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Cisco Applied Mitigation Bulletin

Identifying and Mitigating Exploitation of the Cisco Unified IP Phone Local Kernel System Call Input Validation Vulnerability

 
Threat Type:IntelliShield: Applied Mitigation Bulletin
IntelliShield ID:27763
Version:1
First Published:2013 January 09 11:01 GMT
Last Published:2013 January 09 11:01 GMT
Port: Not available
CVE:CVE-2012-5445
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 Cisco Unified IP Phone Local Kernel System Call Input Validation Vulnerability and provides identification and mitigation techniques that administrators can deploy on Cisco network devices.

Vulnerability Characteristics

The Cisco Unified IP Phones 7900 Series, also known as Cisco TNP phones, contain an input validation vulnerability. This vulnerability is due to a failure to properly validate input that is passed to kernel system calls from applications running in userspace. Such an attack would originate from an unprivileged context. An attacker could exploit this issue by gaining local access via the AUX port located on the rear of the device or by authenticating to the device via SSH and executing an attacker-controlled binary that is designed to exploit the issue. The SSH method is disabled by default on the device once it has been provisioned by a Cisco Unified Call Manager.

The attack vectors for exploitation are through the local console and packets using the following protocols and ports:
  • SSH using TCP port 22
  • TFTP using random UDP ports greater than 1023
An attacker could exploit this vulnerability using spoofed packets.

This vulnerability has been assigned Common Vulnerabilities and Exposures (CVE) identifier CVE-2012-5445.

Mitigation Technique Overview

Cisco devices provide several countermeasures for this vulnerability. Administrators are advised to consider these protection methods to be general security best practices for infrastructure devices and the traffic that transits the network. This section of the document provides an overview of these techniques.

An organization's risk evaluation and mitigation process can consider the following aspects of the vulnerability outlined in this document:

  • To be successful, the SSH authentication must take place over the network or the AUX port located on the rear of the device.
  • SSH is disabled once the Cisco Unified IP Phone has been provisioned.
  • After successful authentication to the phone, the attacker must attempt to force the phone to download a malicious, unsigned binary file by spoofing the IP address of the legitimate TFTP server.
  • Cisco Unified Communications Manager 8.0.1 and later signs configuration files sent to phones via TFTP by default and supports phone configuration file encryption security. The signing and encryption of device configuration files prevents tampering or replacement of the files by spoofing a TFTP server or server response, because the cryptographic signature is verified before the files are utilized by a device.

Effective exploit prevention can be provided by the Cisco Unified Communications Manager with the following methods:

  • Disable the Cisco Unified IP Phone SSH server
  • Configure encrypted phone configuration files

These protection mechanisms prevent console access, as well as verify the integrity and validity of configuration files that are attempting to run on the phones.

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

  • Infrastructure Access Lists (iACLs)
  • VLAN Access Lists (vACLs)
  • Unicast Reverse Path Forwarding (uRPF)
  • DHCP Snooping
  • Dynamic ARP Inspection (DAI)
  • IP Source Guard
  • 802.1x

These protection mechanisms filter and drop, as well as verify the source MAC and IP addresses 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. uRPF should be deployed as close to all traffic sources as possible.

The proper deployment and configuration of DAI provides an effective means of protection against spoofing attacks at layer 2. Deployment of DAI requires the implementation of DHCP snooping. DAI will drop all ARP Packets with invalid IP-to-MAC address bindings.

The proper deployment and configuration of IPSG provides an effective means of protection against spoofing attacks at the access layer

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

  • Transit access control lists (tACLs)
  • Unicast Reverse Path Forwarding (uRPF)

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

Risk Management

Organizations are advised to follow their standard risk evaluation and mitigation processes to determine the potential impact of this vulnerability. Triage refers to sorting projects and prioritizing efforts that are most likely to be successful. Cisco has provided documents that can help organizations develop a risk-based triage capability for their information security teams. Risk Triage for Security Vulnerability Announcements and Risk Triage and Prototyping can help organizations develop repeatable security evaluation and response processes.

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 Unified Communications Manager

Securing the components in a Cisco Unified Communication System is necessary for protecting the integrity and confidentiality of voice calls. Security guidelines pertaining specifically to Unified Communications technology and the voice network are available to help administrators better secure their Cisco Unified Communications systems. These security guideline resources include:

Specific settings are available within Cisco Unified Communications Manager to mitigate the vulnerability described in this document. The settings are each discussed below.

Mitigation: Disable the Cisco Unified IP Phone PC port

To prevent unauthorized access to the organization's network, administrators are advised to disable the PC port on lobby, conference room, and other physically-unsecured Cisco Unified IP phones. Administrators can perform the following steps to disable the PC port:

Step 1: In the Cisco Unified Communications Manager Administration interface choose Device > Phone

Step 2: List the phones that should have the PC port disabled using the appropriate search criteria and select Find

Step 3: Select the device name to open the Phone Configuration window

Step 4: Locate the PC Port product-specific parameter

Step 5: Select Disabled from the drop-down list for the PC Port parameter

Step 6: Select Save

Step 7: Select Reset

Additional information about disabling the PC port on Cisco Unified IP Phones can be found in the Disabling the PC Port Setting section of the Cisco Unified Communications Manager Security Guide document.

Mitigation: Disable the Cisco Unified IP Phone SSH server

By default the Cisco Unified IP Phone SSH server is disabled. This setting should be verified to prevent remote access attempts to exploit the vulnerability described in this document. Administrators can perform the following steps to verify the setting:

Step 1: In the Cisco Unified Communications Manager Administration interface choose Device > Phone

Step 2: List the phones that should be verified using the appropriate search criteria and select Find

Step 3: Select the device name to open the Phone Configuration window

Step 4: Locate the SSH Access product-specific parameter

Step 5: Verify that Disabled is selected in the drop-down list for the SSH Access parameter

Additional information about disabling the SSH server on Cisco Unified IP Phones is in the Enabling and Disabling SSH section of the Cisco Unified IP Phone 8961, 9951, and 9971 Administration Guide for Cisco Unified Communications Manager document.

Mitigation: Configuring Encrypted Phone Configuration Files

Administrators must configure encryption of the Cisco Unified IP Phone configuration file to ensure privacy. The configuration files are downloaded from the Unified Communications Manager TFTP server. Configuring encryption is also a mitigation for the vulnerability described in this document because any configuration files that are not encrypted, digitally signed, and received from an authenticated source will be ignored and discarded.

Cisco Unified Communications Manager software release 8.0(1) and later provides the ability to sign phone configuration files and support for phone configuration file encryption security by default. Additional information about security by default features is in the Security by Default section of the Cisco Unified Communications Manager Security Guide document.

The steps to configure encrypted phone configuration files are fully covered in the Encryption Configuration File Configuration Checklist. Additional information about Encrypted Phone Configuration Files is in the Configuring Encrypted Phone Configuration Files and Configuring a Phone Security Profile sections of the Cisco Unified Communications Manager Security Guide document.

Administrators can perform the following steps to verify encrypted phone configuration files:

Step 1: In the Cisco Unified Communications Manager Administration interface choose System > Security Profile > Phone Security Profile

Step 2: Enter search criteria to locate the appropriate record in the database and select Find

Step 3: Select the record to be reviewed

Step 4: Locate the TFTP Encrypted Config security profile parameter

Step 5: Verify that the check box for the TFTP Encrypted Config parameter is checked

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

Cisco VLAN technology separates the physical network into multiple logical networks. Separate voice and data VLANs are recommended for the following reasons:

  • Address space conservation and voice device protection from external networks
  • Private addressing of phones on the voice or auxiliary VLAN ensures address conservation and that phones are not directly accessible through public networks. PCs and servers are typically addressed with publicly routed subnet addresses; however, voice endpoints may be addressed using RFC 1918 private subnet addresses.
  • Quality of Service (QoS) trust boundary extension to voice devices
  • Cisco switches can discover IP Phones using Cisco Discovery Protocol and automatically extend QoS trust to the phone port dynamically.
  • Protection from malicious network attacks
  • VLAN access control, 802.1Q, and 802.1p tagging can provide protection for voice devices from malicious internal and external network attacks such as worms, denial of service (DoS) attacks, and attempts by data devices to gain access to priority queues through packet tagging.
  • Ease of management and configuration
  • Separate VLANs for voice and data devices at the access layer provide ease of management and simplified QoS configuration.

In the iACL examples below, access lists Infrastructure-ACLdata-Policy and IPv6-Infrastructure-ACLdata-Policy deny unauthorized SSH IPv4 and IPv6 packets on TCP port 22 and TFTP IPv4 and IPv6 packets on UDP ports above 1023 that are sent from the data network to affected devices on the voice network. The access lists Infrastructure-ACLvoice-Policy and IPv6-Infrastructure-ACLvoice-Policy deny unauthorized RTP IPv4 and IPv6 packets on UDP ports above 1023 that may be sent from affected devices on the voice network to the data network in an attempt to exfiltrate voice communications. 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 on the voice network, and interface GigabitEthernet0/0 is the interface facing the data network side of a router that is deployed between the data and voice networks. Interface GigabitEthernet0/1 is the interface facing the voice network side of the router that is deployed between the data and voice networks. 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 that is 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-ACLdata-Policy
  !
  !-- Include explicit permit statements for trusted sources
  !-- that require access on the vulnerable protocols and ports
  !-- This includes softphones, web access to the phone (if allowed), 
  !-- the Attendant Console or other applications communication end-points 
  !-- that need access to the voice VLAN subnets
  !
  permit tcp host 192.168.100.1 192.168.60.0 0.0.0.255 eq 22
  permit udp host 192.168.100.1 gt 1023 192.168.60.0 0.0.0.255 gt 1023
  !
  !-- The following vulnerability-specific access control entries
  !-- (ACEs) can aid in identification of attacks
  !
  deny tcp any 192.168.60.0 0.0.0.255 eq 22
  deny udp any 192.168.60.0 0.0.0.255 gt 1023
  !
  !-- 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-ACLdata-Policy
  !
  !-- Include explicit permit statements for trusted sources
  !-- that require access on the vulnerable protocols and ports
  !-- This includes softphones, web access to the phone (if allowed), 
  !-- the Attendant Console or other applications communication end-points 
  !-- that need access to the voice VLAN subnets
  !
  permit tcp host 2001:DB8::100:1 2001:DB8:1:60::/64 eq 22
  permit udp host 2001:DB8::100:1 gt 1023 2001:DB8:1:60::/64 gt 1023
  !
  !-- The following vulnerability-specific access control entries
  !-- (ACEs) can aid in identification of attacks to global and
  !-- link-local addresses
  !
  deny tcp any 2001:DB8:1:60::/64 eq 22
  deny udp any gt 1023 2001:DB8:1:60::/64 gt 1023
  !
  !-- 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-ACLdata-Policy in ipv6 traffic-filter IPv6-Infrastructure-ACLdata-Policy in
  ip access-list extended Infrastructure-ACLvoice-Policy
  !
  !-- Include explicit permit statements for trusted sources
  !-- that require access on the vulnerable protocols and ports
  !-- This includes softphones, web access to the phone (if allowed), 
  !-- the Attendant Console or other applications communication end-points 
  !-- that need access to the voice VLAN subnets
  !
  permit udp 192.168.60.0 0 0.0.0.255 gt 1023 host 192.168.100.1 gt 1023
  !
  !-- The following vulnerability-specific access control entries
  !-- (ACEs) can aid in identification of attacks
  !
  deny udp 192.168.60.0 0.0.0.255 gt 1023 any gt 1023
  !
  !
  !-- 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-ACLvoice-Policy
  !
  !-- Include explicit permit statements for trusted sources
  !-- that require access on the vulnerable protocols and ports
  !-- This includes softphones, web access to the phone (if allowed), 
  !-- the Attendant Console or other applications communication end-points 
  !-- that need access to the voice VLAN subnets
  !
  permit udp 2001:DB8:60::/64 gt 1023 2001:DB8::100::1 gt 1023
  !
  !-- The following vulnerability-specific access control entries
  !-- (ACEs) can aid in identification of attacks to global and
  !-- link-local addresses
  !
  deny udp 2001:DB8::60::/64 gt 1023 any gt 1023
  !
  !-- 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
  !  
  !--  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 GigabitEthernet0/1
   ip access-group Infrastructure-ACLvoice-Policy in
   ipv6 traffic-filter IPv6-Infrastructure-ACLvoice-Policy in

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 SSH packets on TCP port 22 and TFTP packets on UDP ports greater than 1023 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-ACLdata-Policy follows:

router#show ip access-lists Infrastructure-ACLdata-Policy
Extended IP access list Infrastructure-ACLdata-Policy
    10 permit tcp host 192.168.100.1 192.168.60.0 0.0.0.255 eq 22
    20 permit udp host 192.168.100.1 gt 1023 192.168.60.0 0.0.0.255 gt 1023
    30 deny tcp any 192.168.60.0 0.0.0.255 eq 22 (11 matches)
    40 deny udp any gt 1023 192.168.60.0 0.0.0.255 gt 1023 (43 matches)
    50 deny ip any 192.168.60.0 0.0.0.255
router#

In the preceding example, access list Infrastructure-ACLdata-Policy has dropped 11 SSH packets on TCP port 22 for access control list entry (ACE) line 30 and has dropped 43 TFTP packets on UDP ports greater than 1023 for access control list entry (ACE) line 40.

After the administrator applies the iACL to an interface, the show ipv6 access-list IPv6-Infrastructure-ACLdata-Policy command will identify the number of SSH packets on TCP port 22 and TFTP packets on UDP ports greater than 1023 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 IPv6-Infrastructure-ACLdata-Policy follows:

router#show ipv6 access-list IPv6-Infrastructure-ACLdata-Policy
IPv6 access list IPv6-Infrastructure-ACLdata-Policy
    permit tcp host 2001:DB8::100:1 2001:DB8:1:60::/64 eq 22 sequence 10
    permit udp host 2001:DB8::100:1 gt 1023 2001:DB8:1:60::/64 gt 1023 sequence 20
    deny tcp any 2001:DB8:1:60::/64 eq 22 (1240 matches) sequence 30 
    deny udp any gt 1023 2001:DB8:1:60::/64 gt 1023 (740 matches) sequence 40
    permit icmp any any nd-ns sequence 50
    permit icmp any any nd-na sequence 60
    deny ipv6 any 2001:DB8:1:60::/64 sequence 70
router#

In the preceding example, access list IPv6-Infrastructure-ACL-Policy has dropped the following packets received from an untrusted host or network:

  • 1240 IPv6 SSH packets on TCP port 22 for ACE line 30
  • 740 IPv6 TFTP packets on UDP ports greater than 1023 for ACE line 40

After the administrator applies the iACL to an interface, the show ip access-lists command will identify the number of RTP packets on UDP ports greater than 1023 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-ACLvoice-Policy follows:

router#show ip access-lists Infrastructure-ACLvoice-Policy
Extended IP access list Infrastructure-ACL-Policy
    10 permit udp 192.168.60.0 0 0.0.0.255 gt 1023 host 192.168.100.1 gt 1023
    20 deny udp 192.168.60.0 0.0.0.255 gt 1023 any gt 1023 (87 matches)
30 deny ip any 192.168.60.0 0.0.0.255
router#

In the preceding example, access list Infrastructure-ACLvoice-Policy has dropped 87 RTP packets on UDP ports greater than 1023 for access control list entry (ACE) line 20.

After the administrator applies the iACL to an interface, the show ipv6 access-lists command will identify the number of RTP packets on UDP ports greater than 1023 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 ipv6 access-list IPv6-Infrastructure-ACLvoice-Policy follows:

router#show ipv6 access-list IPv6-Infrastructure-ACLvoice-Policy
IPv6 access list IPv6-Infrastructure-ACLvoice-Policy
    permit udp 2001:DB8:60::/64 gt 1023 2001:DB8::100::1 gt 1023 sequence 10
    deny udp 2001:DB8::60::/64 gt 1023 any gt 1023 (52 matches) sequence 20
    permit icmp any any nd-ns sequence 30
    permit icmp any any nd-na sequence 40
    deny ipv6 any 2001:DB8:1:60::/64 sequence 50
router#

In the preceding example, access list IPv6-Infrastructure-ACLvoice-Policy has dropped 52 RTP packets on UDP ports greater than 1023 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.

Mitigation: VLAN Access Control Lists(VACLs)

To protect the network from all packets that are bridged within a VLAN or that are routed into or out of a VLAN, administrators are advised to deploy VLAN access control lists (VACLs) to perform policy enforcement. If you plan to deploy a VACL, you should verify which ports are needed to allow the phones to function with each application that is used in your IP Telephony network. Normally any VACL would be applied to the VLAN that the phones use, which would allow control at the access port. Administrators can construct a VACL by explicitly permitting only authorized traffic to be bridged within a VLAN or routed into or out of a VLAN in accordance with existing security policies and configurations. A VACL workaround cannot provide complete protection against vulnerabilities that have an IPv4 network attack vector when the attack comes from a trusted source address.

Note: VACLs differ from traditional Cisco IOS ACLs in that they apply to all packets as opposed to routed packets only. In addition to unicast traffic, VACLs also apply to multicast and broadcast traffic; these types of packets must be accounted for in the VACL configuration.

The VACL policy denies unauthorized SSH packets on TCP port 22 that are sent to affected devices within the voice vlan (as is the case when a compromised device attempts to SSH into a phone). In the following example, 192.168.60.0/24 is the IP address space that is used by the affected devices in VLAN 60. Care should be taken to allow required traffic for routing and administrative access prior to denying all unauthorized traffic.

Additional information about VACLs is available in the Configuring Port ACLs and VLAN ACLs chapter of the Catalyst 6500 Release 12.2SXH and Later Software Configuration Guide document.

!
!-- Create an access list policy that will be applied in a
!-- VLAN access map that includes vulnerability-specific 
!-- access control entries (ACEs) that can aid in  
!-- identification of attacks 
!
ip access-list extended VACL-deny
  permit TCP any 192.168.60.0 0.0.0.255 eq 22
!
!-- Create an access list policy that will be applied 
!-- in a VLAN access map that permits all other
!-- Layer 2 and Layer 3 traffic in accordance with
!-- existing security polices and configurations.
!
!-- Broadcast and multicast traffic must be accounted 
!-- for as well
!--
!-- This access list policy could be implemented as an
!-- explicit permit statement allowing all other IP traffic
!-- as shown here
!
ip access-list extended Permit-All
  permit ip any any
!
!-- Define the VLAN access map that will be used to 
!-- perform policy enforcement (either permit or deny)  
!-- on the traffic matched by the previously defined 
!-- access control lists
!
!
vlan access-map VACL 10
  match ip address VACL-deny
  action drop
!
vlan access-map VACL 20
  match ip address Permit-All
  action forward
!
!-- Apply the VACL policy to the VLAN or list of VLANs 
!-- to filter traffic (Note:  The 'vlan-list' operator 
!-- is a single VLAN or comma separated list of VLANs)
!
vlan filter VACL vlan-list 60
!

Note: In the preceding VACL example, the access control list entries (ACEs) that match the potential exploit packets with the permit statement result in these packets being discarded by the VLAN access-map drop statement.

Identification: VLAN Access Control Lists

After the administrator applies the VACL to a VLAN or list of VLANs, the show tcam interface vlan vlan-interface-number acl in ip command will identify the number of packets 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 tcam interface vlan 60 acl in ip follows:

Note: An administrator can see additional access control entry statistics using the show tcam interface vlan vlan-interface-number acl in ip detail command.

switch#show tcam interface vlan 60 acl in ip

* Global Defaults shared

Entries from Bank 0

Entries from Bank 1

    deny         tcp any 192.168.60.0 0.0.0.255 eq 22 (35 matches)
    permit       ip any any (379 matches)
switch#

In the preceding example, rules for VACL policy VACL have dropped 35 SSH packets on TCP port 22.

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.

Mitigation: Spoofing Protection

Administrators can use the following spoofing protections to protect against compromised devices that are attempting to spoof TFTP server traffic.

Unicast Reverse Path Forwarding

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.

Identification: Spoofing Protection Using Unicast Reverse Path Forwarding

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

Note: Beginning with Cisco IOS Software Release 12.4(20)T, the command show ip cef switching has been replaced by show ip cef switching statistics feature.

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

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

Note: show cef interface type slot/port internal is a hidden command that must be fully entered at the command-line interface. Command completion is not available for it.

router#show 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 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 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 ip traffic | include RPF 18 no route, 18 unicast RPF, 0 forced drop router#

In the preceding show cef interface GigabitEthernet 0/0 internal, show ip interface GigabitEthernet 0/0, show cef drop, show ip cef switching statistics feature, and show ip traffic examples, uRPF has dropped 18 IP packets received globally on all interfaces with uRPF configured because of the inability to verify the source address of the IP packets within the forwarding information base of Cisco Express Forwarding.

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 features in this document that are used against a Layer 2 attack are DHCP snooping and Dynamic ARP Inspection (DAI). Note that configuring DHCP snooping is a prerequisite to configure DAI.

DHCP Snooping

DHCP snooping is a security feature that intercepts DHCP messages crossing a switch and blocks untrusted DHCP offers. DHCP snooping uses the concept of trusted and untrusted ports. Typically, trusted ports are used to reach DHCP servers or relay agents, while untrusted ports are used to connect to clients. All DHCP messages are allowed on trusted ports, while only DHCP client messages are accepted on untrusted ports. As neither servers nor relay agents are supposed to connect to untrusted ports, server messages like DHCPOFFER, DHCPACK, and DHCPNAK are dropped on untrusted ports. Additionally, DHCP snooping builds and maintains a MAC-to-IP binding table that is used to validate DHCP packets received from untrusted ports. The DHCP snooping binding table contains the MAC address, IP address, lease time in seconds, and VLAN port information for the DHCP clients on the untrusted ports of a switch. The information that is contained in a DHCP snooping binding table is removed from the binding table when its lease expires or DHCP snooping is disabled in the VLAN.

Use the show ip dhcp snooping command to display the DHCP snooping settings and the show ip dhcp snooping binding command to display the binding entries corresponding to untrusted ports.

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

Dynamic ARP Inspection (DAI)

The proper deployment and configuration of DAI provides an effective means of protection against spoofing attacks at layer 2. Deployment of DAI requires the implementation of DHCP snooping. DAI will drop all ARP Packets with invalid IP-to-MAC address bindings that fail the inspection.

DAI relies on the entries in the DHCP snooping binding database to verify IP-to-MAC address bindings. Configure each secure interface as trusted using the ip arp inspection trust interface configuration command. The trusted interfaces bypass the ARP inspection validation checks and all other packets are subject to inspection when they arrive on untrusted interfaces. The following example shows how to configure an interface as trusted and how to enable DAI for VLANs 5 through 10.

DAI Configuration Example:

Switch(config)#interface GigabitEthernet 1/0/1
Switch(config-if)#ip arp inspection trust
Switch(config)#ip arp inspection vlan 5-10

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 vulnerability that is described in this document.

The IP Source Guard Feature is enabled in combination with the DHCP snooping feature on Layer 2 interfaces, including access and trunk Ports.

IP Source Guard Configuration Example
Switch(config)#interface GigabitEthernet 1/0/1
Switch(config-if)#ip verify source port-security

The example above shows how to enable the IPSG with dynamic source IP and MAC address filtering.

Use the show ip verify source command to display the IPSG configuration and the show ip source binding command to display the IP source bindings on the switch.

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

Identity-Based Networking with IEEE 802.1X-Enabled Networks

IEEE 802.1x is a protocol standard framework for wired and wireless LANs that authenticates users or network devices and provides policy enforcement features at the port-level to provide secure network access control. The IEEE 802.1x standard, combined with the capability of network devices to communicate using existing protocols such as EAP and RADIUS, provides increased security and control of access to network segments and resources by associating the identity of a network-connected entity to a corresponding set of control policies.

Additional information about the deployment and configuration of 802.1x is in Identity-Based Networking Services: IP Telephony In IEEE 802.1X-Enabled Networks Deployment and Configuration Guide.

Cisco IOS NetFlow and Cisco IOS Flexible NetFlow

Identification: IPv4 Traffic Flow Identification Using Cisco IOS NetFlow

Administrators can configure Cisco IOS NetFlow on Cisco IOS routers and switches that forward traffic between the Data and Voice networks to aid in the identification of IPv4 traffic flows that may be attempts to exploit this vulnerability by a device on the Data network or flows originated by a compromised Voice network phone. Administrators are advised to investigate these flows to determine whether they are attempts to exploit these vulnerabilities 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 1023 1536 2048 2560 3072 3584 4096 4608
   .000 .001 .256 .000 .010 .000 .000 .000 .000 .000 .000

IP Flow Switching Cache, 4456704 bytes
  1885 active, 63651 inactive, 59960004 added
  129803821 ager polls, 0 flow alloc failures
  Active flows timeout in 30 minutes
  Inactive flows timeout in 15 seconds
IP Sub Flow Cache, 402056 bytes
  0 active, 16384 inactive, 0 added, 0 added to flow
  0 alloc failures, 0 force free
  1 chunk, 1 chunk added
  last clearing of statistics never
Protocol         Total    Flows   Packets Bytes  Packets Active(Sec) Idle(Sec)
--------         Flows     /Sec     /Flow  /Pkt     /Sec     /Flow     /Flow
TCP-Telnet    11393421      2.8         1    48      3.1       0.0       1.4
TCP-FTP            236      0.0        12    66      0.0       1.8       4.8
TCP-FTPD            21      0.0     13726  1294      0.0      18.4       4.1
TCP-WWW          22282      0.0        21  1020      0.1       4.1       7.3
TCP-X              719      0.0         1    40      0.0       0.0       1.3
TCP-BGP              1      0.0         1    40      0.0       0.0      15.0
TCP-Frag         70399      0.0         1   688      0.0       0.0      22.7
TCP-other     47861004     11.8         1   211     18.9       0.0       1.3
UDP-DNS            582      0.0         4    73      0.0       3.4      15.4
UDP-NTP         287252      0.0         1    76      0.0       0.0      15.5
UDP-other       310347      0.0         2   230      0.1       0.6      15.9
ICMP             11674      0.0         3    61      0.0      19.8      15.5
IPv6INIP            15      0.0         1  1132      0.0       0.0      15.4
GRE                  4      0.0         1    48      0.0       0.0      15.3 
Total:        59957957     14.8         1   196     22.5       0.0       1.5

SrcIf         SrcIPaddress    DstIf         DstIPaddress    Pr SrcP DstP  Pkts
Gi0/0         192.168.60.10   Gi0/1         192.168.60.102  11 1785 1941    12
Gi0/1         192.168.60.28   Gi0/0         192.168.13.97   11 1B3E 2A53  5391
Gi0/1         192.168.150.60  Gi0/0         10.89.16.226    06 0016 12CA     1
Gi0/0         192.168.10.17   Gi0/1         192.168.60.97   06 2B89 0016    45
Gi0/1         192.168.60.158  Gi0/0         192.168.11.54   06 0911 3628  1612
Gi0/0         10.88.226.1     Gi0/1         192.168.202.22  11 007B 007B     1
router#

In the preceding example, we can see flows with

  • 12 packets on high UDP ports (hex values 1785 and 1941) from 192.168.60.10 destined to 192.168.60.102. If these are two phone endpoints that are expected to be communicating over UDP high ports (i.e. RTP voice streams) then these could be legitimate transactions.
  • 5391 packets on high UDP ports (hex values 1B3E and 2A53) from 192.168.60.28 destined to 192.168.13.97. If 192.168.13.97 is not a TFTP server or a voice endpoint or gateway that is expected to be communicating with phone 192.168.60.28, then this could be an attempt to exploit this vulnerability by spoofing a TFTP transaction to exfiltrate data from a compromized phone (192.168.60.28). Further investigation will be needed.
  • 45 SSH packets on TCP port 22 (hex value 0016) between 192.168.10.17 and phone 192.168.60.97. If 192.168.10.17 is not expected to be managing phone 192.168.60.97 via SSH, then this coulde be an attempt to exploit this vulnerability. Further investigation will be needed.
  • 1612 packets high TCP port (hex values 0911 and 3628) between 192.168.60.158 and 192.168.11.54. If 192.168.11.54 is not a voice endpoint or gateway that is expected to be communicating with phone 192.168.60.158 over these high TCP ports, then the phone may be compromized and trying to expfiltrate data. Further investigation will be needed.

Legitimate TFTP data traffic that is sent from the data to voice network is sourced from the ip address of the trusted voice TFTP server and is sent to addresses within the 192.168.60.0/24 address block, which is used by affected 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 network traffic sent from the data to voice networks 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 SSH packets on TCP port 22 (hex value 16), use the show ip cache flow | include SrcIf|_06_.*0016 command to display the related Cisco NetFlow records:

router#show ip cache flow | include SrcIf|_06_.*0016
SrcIf         SrcIPaddress    DstIf         DstIPaddress    Pr SrcP DstP  Pkts
Gi0/0         192.168.10.17   Gi0/1         192.168.60.97   06 2B89 0016    45
router#

As shown in the following example, to view only packets transactions involving phone addesses use show ip cache flow | include Src_If|192.168.60 command to display the related Cisco NetFlow records:

router#show ip cache flow | include Src_If|192.168.60
SrcIf         SrcIPaddress    DstIf         DstIPaddress    Pr SrcP DstP  Pkts
Gi0/0         192.168.60.10   Gi0/1         192.168.60.102  11 1785 1941    12
Gi0/1         192.168.60.28   Gi0/0         192.168.13.97   11 1B3E 2A53  5391
Gi0/0         192.168.10.17   Gi0/1         192.168.60.97   06 2B89 0016    45
Gi0/1         192.168.60.158  Gi0/0         192.168.11.54   06 0911 3628  1612
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 this vulnerability 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 1023 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...128::201 Gi0/0  2001:DB..128::20   Gi0/1    0x11 0x16C4 0x13C4   12
2001:DB...6A:5BA6  Gi0/0  2001:DB...28::21   Gi0/1    0x3A 0x0000 0x8000  841
2001:DB...128::10  Gi0/1  1445:DB...17::121  Gi0/0    0x11 0x4281 0x4510 5101    
2001:DB...6A:5BA6  Gi0/0  2001:DB...134::3   Gi0/1    0x3A 0x0000 0x8000 1191
2001:DB...128::15  Gi0/1  3411:DB...22::44   Gi0/0    0x10 0x4281 0x0016 6814   
2001:DB...6A::15B  Gi0/0  2001:DB...128::2   Gi0/1    0x06 0x160A 0x134A 1597
2001:DB...6A:5BA6  Gi0/0  2001:DB...146::3   Gi0/1    0x3A 0x0000 0x8000 1092 

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

In the preceding example, we can see flows with

  • 12 packets on high UDP ports (hex values 16C4 and 13C4) from 2001:DB8:1:60:128::201 destined to 2001:DB8:1:60:128::20. If these are two phone endpoints that are expected to be communicating over high UDP ports (i.e. RTP voice streams) then these could be legitimate transactions.
  • 5101 packets on high UDP ports (hex values 1B3E and 2A53) from 2001:DB8:1:60:128::10 destined to 2001:DB8:17::121. If 2001:DB8:17::121 is not a TFTP server or a voice endpoint or gateway that is expected to be communicating with phone 2001:DB8:1:60:128::10, then this could be an attempt to exploit this vulnerability by spoofing a TFTP transaction to exfiltrate data from a compromized phone (2001:DB8:1:60:128::10). Further investigation will be needed.
  • 6814 SSH packets on TCP port 22 (hex value 0016) between 2001:DB8:1:128::15 and phone 2001:DB8:1:60:22::44. If 2001:DB8:1:128::15 is not expected to be managing phone 2001:DB8:1:60:22::44 via SSH, then this coulde be an attempt to exploit this vulnerability. Further investigation will be needed.
  • 1597 packets on high TCP port (hex values 160A and 134A) between 2001:DB8:1:60:6A::15B and 2001:DB8:3:128::2. If 2001:DB8:3:128::2 is not a voice endpoint or gateway that is expected to be communicating with phone 2001:DB8:1:60:6A::15B over these high TCP ports, then the phone may be compromized and trying to expfiltrate data. Further investigation will be needed.

The reader should note that legitimate TFTP data traffic sent from the data to voice network is sourced from the ip address of the trusted voice TFTP server and is sent to addresses within the2001:DB8:1:60::/64 address block, which is used by affected 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 network traffic sent from the data to voice networks 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 SSH packets on TCP port 22 (hex value 16), use the show ip cache flow | include SrcIf|_06_.*0016 command to display the related Cisco NetFlow records:

router#show ip cache flow | include SrcIf|_06_.*0016
SrcAddress         InpIf  DstAddress         OutIf    Prot SrcPrt DstPrt Packets
2001:DB...128::15  Gi0/1  3411:DB...22::44   Gi0/0    0x10 0x4281 0x0016 6814  
router#

As shown in the following example, to view only packets transactions involving phone addesses use show ip cache flow | include Src_If|2001:DB8:1:60 command to display the related Cisco NetFlow records:

router#show ip cache flow | include Src_If|2001:DB8:1:60
SrcAddress         InpIf  DstAddress         OutIf    Prot SrcPrt DstPrt Packets
2001:DB...128::201 Gi0/0  2001:DB..128::20   Gi0/1    0x11 0x16C4 0x13C4     12
2001:DB...128::10  Gi0/1  1445:DB...17::121  Gi0/0    0x11 0x4281 0x4510   5101    
2001:DB...128::15  Gi0/1  3411:DB...22::44   Gi0/0    0x10 0x4281 0x0016   6814   
2001:DB...6A::15B  Gi0/0  2001:DB...128::2   Gi0/1    0x06 0x160A 0x134A   1597 

    

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
==============  ============= ============= ============= ========== ===========  ==== =======
 192.168.60.10  192.168.60.201         1456          7631      Gi0/0       Gi0/1  1128      17
 192.168.60.28   192.168.13.97         8123         33112      Gi0/0       Gi0/1  5391      17
192.168.150.60    10.89.16.226         2567           443      Gi0/0       Gi0/1    13       6
 192.168.10.17   192.168.60.97         1289            22      Gi0/0       Gi0/1    45       6
192.168.60.158   192.168.11.54        32211          3628      Gi0/0       Gi0/1  1612       6

To only view the SSH flows on TCP port 22, use the show flow monitor FLOW-MONITOR-ipv4 cache format table | include IPV4 DST ADDR |_22_.*_6_ command to display the related NetFlow records.

To view only packet transactions involving phone addresses use the show ip cache flow | include Src_If|192.168.60 command to display the related Cisco 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 prt trns dst prt intf inpt intf outpt  pkts ip prot
================   =============== ============ ============ ========= ==========  ==== =======
2001:DB...128::201 2001:DB..128::20       Gi0/0        Gi0/1     21345      12134   222      17
2001:DB...6A:5BA6  2001:DB...28::21       Gi0/0        Gi0/1     12134         23   841       6
2001:DB...128::10  20015:DB...17::121     Gi0/1        Gi0/0     23254      45234    56      17
2001:DB...6A:5BA6  2001:DB...134::3       Gi0/0        Gi0/1      1231         53   135      17
2001:DB...128::15  2001:DB...22::44       Gi0/1        Gi0/0      4234         22   234       6
2001:DB...6A::15B  2001:DB...128::2       Gi0/0        Gi0/1     21431      12134   843       6
2001:DB...6A:5BA6  2001:DB...146::3       Gi0/0        Gi0/1     48212        121  1092      17

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

To view only the SSH flows on TCP port 22, use the show flow monitor FLOW-MONITOR-ipv6 cache format table | include IPV6 DST ADDR|_22_.*_6_ command to display the related Cisco IOS Flexible NetFlow records.

To view only packet transactions involving phone addesses use show ip cache flow | includeSrc_If|2001:DB8:1:60 command to display the related Cisco NetFlow records.

Cisco ASA and Cisco FWSM Firewalls

Mitigation: Transit Access Control Lists

To protect the network from traffic that enters the network at ingress access points, which may include Internet connection points, partner and supplier connection points, or VPN connection points, administrators are advised to deploy transit access control lists (tACLs) to perform policy enforcement. Administrators can construct a tACL by explicitly permitting only authorized traffic to enter the network at ingress access points or permitting authorized traffic to transit the network in accordance with existing security policies and configurations. A tACL workaround cannot provide complete protection against this vulnerability when the attack originates from a trusted source address.

The first access lists tACL-Policy-Data and IPv6-tACL-Policy-Data deny unauthorized SSH IPv4 and IPv6 packets on TCP ports 22 and TFTP IPv4 and IPv6 packets on UDP ports greater than 1023 that are sent from the data network to affected devices on the voice network. The second access lists tACL-Policy-Voice and IPv6-tACL-Policy-Voice deny unauthorized RTP IPv4 and IPv6 packets on UDP ports greater than 1023 that are sent from affected devices on the voice network to the data network and that may be an attempt to exfiltrate voice communications. 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 on the voice network, 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.

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 TCP and UDP ports
!-- This includes softphones and other communication end-points !-- deployed on the data network
!

access-list tACL-Policy-Data extended permit tcp host 192.168.100.1 192.168.60.0 255.255.255.0 eq 22 access-list tACL-Policy-Data extended permit udp host 192.168.100.1 gt 1023 192.168.60.0 255.255.255.0 gt 1023 !
!-- The following vulnerability-specific access control entries
!-- (ACEs) can aid in identification of attacks
!
access-list tACL-Policy-Data extended deny tcp any 192.168.60.0 255.255.255.0 eq 22 access-list tACL-Policy-Data extended deny udp any gt 1023 192.168.60.0 255.255.255.0 gt 1023 !
!-- 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-Data extended deny ip any any !
!-- Create the corresponding IPv6 tACL
!
!
!-- Include explicit permit statements for trusted sources
!-- that require access on the vulnerable TCP and UDP ports !-- This includes softphones and other communication end-points !-- deployed on the data network
!
ipv6 access-list IPv6-tACL-Policy-Data permit tcp host 2001:DB8::100:1 2001:DB8:1:60::/64 eq 22 ipv6 access-list IPv6-tACL-Policy-Data permit udp host 2001:DB8::100:1 gt 1023 2001:DB8:1:60::/64 gt 1023
!
!-- The following vulnerability-specific ACEs can
!-- aid in identification of attacks to global and
!-- link-local addresses
!
ipv6 access-list IPv6-tACL-Policy-Data deny tcp any 2001:DB8:1:60::/64 eq 22 ipv6 access-list IPv6-tACL-Policy-Data deny udp any gt 1023 2001:DB8:1:60::/64 gt 1023
!
!-- Permit or deny all other Layer 3 and Layer 4 traffic in
!-- accordance with existing security policies and configurations
!
!-- Explicit deny for all other IPv6 traffic
!

ipv6 access-list IPv6-tACL-Policy-Data deny ip any any !
!
!-- Apply tACLs to interfaces in the ingress direction
!
access-group tACL-Policy-Data in interface outside access-group IPv6-tACL-Policy-Data in interface outside
!
!-- Include explicit permit statements for trusted sources
!-- that require access on the vulnerable TCP and UDP ports

!-- This includes softphones and other communication end-points !-- deployed on the data network.
!
access-list tACL-Policy-Voice permit udp 192.168.60.0 255.255.255.0 gt 1023 host 192.168.100.1 gt 1023 !
!-- The following vulnerability-specific access control entries
!-- (ACEs) can aid in identification of attacks
!
access-list tACL-Policy-Voice deny udp 192.168.60.0 255.255.255.0 gt 1023 any gt 1023 !
!-- 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-Voice deny ip any any !
!-- Create the corresponding IPv6 tACL
!

!-- Include explicit permit statements for trusted sources
!-- that require access on the vulnerable TCP and UDP ports !-- This includes softphones and other communication end-points !-- deployed on the data network
! ipv6 access-list IPv6-tACL-Policy-Voice permit udp 2001:DB8:60::/64 gt 1023 host 2001:DB8:1:100::1 gt 1023
!
!-- The following vulnerability-specific ACEs can
!-- aid in identification of attacks to global and
!-- link-local addresses
!
ipv6 access-list IPv6-tACL-Policy-Voice deny udp 2001:DB8:60::/64 gt 1023 any gt 1023
!
!-- Permit or deny all other Layer 3 and Layer 4 traffic in
!-- accordance with existing security policies and configurations
!

!-- Explicit deny for all other IPv6 traffic
!

ipv6 access-list IPv6-tACL-Policy-Voice deny ip any any !
!-- Apply tACLs to interfaces in the ingress direction
!
access-group tACL-Policy-Voice in interface inside
access-group IPv6-tACL-Policy-Voice in interface inside
    

Identification: Access List Logging

After the tACLs have been applied to an interface, administrators can use the show ip access-lists to identify the number of SSH IPv4 and IPv6 packets on TCP ports 22 and TFTP IPv4 and IPv6 packets on UDP ports greater than 1023 that have been filtered. Administrators are advised to investigate filtered packets to determine whether they are attempts to exploit this vulnerability. Example output for show ip access-lists tACL-Policy-Data and show access-list IPv6-tACL-Policy-Data follows:

Firewall#show ip access-lists tACL-Policy-Data
access-list tACL-Policy-Data; 5 elements; name hash: 0x47502853
access-list tACL-Policy-Data line 1 extended permit tcp host 192.168.100.1 
     gt 1023 192.168.60.0 0.0.0.255 eq 22
access-list tACL-Policy-Data line 2 extended permit udp host 192.168.100.1 
     gt 1023 192.168.60.0 0.0.0.255 gt 1023
access-list tACL-Policy-Data line 3 extended deny tcp any 192.168.60.0 0.0.0.255 
     eq 22 (hitcnt=57)
access-list tACL-Policy-Data line 4 extended deny udp any gt 1023 192.168.60.0 0.0.0.255 
     gt 1023 (hitcnt=40)
access-list tACL-Policy-Data line 5 extended deny ip any any
Firewall#

In the preceding example, access list tACL-Policy-Data has dropped the following packets received from an untrusted host or network:

  • 57 SSH packets on TCP port 22 for ACE line 3
  • 40 TFTP packets on UDP ports greater than 1023 for ACE line 4
Firewall#show ipv6 access-list IPv6-tACL-Policy-Data 
ipv6 access-list IPv6-tACL-Policy-Data; 5 elements; name hash: 0x342aba4c
Ipv6 access-list IPv6-tACL-Policy-Data line 1 permit tcp host 2001:DB8::100:1 
     2001:DB8:1:60::/64 eq 22 (hitcnt=55)
Ipv6 access-list IPv6-tACL-Policy-Data line 2 permit udp host 2001:DB8::100:1 
     gt 1023 2001:DB8:1:60::/64 gt 1023 (hitcnt=38)
Ipv6 access-list IPv6-tACL-Policy-Data line 3 deny tcp any 2001:DB8:1:60::/64 
     eq 22 (hitcnt=18)
Ipv6 access-list IPv6-tACL-Policy-Data line 4 deny udp any gt 1023 2001:DB8:1:60::/64 
     gt 1023 (hitcnt=21)
Ipv6 access-list IPv6-tACL-Policy-Data line 5 deny ip any any (hitcnt=21)

In the preceding example, access list IPv6-tACL-Policy-Data has dropped the following packets received from an untrusted host or network:

  • 18 SSH packets on TCP port 22 for ACE line 3
  • 21 TFTP packets on TCP ports greater than 1023 for ACE line 4
Firewall#show access-list tACL-Policy-Voice
access-list tACL-Policy-Voice; 3 elements; name hash: 0xef0bf5e
access-list tACL-Policy-Voice line 1 extended permit udp 192.168.60.0 
     255.255.255.0 gt 1023 host 192.168.100.1 gt 1023 (hitcnt=0)
access-list tACL-Policy-Voice line 2 extended deny udp 192.168.60.0 
     255.255.255.0 gt 1023 any gt 1023 (hitcnt=29) 
access-list tACL-Policy-Voice line 3 extended deny ip any any (hitcnt=0)
Firewall#

In the preceding example, access list IPv6-tACL-Policy-Voice has dropped the following packets received from an untrusted host or network:

  • 29 RTP packets on UDP ports greater than 1023 for ACE line 2
Firewall#router#show ipv6 access-list IPv6-tACL-Policy-Voice 
ipv6 access-list IPv6-tACL-Policy-Voice line 1 permit udp 2001:db8:60::/64 
     gt 1023 host 2001:db8:1:100::1 gt 1023 (hitcnt=0) 
ipv6 access-list IPv6-tACL-Policy-Voice line 2 deny udp 2001:db8:60::/64 gt 
     1023 any gt 1023 (hitcnt=44)  
ipv6 access-list IPv6-tACL-Policy-Voice line 3 deny ip any any (hitcnt=0)

In the preceding example, access list IPv6-tACL-Policy-Voice has dropped the following packets received from an untrusted host or network:

  • 44 RTP packets on UDP ports greater than 1023 for ACE line 2

Identification: Firewall Access List Syslog Messages

Firewall syslog message 106023 will be generated for packets denied by an access control entry (ACE) that does not have the log keyword present. Additional information about this syslog message is in Cisco ASA 5500 Series System Log Message, 8.2 - 106023.

Information about configuring syslog for the Cisco ASA 5500 Series Adaptive Security Appliance is in Monitoring - Configuring Logging. Information about configuring syslog on the Cisco Catalyst 6500 Series ASA Services Module is in Configuring Logging. Information about configuring syslog on the FWSM for Cisco Catalyst 6500 Series Switches and Cisco 7600 Series Routers is in Monitoring the Firewall Services Module.

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

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

firewall#show logging | grep 106023
  Jan 09 2013 00:15:13: %ASA-4-106023: Deny udp src outside:192.0.2.18/2944 
         dst inside:192.168.60.191/6281 by access-group "tACL-Policy-Data"
  Jan 09 2013 00:15:13: %ASA-4-106023: Deny udp src inside:192.168.60.33/5298 
         dst outside:192.0.2.200/2834  by access-group "tACL-Policy-Voice"
  Jan 09 2013 00:15:13: %ASA-4-106023: Deny tcp src outside:2001:db8:2::2:172/2951
         dst inside:2001:db8:1:60::23/22 by access-group "IPv6-tACL-Policy-Data"
  Jan 09 2013 00:15:13: %ASA-4-106023: Deny udp src inside:2001:db8:1:60::134/7528
         dst outside:2001:db8:d::a85e:172/4752 by access-group "IPv6-tACL-Policy-Voice"
firewall#

Additional information about syslog messages for Cisco ASA Series Adaptive Security Appliances is in Cisco ASA 5500 Series System Log Messages, 8.2. Additional information about syslog messages for Cisco Catalyst 6500 Series ASA Services Module is in the Analyzing Syslog Messages section of the Cisco ASASM CLI Configuration Guide. Additional information about syslog messages for the Cisco FWSM is in Catalyst 6500 Series Switch and Cisco 7600 Series Router Firewall Services Module Logging System Log Messages.

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

Mitigation: Spoofing Protection Using Unicast Reverse Path Forwarding

The vulnerabilities that are described in this document can be exploited by spoofed IP packets. Administrators can deploy and configure uRPF as a protection mechanism against spoofing.

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

For additional information about the configuration and use of uRPF, reference the Cisco Security Appliance Command Reference for ip verify reverse-path and the Understanding Unicast Reverse Path Forwarding Cisco Security Intelligence Operations white paper.

Identification: Spoofing Protection Using Unicast Reverse Path Forwarding

Firewall syslog message 106021 will be generated for packets denied by uRPF. Additional information about this syslog message is in Cisco ASA 5500 Series System Log Message, 8.2 - 106021.

Information about configuring syslog for the Cisco ASA 5500 Series Adaptive Security Appliance is in Monitoring - Configuring Logging. Information about configuring syslog for the Cisco Catalyst 6500 Series ASA Services Module is in Configuring Logging. Information about configuring syslog on the FWSM for Cisco Catalyst 6500 Series Switches and Cisco 7600 Series Routers is in Monitoring the Firewall Services Module.

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

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

firewall#show logging | grep 106021
  Jan 09 2013 00:15:13: %ASA-1-106021: Deny UDP reverse path check from
         192.168.60.1 to 192.168.60.99 on interface outside
  Jan 09 2013 00:15:13: %ASA-1-106021: Deny UDP reverse path check from
         192.168.60.1 to 192.168.60.99 on interface outside
  Jan 09 2013 00:15:13: %ASA-1-106021: Deny TCP reverse path check from
         192.168.60.1 to 192.168.60.99 on interface outside

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

firewall#show asp drop frame rpf-violated
  Reverse-path verify failed                          11
firewall#

In the preceding example, uRPF has dropped 11 IP packets received on interfaces with uRPF configured. Absence of output indicates that the uRPF feature on the firewall has not dropped packets.

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

Cisco Security Manager

Identification: Cisco Security Manager

Cisco Security Manager, Event Viewer

Beginning in software version 4.0, Cisco Security Manager can collect syslogs from Cisco firewalls and Cisco IPS devices and provides the Event Viewer, which can query for events that are related to the vulnerabilities that are described in this document.

Using the following filters in the Firewall Denied Events predefined view in the Event Viewer provides all captured Cisco firewall access list deny syslog messages that could indicate potential attempts to exploit the vulnerabilities that are described in this document.

  • Use the Destination event filter to filter network objects that contain the IP address space that is used by the affected devices (for example, IPv4 address range 192.168.60.0/24 and IPv6 address range 2001:DB8:1:60::/64)
  • Use the Destination Service event filter to filter objects that contain TCP port 22 and UDP ports 1023 - 65535

An Event Type ID filter can be used with the Firewall Denied Events predefined view in the Event Viewer to filter the syslog IDs shown in the following list to provide all captured Cisco firewall deny syslog messages that could indicate potential attempts to exploit the vulnerabilities that are described in this document:

  • ASA-4-106021 (uRPF spoofing)
  • ASA-4-106023 (ACL deny)

For more information about Cisco Security Manager Events, refer to the Filtering and Querying Events section of the Cisco Security Manager User Guide.

Identification: Event Management System Partner Events

Cisco works with industry-leading Security Information and Event Management (SIEM) companies through the Cisco Developer Network. This partnership helps Cisco deliver validated and tested SIEM systems that address business concerns such as long-term log archiving and forensics, heterogeneous event correlation, and advanced compliance reporting. Security Information and Event Management partner products can be leveraged to collect events from Cisco devices and then query the collected events for the incidents created by a Cisco IPS signature or deny syslog messages from firewalls that could indicate potential attempts to exploit the vulnerabilities that are described in this document. The queries can be made by Sig ID and Syslog ID as shown in the following list:

  • ASA-4-106021 (uRPF spoofing)
  • ASA-4-106023 (ACL deny)

For more information about SIEM partners, refer to the Security Management System website.


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 Unified IP Phone 7906G 7.2 (3) | 8.0 (3), (4), (4)SR1, (4)SR2, (4)SR3A | 8.2 (1), (2), (2) SR1, (2) SR2, (2) SR4, (2) SR3 | 8.3 (1), (2), (2) SR1, (2) SR2, (2) SR3, (2) SR4, (3), (3) SR2 | 9.0 9.0(2)SR1, 9.0(2)SR2, 9.0(3), 9.1(1)SR1, 9.2(1)
CiscoCisco Unified IP Phone 7911G 7.2 (1), (2)SR2, (3) | 8.0 (1), (2)SR2, (3), (4)SR1, (4)SR2, (4)SR3A | 8.2 (1), (2) SR1, (2) SR2, (2) SR3, (2) SR4 | 8.3 (1), (2), (2) SR1, (3), (3) SR2 | 9.0 9.0(2)SR1, 9.0(2)SR2, 9.0(3), 9.1(1)SR1, 9.2(1)
CiscoCisco Unified IP Phone 7931G 8.3 (1), (2), (2)SR1, (3), (3) SR2 | 9.0 9.0(2)SR1, 9.0(2)SR2, 9.0(3), 9.1(1)SR1, 9.2(1)
CiscoCisco Unified IP Phone 7941G 7.0 (2), (2)SR1, (3) | 8.0 (1), (2)SR1, (3), (4), (4)SR1, (4)SR2, (4)SR3A | 8.2 (1), (2) SR1, (2) SR2, (2) SR3, (2) SR4 | 8.3 (1), (2), (2) SR1, (3), (3) SR2 | 9.0 9.0(2)SR1, 9.0(2)SR2, 9.0(3), 9.1(1)SR1, 9.2(1)
CiscoCisco Unified IP Phone 7942G 8.3 (2), (2) SR1, (3), (3) SR2 | 9.0 9.0(2)SR1, 9.0(2)SR2, 9.0(3), 9.1(1)SR1, 9.2(1)
CiscoCisco Unified IP Phone 7945G 8.3 (2), (2) SR1, (3), (3) SR2 | 9.0 9.0(2)SR1, 9.0(2)SR2, 9.0(3), 9.1(1)SR1, 9.2(1)
CiscoCisco Unified IP Phone 7961G 7.0 (2), (2) SR1, (3) | 8.0 (1), (2)SR1, (3), (4)SR1, (4)SR2, (4)SR3A | 8.2 (1), (2) SR1, (2) SR2, (2) SR3, (2) SR4 | 8.3 (1), (2), (2) SR1, (3), (3) SR2 | 9.0 9.0(2)SR1, 9.0(2)SR2, 9.0(3), 9.1(1)SR1, 9.2(1)
CiscoCisco Unified IP Phone 7962G 8.3 (2), (2) SR1, (3), (3) SR2 | 9.0 9.0(2)SR1, 9.0(2)SR2, 9.0(3), 9.1(1)SR1, 9.2(1)
CiscoCisco Unified IP Phone 7965G 8.3 (2), (2) SR1, (3), (3) SR2 | 9.0 9.0(2)SR1, 9.0(2)SR2, 9.0(3), 9.1(1)SR1, 9.2(1)
CiscoCisco Unified IP Phone 7970G 5.0 (1), (3) | 6.0 (1), (1)SR1, (2), (2)SR1, (3)SR1 | 7.0 (1), (2), (2)SR1, (3) | 8.0 (1), (2)SR1, (3), (4)SR1, (4)SR2, (4)SR3A | 8.3 (1), (2), (2) SR1, (3), (3) SR2 | 9.0 9.0(2)SR1, 9.0(2)SR2, 9.0(3), 9.1(1)SR1, 9.2(1)
CiscoCisco Unified IP Phone 7971G-GE 9.0 9.0(2)SR1, 9.0(2)SR2, 9.0(3), 9.1(1)SR1, 9.2(1)
CiscoCisco Unified IP Phone 7961G-GE 9.0 9.0(2)SR1, 9.0(2)SR2, 9.0(3), 9.1(1)SR1, 9.2(1)
CiscoCisco Unified IP Phone 7941G-GE 9.0 9.0(2)SR1, 9.0(2)SR2, 9.0(3), 9.1(1)SR1, 9.2(1)
CiscoCisco Unified IP Phone 7975G 8.3(2) Base, SR1 | 8.3(3) Base, SR2 | 8.3(4) Base, SR1 | 8.3(5) Base | 8.4(1) Base, SR1, SR2 | 8.4(2) Base | 8.4(3) Base | 8.4(4) Base | 8.5(2) Base, SR1 | 8.5(3) Base, SR1 | 8.5(4) Base | 9.0(2) Base, SR1, SR2 | 9.0(3) Base | 9.1(1) Base, SR1 | 9.2(1) Base, SR2 | 9.2(3) Base | 9.3(1) Base, SR1

Associated Products:
N/A




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LEGAL DISCLAIMER
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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