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Identifying and Mitigating Exploitation of the Denial of Service Vulnerability in Cisco TelePresence Codecs

Advisory ID: cisco-amb-20110831-tandberg

http://tools.cisco.com/security/center/content/CiscoAppliedMitigationBulletin/cisco-amb-20110831-tandberg

Revision 1.0

For Public Release 2011 August 31 16:00  UTC (GMT)


Contents

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

Cisco Response

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

Vulnerability Characteristics

Cisco TelePresence E/EX Personal Video units and MXP and C Series Codecs contain a vulnerability when processing a specially crafted Session Initiation Protocol (SIP) packet. This vulnerability can be exploited remotely without authentication and without end-user interaction. Successful exploitation of this vulnerability may cause the affected device to crash resultig in a denial of service (DoS) condition. Repeated attempts to exploit this vulnerability could result in a sustained DoS condition. An attacker could exploit this vulnerability using spoofed packets.

The attack vectors for exploitation are through a packet using the following protocols and ports:

  • SIP using TCP port 5060
  • SIP using UDP port 5060
  • SIP TLS using TCP port 5061
  • SIP TLS using UDP port 5061

This vulnerability has been assigned CVE identifier CVE-2011-2577.

Vulnerability Overview

Information about vulnerable, unaffected, and fixed software is available in the PSIRT Security Advisory, which is available at the following link: http://www.cisco.com/warp/public/707/cisco-sa-20110831-tandberg.shtml.

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.

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

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

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

The proper deployment and configuration of Unicast RPF 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 spoofing attacks 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

  • Transit access control lists (tACLs)
  • Unicast RPF

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

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

Cisco IOS Software, Cisco ASA, and FWSM firewalls can provide visibility through syslog messages and counter values displayed in the output from show commands.

Risk Management

Organizations are advised to follow their standard risk evaluation and mitigation processes to determine the potential impact of [this vulnerability|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 is dependent on specific customer situations such as product mix, network topology, traffic behavior, and organizational mission. As with any configuration change, evaluate the impact of this configuration prior to applying the change.

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

The iACL policy denies unauthorized SIP packets on TCP and UDP ports 5060, and SIP TLS packets on TCP and UDP ports 5061that are sent to affected devices. In the following example, 192.168.60.0/24 is the IP address space that is used by the affected devices, and the host at 192.168.100.1 is considered a trusted source that requires access to the affected devices. Care should be taken to allow required traffic for routing and administrative access prior to denying all unauthorized traffic. 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 ports !

permit tcp host 192.168.100.1 192.168.60.0 0.0.0.255 eq 5060 permit udp host 192.168.100.1 192.168.60.0 0.0.0.255 eq 5060 permit tcp host 192.168.100.1 192.168.60.0 0.0.0.255 eq 5061 permit udp host 192.168.100.1 192.168.60.0 0.0.0.255 eq 5061
!
!-- 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 5060 deny udp any 192.168.60.0 0.0.0.255 eq 5060 deny tcp any 192.168.60.0 0.0.0.255 eq 5061 deny udp any 192.168.60.0 0.0.0.255 eq 5061 !
!-- 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
!
!-- Apply iACL to interfaces in the ingress direction !

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

Mitigation: Spoofing Protection

Unicast Reverse Path Forwarding

The vulnerability that is described in this document can be exploited by spoofed IP packets. Administrators can deploy and configure Unicast Reverse Path Forwarding (Unicast RPF) as a protection mechanism against spoofing.

Unicast RPF is configured at the interface level and can detect and drop packets that lack a verifiable source IP address. Administrators should not rely on Unicast RPF to provide complete spoofing protection because spoofed packets may enter the network through a Unicast RPF-enabled interface if an appropriate return route to the source IP address exists. Administrators are advised to take care to ensure that the appropriate Unicast RPF mode (loose or strict) is configured during the deployment of this feature because it can drop legitimate traffic that is transiting the network. In an enterprise environment, Unicast RPF might be enabled at the Internet edge and the internal access layer on the user-supporting Layer 3 interfaces.

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

For additional information about the configuration and use of Unicast RPF, reference the Understanding Unicast Reverse Path Forwarding Applied Intelligence white paper.

IP Source Guard

IP source guard (IPSG) is a security feature that restricts IP traffic on nonrouted, Layer 2 interfaces by filtering packets based on the DHCP snooping binding database and manually configured IP source bindings. Administrators can use IPSG to prevent attacks from an attacker who attempts to spoof packets by forging the source IP address and/or the MAC address. When properly deployed and configured, IPSG coupled with strict mode Unicast RPF provides the most effective means of spoofing protection for the vulnerability that is described in this document.

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

Identification: Infrastructure Access Control Lists

After the administrator applies the iACL to an interface, the show ip access-lists command will identify the number of SIP packets on TCP and UDP ports 5060, and SIP TLS packets on TCP and UDP ports 5061 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 follows:

router#show ip access-lists Infrastructure-ACL-Policy
Extended IP access list Infrastructure-ACL-Policy
10 permit tcp host 192.168.100.1 192.168.60.0 0.0.0.255 eq 5060 (11 matches) 20 permit udp host 192.168.100.1 192.168.60.0 0.0.0.255 eq 5060 (63 matches) 30 permit tcp host 192.168.100.1 192.168.60.0 0.0.0.255 eq 5061 (17 matches) 40 permit udp host 192.168.100.1 192.168.60.0 0.0.0.255 eq 5061 (11 matches)
50 deny tcp any 192.168.60.0 0.0.0.255 eq 5060 (13 matches) 60 deny udp any 192.168.60.0 0.0.0.255 eq 5060 (17 matches) 70 deny tcp any 192.168.60.0 0.0.0.255 eq 5061 (36 matches) 80 deny udp any 192.168.60.0 0.0.0.255 eq 5061 (10 matches)
90 deny ip any 192.168.60.0 0.0.0.255
router#

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

  • 13 SIP packets on TCP port 5060 for ACE line 50
  • 17 SIP packets on UDP port 5060 for ACE line 60
  • 36 SIP TLS packets on TCP port 5061 for ACE line 70
  • 10 SIP TLS packets on UDP port 5061 for ACE line 80

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

Administrators can use Embedded Event Manager to provide instrumentation when specific conditions are met, such as ACE counter hits. The Applied Intelligence white paper Embedded Event Manager in a Security Context provides additional details about how to use this feature.

Identification: Access List Logging

The log 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 Applied Intelligence white paper.

Identification: Spoofing Protection Using Unicast Reverse Path Forwarding

With Unicast RPF properly deployed and configured throughout the network infrastructure, administrators can use the show 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 Unicast RPF has dropped.

Note: Beginning with Cisco IOS Software version 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 drop, show ip cef switching statistics feature and show ip traffic examples, Unicast RPF has dropped 18 IP packets received globally on all interfaces with Unicast RPF configured because of the inability to verify the source address of the IP packets within the Forwarding Information Base of Cisco Express Forwarding.

Cisco IOS NetFlow

Identification: Traffic Flow Identification Using NetFlow Records

Administrators can configure Cisco IOS NetFlow on Cisco IOS routers and switches to aid in the identification of traffic flows that may be attempts to exploit the 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             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         192.168.10.201  Gi0/1         192.168.60.102  06 0984 13C4     7
Gi0/0         192.168.11.54   Gi0/1         192.168.60.158  06 0911 13C5     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 13C4     5
Gi0/0         192.168.10.17   Gi0/1         192.168.60.97   06 0B89 13C5     1
Gi0/0         10.88.226.1     Gi0/1         192.168.202.22  11 007B 007B     1
Gi0/0         192.168.12.185  Gi0/1         192.168.60.239  11 0BD7 13C5     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 SIP on TCP and UDP ports 5060 (hex value 13C4), and SIP TLS on TCP and UDP ports 5061 (hex value 13C5).

The SIP packets on UDP ports 5060 and 5061 are sourced from and sent to addresses within the 192.168.60.0/24 address block, which is used by infrastructure devices. The packets in these UDP flows may be spoofed and may indicate an attempt to exploit this vulnerability. Administrators are advised to compare these flows to baseline utilization for SIP traffic sent on UDP ports 5060 and 5061, and also investigate the flows to determine whether they are sourced from untrusted hosts or networks.

To view only the traffic flows for SIP and SIP TLS on TCP ports 5060 (hex value 13C4) and 5061 (hex value 13C5) use the command show ip cache flow | include SrcIf|_06_.*(13C4|13C5)_ . To view only the traffic flows for SIP and SIP TLS on UDP ports 5060 (hex value 13C4) and 5061 (hex value 13C5) use the command show ip cache flow | include SrcIf|_11_.*(13C4|13C5)_. Both respective outputs of the related TCP and UDP NetFlow records are shown here:

TCP Flows

router#show ip cache flow | include SrcIf|_06_.*(13C4|13C5)_
SrcIf         SrcIPaddress     DstIf         DstIPaddress    Pr SrcP DstP  Pkts
Gi0/0         192.168.10.201   Gi0/1         192.168.60.102  06 0984 13C4     7
Gi0/0         192.168.11.54    Gi0/1         192.168.60.158  06 0911 13C5     3
Gi0/0         192.168.10.17    Gi0/1         192.168.60.97   06 0B89 13C5     1
router#
UDP Flows
router#show ip cache flow | include SrcIf|_11_.*(13C4|13C5)_
SrcIf         SrcIPaddress     DstIf         DstIPaddress    Pr SrcP DstP  Pkts
Gi0/0         192.168.13.97    Gi0/1         192.168.60.28   11 0B3E 13C4     5
Gi0/0         192.168.12.185   Gi0/1         192.168.60.239  11 0BD7 13C5     1
router#

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 SIP packets on TCP and UDP ports 5060, and SIP TLS packets on TCP and UDP ports 5061that are sent to affected devices. In the following example, 192.168.60.0/24 is the IP address space that is used by the affected devices, and the host at 192.168.100.1 is considered a trusted source that requires access to the affected devices. Care should be taken to allow required traffic for routing and administrative access prior to denying all unauthorized traffic.

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

! !-- Include explicit permit statements for trusted sources !-- that require access on the vulnerable ports !
access-list tACL-Policy extended permit tcp host 192.168.100.1 
     192.168.60.0 255.255.255.0 eq 5060
access-list tACL-Policy extended permit udp host 192.168.100.1 
     192.168.60.0 255.255.255.0 eq 5060
access-list tACL-Policy extended permit tcp host 192.168.100.1 
     192.168.60.0 255.255.255.0 eq 5061
access-list tACL-Policy extended permit udp host 192.168.100.1 
     192.168.60.0 255.255.255.0 eq 5061
! !-- The following vulnerability-specific access control entries !-- (ACEs) can aid in identification of attacks !
access-list tACL-Policy extended deny tcp any 192.168.60.0 255.255.255.0 eq 5060
access-list tACL-Policy extended deny udp any 192.168.60.0 255.255.255.0 eq 5060
access-list tACL-Policy extended deny tcp any 192.168.60.0 255.255.255.0 eq 5061
access-list tACL-Policy extended deny udp any 192.168.60.0 255.255.255.0 eq 5061
! !-- 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
! !-- Apply tACL to interface(s) in the ingress direction !
access-group tACL-Policy in interface outside

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 Unicast RPF as a protection mechanism against spoofing.

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

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

Identification: Transit Access Control Lists

After the tACL has been applied to an interface, administrators can use the show access-list command to identify the number of SIP packets on TCP and UDP ports 5060, and SIP TLS packets on TCP and UDP ports 5061that 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 follows:

firewall#show access-list tACL-Policy
access-list tACL-Policy; 9 elements
access-list tACL-Policy line 1 extended permit tcp host 192.168.100.1 
     192.168.60.0 255.255.255.0 eq sip (hitcnt=3)
access-list tACL-Policy line 2 extended permit udp host 192.168.100.1 
     192.168.60.0 255.255.255.0 eq sip (hitcnt=7)
access-list tACL-Policy line 3 extended permit tcp host 192.168.100.1 
     192.168.60.0 255.255.255.0 eq 5061 (hitcnt=21)
access-list tACL-Policy line 4 extended permit udp host 192.168.100.1 
     192.168.60.0 255.255.255.0 eq 5061 (hitcnt=27)
access-list tACL-Policy line 5 extended deny tcp any 
     192.168.60.0 255.255.255.0 eq sip (hitcnt=11)
access-list tACL-Policy line 6 extended deny udp any 
     192.168.60.0 255.255.255.0 eq sip (hitcnt=12)
access-list tACL-Policy line 7 extended deny tcp any 
     192.168.60.0 255.255.255.0 eq 5061 (hitcnt=1)
access-list tACL-Policy line 8 extended deny udp any 
     192.168.60.0 255.255.255.0 eq 5061 (hitcnt=1)
access-list tACL-Policy line 9 extended deny ip any any (hitcnt=8)
firewall#

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

  • 11 SIP packets on TCP port 5060 for ACE line 5
  • 12 SIP packets on UDP port 5060 for ACE line 6
  • 1 SIP TLS packet on TCP port 5061 for ACE line 7
  • 1 SIP TLS packet on UDP port 5061 for ACE line 8

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
  Aug 31 2011 00:15:13: %ASA-4-106023: Deny tcp src outside:192.0.2.18/2944 
         dst inside:192.168.60.191/5060 by access-group "tACL-Policy"
  Aug 31 2011 00:15:13: %ASA-4-106023: Deny udp src outside:192.168.60.200/2945 
         dst inside:192.168.60.33/5060 by access-group "tACL-Policy"
  Aug 31 2011 00:15:13: %ASA-4-106023: Deny udp src outside:192.0.2.99/2946 
         dst inside:192.168.60.240/5061 by access-group "tACL-Policy"
  Aug 31 2011 00:15:13: %ASA-4-106023: Deny tcp src outside:192.168.60.100/2947 
         dst inside:192.168.60.115/5060 by access-group "tACL-Policy"
  Aug 31 2011 00:15:13: %ASA-4-106023: Deny udp src outside:192.0.2.88/2949 
         dst inside:192.168.60.38/5061 by access-group "tACL-Policy"
  Aug 31 2011 00:15:13: %ASA-4-106023: Deny tcp src outside:192.0.2.175/2950 
         dst inside:192.168.60.250/5061 by access-group "tACL-Policy"
firewall#

In the preceding example, the messages logged for the tACL tACL-Policy show potentially spoofed SIP packets for UDP ports 5060, and SIP TLS packets for UDP ports 5061 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 Applied Intelligence 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
  Aug 31 2011 00:15:13: %ASA-1-106021: Deny UDP reverse path check from
         192.168.60.1 to 192.168.60.100 on interface outside
  Aug 31 2011 00:15:13: %ASA-1-106021: Deny UDP reverse path check from
         192.168.60.1 to 192.168.60.100 on interface outside
  Aug 31 2011 00:15:13: %ASA-1-106021: Deny UDP 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 IP 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

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Revision 1.0 2011-August-31 Initial public release

Cisco Security Procedures

Complete information on reporting security vulnerabilities in Cisco products, obtaining assistance with security incidents, and registering to receive security information from Cisco, is available on Cisco's worldwide website at http://www.cisco.com/en/US/products/products_security_vulnerability_policy.html. This includes instructions for press inquiries regarding Cisco security notices. All Cisco security advisories are available at http://www.cisco.com/go/psirt.

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