the show flow monitor FLOW-MONITOR-ipv4 cache format table | include ipv4 dst addr |_161_.*_17_ command to display related NetFlow records.

For more information about Cisco IOS Flexible NetFlow, refer to Flexible Netflow Configuration Guide, Cisco IOS Release 15.1M&T and Cisco IOS Flexible NetFlow Configuration Guide, Release 12.4T.

Identification: IPv6 Traffic Flow Identification Using Cisco IOS Flexible NetFlow

The following example output is from a Cisco IOS device that is running a version of Cisco IOS Software in the 15.1T train. Although the syntax will be almost identical for the 12.4T and 15.0 trains, it may vary slightly depending on the actual Cisco IOS release being used. In the following configuration, Cisco IOS Flexible NetFlow will collect information on interface GigabitEthernet0/0 for incoming IPv6 flows based on the source IPv6 address, as defined by the match ipv6 source address key field statement. Cisco IOS Flexible NetFlow will also include nonkey field information about source and destination IPv6 addresses, protocol, ports (if present), ingress and egress interfaces, and packets per flow.

!
!-- Configure key and nonkey fields
!-- in the user-defined flow record
!
flow record FLOW-RECORD-ipv6
 match ipv6 source address
 collect ipv6 protocol
 collect ipv6 destination address
 collect transport source-port
 collect transport destination-port
 collect interface input
 collect interface output
 collect counter packets
!
!-- Configure the flow monitor to
!-- reference the user-defined flow 
!-- record
!
flow monitor FLOW-MONITOR-ipv6
 record FLOW-RECORD-ipv6
!
!-- Apply the flow monitor to the interface
!-- in the ingress direction
!
interface GigabitEthernet0/0
  ipv6 flow monitor FLOW-MONITOR-ipv6 input

The Cisco IOS Flexible NetFlow flow output is as follows:

router#show flow monitor FLOW-MONITOR-ipv6 cache format table
  Cache type:                               Normal
  Cache size:                                 4096
  Current entries:                               6
  High Watermark:                                2

  Flows added:                                   539
  Flows aged:                                    532
    - Active timeout      (  1800 secs)          350
    - Inactive timeout    (    15 secs)          182
    - Event aged                                   0
    - Watermark aged                               0
    - Emergency aged                               0

IPV6 SRC ADDR      ipv6 dst addr    trns src port trns dst port intf input intf output pkts  ip prot
=================  ================ ============= ============= ========== =========== ===== =======
2001:DB...06::201  2001:DB...28::20   123           123           Gi0/0      Gi0/0      17     17
2001:DB...06::201  2001:DB...28::20   4763          161           Gi0/0      Gi0/0      1241   17
2001:DB...06::201  2001:DB...28::20   5542          161           Gi0/0      Gi0/0      2100   17
2001:DB...06::201  2001:DB...28::20   9391          161           Gi0/0      Gi0/0      9123   17
2001:DB...06::201  2001:DB...28::20   2856          80            Gi0/0      Gi0/0      486    6
2001:DB...06::201  2001:DB...28::20   3012          53            Gi0/0      Gi0/0      1016   17
2001:DB...06::201  2001:DB...28::20   2477          53            Gi0/0      Gi0/0      1563   17

To permit display of the full 128-bit IPv6 address, use the terminal width 132 exec mode command.

To view only the SNMP on UDP port 161, use the show flow monitor FLOW-MONITOR-ipv6 cache format table | include ipv6 dst addr |_161_.*_17_ command to display the related Cisco IOS Flexible NetFlow records.

Cisco ASA and 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 this vulnerability when the attack originates from a trusted source address.

The tACL policy denies unauthorized IPv4 and IPv6 SNMP packets on UDP port 161 that are sent to affected devices. It should be noted that blocking access to UDP port 161 does not completely protect devices. If SNMP has been configured with permanent ports, these configured ports should also be added to the iACL. In the following example, 192.168.60.0/24 and 2001:DB8:1:60::/64 represent the IP address space that is used by the affected devices, and the hosts at 192.168.100.1 and 2001:DB8::100:1 are considered trusted sources that requires access to the affected devices. Care should be taken to allow required traffic for routing and administrative access prior to denying all unauthorized traffic.

Additional information about tACLs is in Transit Access Control Lists: Filtering at Your Edge.

!
!-- Include explicit permit statements for trusted sources
!-- that require access on the vulnerable port
!
access-list tACL-Policy extended permit udp host 192.168.100.1
     192.168.60.0 255.255.255.0 eq 161
! !-- The following vulnerability-specific access control entry !-- (ACE) can aid in identification of attacks ! access-list tACL-Policy extended deny udp any 192.168.60.0 255.255.255.0 eq 161
! !-- Permit or deny all other Layer 3 and Layer 4 traffic in accordance !-- with existing security policies and configurations ! !-- Explicit deny for all other IP traffic ! access-list tACL-Policy extended deny ip any any ! !-- Create the corresponding IPv6 tACL ! !-- Include explicit permit statements for trusted sources !-- that require access on the vulnerable protocols and ports ! ipv6 access-list IPv6-tACL-Policy permit udp host 2001:DB8::100:1 2001:db8:1:60::/64 eq 161 ! !-- The following vulnerability-specific access control entries !-- (ACEs) can aid in identification of attacks ! ipv6 access-list IPv6-tACL-Policy deny udp any 2001:db8:1:60::/64 eq 161 ! !-- 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 ! ipv6 access-list IPv6-tACL-Policy deny ip any any ! !-- Apply tACLs to interfaces in the ingress direction ! access-group tACL-Policy in interface outside access-group IPv6-tACL-Policy in interface outside ! !-- Apply tACL to interface(s) 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 SNMP packets on UDP port 161 that have been filtered. Administrators are advised to investigate filtered packets to determine whether they are attempts to exploit this vulnerability. Example output for show access-list tACL-Policy and show access-list IPv6-tACL-Policy follows:

firewall#show access-list tACL-Policy
access-list tACL-Policy; 3 elements
access-list tACL-Policy line 1 extended permit udp host 192.168.100.1
     192.168.60.0 255.255.255.0 eq 161
access-list tACL-Policy line 2 extended deny udp any
     192.168.60.0 255.255.255.0 eq 161 (hitcnt=91)
access-list tACL-Policy line 3 extended deny ip any any
firewall#

In the preceding example, access list tACL-Policy has dropped 91 SNMP packets on UDP port 161 for ACE line 2.

firewall#show access-list IPv6-tACL-Policy 
ipv6 access-list IPv6-tACL-Policy; 3 elements; name hash: 0x566a4229
ipv6 access-list IPv6-tACL-Policy line 1 permit udp host 2001:db8:1:100::1 
     2001:db8:1:60::/64 eq 161 (hitcnt=124) 
ipv6 access-list IPv6-tACL-Policy line 2 deny udp any 
     2001:db8:1:60::/64 eq 161 (hitcnt=216) 
ipv6 access-list IPv6-tACL-Policy line 3 deny ip any any (hitcnt=27)

In the preceding example, access list IPv6-tACL-Policy has dropped 216 SNMP packets on UDP port 161 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.

Mitigation: Spoofing Protection Using Unicast Reverse Path Forwarding

The vulnerability that is 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 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 uRPF, reference the Cisco Security Appliance Command Reference for ip verify reverse-path and the Understanding Unicast Reverse Path Forwarding Security Intelligence Operations white paper.

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 FWSM for Cisco Catalyst 6500 Series switches and Cisco 7600 Series routers is in Monitoring the Firewall Services Module.

In the following example, the show logging | grep regex command extracts syslog messages from the logging buffer on the firewall. These messages provide additional information about denied packets that could indicate potential attempts to exploit the vulnerability that is described in this document. It is possible to use different regular expressions with the grep keyword to search for specific data in the logged messages.

Additional information about regular expression syntax is in Creating a Regular Expression.

firewall#show logging | grep 106023
  Apr 24 2013 00:12:15: %ASA-4-106023: Deny udp src outside:192.0.2.18/5934
         dst inside:192.168.60.191/161 by access-group "tACL-Policy"
  Apr 24 2013 00:12:15: %ASA-4-106023: Deny udp src outside:192.0.2.200/5935
         dst inside:192.168.60.33/161 by access-group "tACL-Policy"
  Apr 24 2013 00:12:15: %ASA-4-106023: Deny udp src outside:192.0.2.99/5936
         dst inside:192.168.60.240/161 by access-group "tACL-Policy"
  Apr 24 2013 00:12:15: %ASA-4-106023: Deny udp src outside:192.0.2.100/5937
         dst inside:192.168.60.115/161 by access-group "tACL-Policy"
  Apr 24 2013 00:12:15: %ASA-4-106023: Deny udp src outside:192.0.2.88/5938
         dst inside:192.168.60.38/161 by access-group "tACL-Policy"
  Apr 24 2013 00:12:15: %ASA-4-106023: Deny udp src outside:192.0.2.175/5939 
         dst inside:192.168.60.250/161 by access-group "tACL-Policy"
  Apr 24 2013 00:12:15: %ASA-4-106023: Deny udp src outside:2001:db8:2::2:172/2951
         dst inside:2001:db8:1:60::23/161 by access-group "IPv6-tACL-Policy"
  Apr 24 2013 00:12:15: %ASA-4-106023: Deny udp src outside:2001:db8:d::a85e:172/2952
         dst inside:2001:db8:1:60::134/161 by access-group "IPv6-tACL-Policy"
firewall#

In the preceding example, the messages logged for the tACL tACL-Policy show potentially spoofed SNMP packets for UDP port 161 sent to the address block assigned to the infrastructure devices.

Additional information about syslog messages for ASA security appliances is in Cisco ASA 5500 Series System Log Messages, 8.2. Additional information about syslog messages for the FWSM is in Catalyst 6500 Series Switch and Cisco 7600 Series Router Firewall Services Module Logging System Log Messages.

For additional information about investigating incidents using syslog events, reference the Identifying Incidents Using Firewall and IOS Router Syslog Events 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 Unicast RPF. 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 on the FWSM for Cisco Catalyst 6500 Series switches and Cisco 7600 Series routers is in Monitoring the Firewall Services Module.

In the following example, the show logging | grep regex command extracts syslog messages from the logging buffer on the firewall. These messages provide additional information about denied packets that could indicate potential attempts to exploit the vulnerability that is described in this document. It is possible to use different regular expressions with the grep keyword to search for specific data in the logged messages.

Additional information about regular expression syntax is in Creating a Regular Expression.

firewall#show logging | grep 106021
  Apr 24 2013 00:12:20: %ASA-1-106021: Deny UDP reverse path check from
         192.168.60.1 to 192.168.60.100 on interface outside
  Apr 24 2013 00:12:20: %ASA-1-106021: Deny UDP reverse path check from
         192.168.60.1 to 192.168.60.100 on interface outside
  Apr 24 2013 00:12:20: %ASA-1-106021: Deny TCP reverse path check from
         192.168.60.1 to 192.168.60.100 on interface outside

The show asp drop command can also identify the number of packets that the Unicast RPF 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, Unicast RPF has dropped 11 SNMP packets received on interfaces with Unicast RPF configured. Absence of output indicates that the Unicast RPF 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.

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.

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

 
Alert History
 

Initial Release



Product Sets
 
The security vulnerability applies to the following combinations of products.

Primary Products:
CiscoCisco NX-OS Software for Nexus 1000 Series Base, 4.0(4)SV1(1), 4.0(4)SV1(2), 4.0(4)SV1(3), 4.0(4)SV1(3a), 4.0(4)SV1(3b), 4.0(4)SV1(3c), 4.0(4)SV1(3d), 4.2(1)SV1(4), 4.2(1)SV1(4a), 4.2(1)SV1(4b) | for Nexus 3000 Series Base, 5.0(3)U1(1), 5.0(3)U1(1a), 5.0(3)U1(1b), 5.0(3)U1(1d), 5.0(3)U1(2), 5.0(3)U1(2a), 5.0(3)U2(1), 5.0(3)U2(2), 5.0(3)U2(2a), 5.0(3)U2(2b), 5.0(3)U2(2c), 5.0(3)U2(2d), 5.0(3)U3(1) | for Nexus 4000 Series Base, 4.1(2)E1(1), 4.1(2)E1(1b), 4.1(2)E1(1d), 4.1(2)E1(1e), 4.1(2)E1(1f), 4.1(2)E1(1g) | for Nexus 5000 Series Base, 4.0(0)N1(1a), 4.0(0)N1(2), 4.0(0)N1(2a), 4.0(1a)N1(1), 4.0(1a)N1(1a), 4.0(1a)N2(1), 4.0(1a)N2(1a), 4.1(3)N1(1), 4.1(3)N1(1a), 4.1(3)N2(1), 4.1(3)N2(1a), 4.2(1)N1(1), 4.2(1)N2(1), 4.2(1)N2(1a), 5.0(2)N1(1), 5.0(3)N1(1c), 5.0(2)N2(1), 5.0(2)N2(1a), 5.0(3)N2(1), 5.0(3)N2(2), 5.0(3)N2(2a), 5.0(3)N2(2b) | for Nexus 7000 Series Base, 4.1.(2), 4.1.(3), 4.1.(4), 4.1.(5), 4.2.(2a), 4.2(3), 4.2(4), 4.2(6), 4.2(8), 5.0(2a), 5.0(3), 5.0(5), 5.1(1), 5.1(1a), 5.1(3), 5.1(4), 5.1(5), 5.1(6), 5.2(1), 5.2(3a), 5.2(4), 6.0(1), 6.0(2), 6.0(3), 6.0(4)
CiscoCisco MDS 9000 NX-OS Software 4.1 Base, (1b), (1c), (3a) | 4.2 Base, (1a), (1b), (3), (3a), (5), (7a), (7b), (7d), (7e) | 5.0 Base, (1a), (4), (4b), (4c), (4d), (7) | 5.2 Base, (1), (2), (2a), (2d)
CiscoCisco Unified Computing System (Managed) 1.0 Base, (2k) | 1.1 Base, (1m) | 1.2 Base, (1d) | 1.3 Base, (1c), (1m), (1n), (1o), (1p), (1q), (1t), (1w), (1y) | 1.4 Base, (1i), (1j), (1m), (3i), (3l), (3m), (3q), (3s), (3u), (3y), (4f), (4g), (4i), (4j), (4k) | 2.0 Base, (1m), (1q), (1s), (1t), (1w), (1x)
CiscoCisco CG-OS CG1 (4), (5) | CG3 (1), (2), (3)

Associated Products:
N/A




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The urgency and severity ratings of this alert are not tailored to individual users; users may value alerts differently based upon their network configurations and circumstances. THE ALERT, AND INFORMATION CONTAINED THEREIN, ARE PROVIDED ON AN "AS IS" BASIS AND DO NOT IMPLY ANY KIND OF GUARANTEE OR WARRANTY, INCLUDING THE WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR USE. YOUR USE OF THE ALERT, AND INFORMATION CONTAINED THEREIN, OR MATERIALS LINKED FROM THE ALERT, IS AT YOUR OWN RISK. INFORMATION IN THIS ALERT AND ANY RELATED COMMUNICATIONS IS BASED ON OUR KNOWLEDGE AT THE TIME OF PUBLICATION AND IS SUBJECT TO CHANGE WITHOUT NOTICE. CISCO RESERVES THE RIGHT TO CHANGE OR UPDATE ALERTS AT ANY TIME.
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