Cisco Applied Mitigation Bulletin

Identifying and Mitigating Exploitation of Multiple DoS Vulnerabilities in Cisco Unified Communications Products

Advisory ID: cisco-amb-20100825-cucm-cup

http://tools.cisco.com/security/center/content/CiscoAppliedMitigationBulletin/cisco-amb-20100825-cucm-cup

Revision 1.0

For Public Release 2010 August 25 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 Advisories Cisco Unified Communications Manager Denial of Service Vulnerabilities and Cisco Unified Presence Denial of Service Vulnerabilities and provides identification and mitigation techniques that administrators can deploy on Cisco network devices.

Vulnerability Characteristics

There are multiple vulnerabilities in the SIP process of the Cisco Unified Communications Manager and Cisco Unified Presence products. The following subsections summarize these vulnerabilities:

Cisco Unified Communications Manager Denial of Service (DoS) Vulnerabilities: These vulnerabilities can be exploited remotely without authentication and without end-user interaction. Successful exploitation of these vulnerabilities may result in a denial of service (DoS) condition. Repeated attempts to exploit these vulnerabilities could result in a sustained DoS condition.

The attack vectors for exploitation are through SIP packets using the following protocols and ports:

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

An attacker could exploit these vulnerabilities using spoofed packets.

These vulnerabilities have been assigned CVE identifiers CVE-2010-2837 and CVE-2010-2838.

Cisco Unified Presence Denial of Service (DoS) Vulnerabilities: These vulnerabilities can be exploited remotely without authentication and without end-user interaction. Successful exploitation of these vulnerabilities may result in a denial of service (DoS) condition. Repeated attempts to exploit these vulnerabilities could result in a sustained DoS condition.

The attack vectors for exploitation are through SIP packets using the following protocols and ports:

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

An attacker could exploit these vulnerabilities using spoofed packets.

These vulnerabilities have been assigned CVE identifiers CVE-2010-2839 and CVE-2010-2840.

Information about vulnerable, unaffected, and fixed software is available in the PSIRT Security Advisories, which are available at the following links: http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20100825-cucm and http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20100825-cup.

Mitigation Technique Overview

Cisco devices provide several countermeasures for these vulnerabilities. Administrators are advised to consider these protection methods to be general security best practices for infrastructure devices and the traffic that transits the network. 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:

  • Transit access control lists (tACLs)
  • 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 these vulnerabilities.

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.

  • tACLs
  • Unicast RPF

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

Effective use of Cisco Intrusion Prevention System (IPS) event actions provides visibility into and protection against attacks that attempt to exploit these vulnerabilities.

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.

The Cisco Security Monitoring, Analysis, and Response System (Cisco Security MARS) appliance can also provide visibility through incidents, queries, and event reporting.

Risk Management

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

Device-Specific Mitigation and Identification

Caution: The effectiveness of any mitigation technique depends on specific customer situations such as product mix, network topology, traffic behavior, and organizational mission. As with any configuration change, evaluate the impact of this configuration prior to applying the change.

Specific information about mitigation and identification is available for these devices:

Cisco IOS Routers and Switches

Mitigation: 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 these vulnerabilities when the attack originates from a trusted source address.

The tACL policy denies unauthorized SIP packets on TCP ports 5060 and 5061 and UDP ports 5060 and 5061 that 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 150 permit tcp host 192.168.100.1 192.168.60.0 0.0.0.255 eq 5060
access-list 150 permit tcp host 192.168.100.1 192.168.60.0 0.0.0.255 eq 5061
access-list 150 permit udp host 192.168.100.1 192.168.60.0 0.0.0.255 eq 5060
access-list 150 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
!

access-list 150 deny tcp any 192.168.60.0 0.0.0.255 eq 5060
access-list 150 deny tcp any 192.168.60.0 0.0.0.255 eq 5061
access-list 150 deny udp any 192.168.60.0 0.0.0.255 eq 5060
access-list 150 deny udp any 192.168.60.0 0.0.0.255 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 150 deny ip any any

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

interface GigabitEthernet0/0
 ip access-group 150 in

Note that filtering with an interface access list will elicit the transmission of ICMP unreachable messages back to the source of the filtered traffic. Generating these messages could have the undesired effect of increasing CPU utilization on the device. In Cisco IOS Software, ICMP unreachable generation is limited to one packet every 500 milliseconds by default. ICMP unreachable message generation can be disabled using the interface configuration command no ip unreachables. ICMP unreachable rate limiting can be changed from the default using the global configuration command ip icmp rate-limit unreachable interval-in-ms.

Mitigation: Spoofing Protection

Unicast Reverse Path Forwarding

The vulnerabilities that are 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 vulnerabilities that are described in this document.

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

Identification: Transit Access Control Lists

After the administrator applies the tACL to an interface, the show ip access-lists command will identify the number of SIP packets on TCP ports 5060 and 5061 and UDP ports 5060 and 5061 that have been filtered. Administrators are advised to investigate filtered packets to determine whether they are attempts to exploit these vulnerabilities. Example output for show ip access-lists 150 follows:

router#show ip access-lists 150
Extended IP access list 150
    10 permit tcp host 192.168.100.1 192.168.60.0 0.0.0.255 eq 5060 (1 match)
    20 permit tcp host 192.168.100.1 192.168.60.0 0.0.0.255 eq 5061 (31 matches)
    30 permit udp host 192.168.100.1 192.168.60.0 0.0.0.255 eq 5060 (15 matches)
    40 permit udp host 192.168.100.1 192.168.60.0 0.0.0.255 eq 5061 (5 matches)
    50 deny tcp any 192.168.60.0 0.0.0.255 eq 5060 (227 matches)
    60 deny tcp any 192.168.60.0 0.0.0.255 eq 5061 (257 matches)
    70 deny udp any 192.168.60.0 0.0.0.255 eq 5060 (130 matches)
    80 deny udp any 192.168.60.0 0.0.0.255 eq 5061 (175 matches)
    90 deny ip any any (5219 matches)

In the preceding example, access list 150 has dropped the following packets that are received from an untrusted host or network:

  • 227 SIP packets on TCP port 5060 for ACE line 50
  • 257 SIP packets on TCP port 5061 for ACE line 60
  • 130 SIP packets on UDP port 5060 for ACE line 70
  • 175 SIP 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 these vulnerabilities. Administrators are advised to investigate 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 (54955 total packets):
   1-32   64   96  128  160  192  224  256  288  320  352  384  416  448  480
   .082 .531 .375 .000 .000 .000 .000 .000 .000 .000 .000 .000 .000 .000 .000

    512  544  576 1024 1536 2048 2560 3072 3584 4096 4608
   .000 .000 .000 .000 .009 .000 .000 .000 .000 .000 .000

IP Flow Switching Cache, 278544 bytes
  167 active, 3929 inactive, 32741 added
  607632 ager polls, 0 flow alloc failures
  Active flows timeout in 30 minutes
  Inactive flows timeout in 15 seconds
IP Sub Flow Cache, 34056 bytes
  0 active, 1024 inactive, 0 added, 0 added to flow
  0 alloc failures, 0 force free
  1 chunk, 1 chunk added
  last clearing of statistics never
Protocol         Total    Flows   Packets Bytes  Packets Active(Sec) Idle(Sec)
--------         Flows     /Sec     /Flow  /Pkt     /Sec     /Flow     /Flow
TCP-WWW            109      0.0         3    40      0.0       0.0      15.4
TCP-BGP          28425      0.0         1    68      0.0       2.9      15.4
TCP-other         1111      0.0         6    40      0.0       0.0      15.4
UDP-NTP           2221      0.0         1    76      0.0       0.0      15.6
UDP-TFTP            95      0.0         4    28      0.0       0.0      15.6
UDP-other          589      0.0         6    28      0.0       0.0      15.4
ICMP                24      0.0        31  1009      0.0      19.9      15.4
Total:           32574      0.0         1    75      0.0       2.5      15.5

SrcIf         SrcIPaddress    DstIf         DstIPaddress    Pr SrcP DstP  Pkts
Et0/0         192.168.68.44   Et0/1          192.168.60.212  06 F208 098B     4 
Et0/0         192.168.38.121  Et0/1          192.168.60.6    06 A826 01BB     3 
Et0/0         192.168.224.241 Et0/1          192.168.60.182  06 7536 13C5     5 
Et0/0         192.168.212.211 Et0/1          192.168.60.114  06 AB5E 01BB     2 
Et0/0         192.168.205.69  Et0/1          192.168.60.110  06 98A5 0ABC    10 
Et0/0         192.168.40.45   Et0/1          192.168.60.42   06 5FA7 01BB     2 
Et0/0         192.168.4.192   Et0/1          192.168.93.248  11 FFFE 8002    15 
Et0/0         192.168.44.66   Et0/1          192.168.178.29  06 A30D 0F4A     3 
Et0/0         192.168.36.239  Et0/1          192.168.60.214  11 BCA3 0045     3 
Et0/0         192.168.60.164  Et0/1          192.168.60.26   11 1EFB 13C4     2 
Et0/0         192.168.234.206 Et0/1          192.168.147.20  11 C959 9972    17 
Et0/0         192.168.148.143 Et0/1          192.168.60.25   11 CD48 0045     2 
Et0/0         192.168.250.187 Et0/1          192.168.60.41   06 C5B3 098B     3 
Et0/0         192.168.227.167 Et0/1          192.168.125.75  06 1048 23FC     3 
Et0/0         192.168.107.126 Et0/1          192.168.194.53  06 3767 139B    13 
Et0/0         192.168.1.194   Et0/0          192.168.60.155  06 CE95 098B   192 
Et0/0         192.168.118.14  Et0/1          192.168.226.46  11 3966 FF31     8 
Et0/0         192.168.35.154  Et0/1          192.168.60.77   06 3C5C 0ABC     1 
Et0/0         192.168.145.167 Et0/1          192.168.60.74   11 B06D 0045     7 
Et0/0         192.168.56.109  Et0/1          192.168.247.33  11 3F4C 9E2C     6 
Et0/0         192.168.28.223  Et0/1          192.168.60.154  06 B35D 13C4     1 
Et0/0         192.168.139.201 Et0/1          192.168.60.229  06 8E56 07D0     2 
Et0/0         192.168.60.199  Et0/1          192.168.60.242  11 37AF 13C4     5 
Et0/0         192.168.212.244 Et0/1          192.168.59.244  06 9CB9 95F7    12 
Et0/0         192.168.133.250 Et0/1          192.168.60.49   06 41A2 098B     4 
Et0/0         192.168.92.118  Et0/1          192.168.13.136  11 82E2 95B8     2 
Et0/0         192.168.206.122 Et0/1          192.168.54.12   06 A09B 7514    11 
Et0/0         192.168.164.86  Et0/1          192.168.60.44   11 4ED8 0045     7 
Et0/0         192.168.144.222 Et0/1          192.168.60.188  06 770C 13C4     1 
Et0/0         192.168.138.85  Et0/1          192.168.60.38   11 9B7D 13C4    11 
Et0/0         192.168.185.139 Et0/1          192.168.97.208  11 A25E FE8C     8 
Et0/0         192.168.78.45   Et0/1          192.168.92.184  11 08B5 BD08    13 
Et0/0         192.168.2.81    Et0/1          192.168.60.138  11 3258 13C5     2 
Et0/0         192.168.144.96  Et0/1          192.168.99.50   06 9D6D 4E7E    15 
router#

In the preceding example, there are multiple flows for SIP on TCP (Protocol hex value 06) ports 5060 (hex value 13C4) and 5061 (hex value 13C5) and UDP (Protocol hex value 11) ports 5060 (hex value 13C4) and 5061 (hex value 13C5).

Some of this traffic is sourced from and 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 these vulnerabilities. Administrators are advised to compare these flows to baseline utilization for SIP traffic sent on TCP ports 5060 and 5061 and 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 packets on TCP (Protocol hex value 06) ports 5060 (hex value 13C4) and 5061 (hex value 13C5) and UDP (Protocol hex value 11) ports 5060 (hex value 13C4) and 5061 (hex value 13C5) , the commands show ip cache flow | include SrcIf|_06_.*(13C4|13C5) and show ip cache flow | include SrcIf|_11_.*(13C4|13C5) will display the related TCP and UDP NetFlow records as shown here:

TCP Flows

router#show ip cache flow | include SrcIf|_06_.*(13C4|13C5)
SrcIf         SrcIPaddress    DstIf         DstIPaddress    Pr SrcP DstP  Pkts
Et0/0         192.168.114.191 Et0/1          192.168.60.53   06 1713 13C4     4 
Et0/0         192.168.40.246  Et0/1          192.168.60.145  06 CC2D 13C5     9 
Et0/0         192.168.147.251 Et0/1          192.168.60.183  06 E2E1 13C4     1 
Et0/0         192.168.88.150  Et0/1          192.168.60.197  06 6E1D 13C5    10 
Et0/0         192.168.16.232  Et0/1          192.168.60.235  06 BD24 13C4     4 
Et0/0         192.168.30.204  Et0/1          192.168.60.16   06 1A93 13C4     3 
Et0/0         192.168.65.79   Et0/1          192.168.60.223  06 3FD5 13C5     2 
Et0/0         192.168.82.123  Et0/1          192.168.60.100  06 ACA7 13C4     2 
Et0/0         192.168.224.47  Et0/1          192.168.60.178  06 5BD7 13C4     3 
Et0/0         192.168.87.54   Et0/1          192.168.60.49   06 D55B 13C5     2 
router#

UDP Flows

router#show ip cache flow | include SrcIf|_11_.*(13C4|13C5)
SrcIf         SrcIPaddress    DstIf         DstIPaddress    Pr SrcP DstP  Pkts
Et0/0         192.168.151.1   Et0/1          192.168.60.96   11 2C2D 13C5     3 
Et0/0         192.168.237.123 Et0/1          192.168.60.131  11 5712 13C5     4 
Et0/0         192.168.246.100 Et0/1          192.168.60.37   11 FCBC 13C5     4 
Et0/0         192.168.126.21  Et0/1          192.168.60.103  11 9716 13C4     1 
Et0/0         192.168.60.28   Et0/1          192.168.60.244  11 E40B 13C4   192 
Et0/0         192.168.56.139  Et0/1          192.168.60.218  11 4EE8 13C4    10 
Et0/0         192.168.51.212  Et0/1          192.168.60.209  11 835D 13C4     3 
Et0/0         192.168.252.73  Et0/1          192.168.60.115  11 521E 13C4     3 
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 these vulnerabilities when the attack originates from a trusted source address.

The tACL policy denies unauthorized SIP packets on TCP ports 5060 and 5061 and UDP ports 5060 and 5061 that 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 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 5060
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 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 5060
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 vulnerabilities that are 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 ports 5060 and 5061 and UDP ports 5060 and 5061 that have been filtered. Administrators are advised to investigate filtered packets to determine whether they are attempts to exploit these vulnerabilities. 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=224)
access-list tACL-Policy line 2 extended permit tcp host 192.168.100.1 
                192.168.60.0 255.255.255.0 eq 5061 (hitcnt=28)
access-list tACL-Policy line 3 extended permit udp host 192.168.100.1 
                192.168.60.0 255.255.255.0 eq sip (hitcnt=36)
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=41)
access-list tACL-Policy line 5 extended deny tcp any 
                192.168.60.0 255.255.255.0 eq sip (hitcnt=78)
access-list tACL-Policy line 6 extended deny tcp any 
                192.168.60.0 255.255.255.0 eq 5061 (hitcnt=39)
access-list tACL-Policy line 7 extended deny udp any 
                192.168.60.0 255.255.255.0 eq sip (hitcnt=437)
access-list tACL-Policy line 8 extended deny udp any 
                192.168.60.0 255.255.255.0 eq 5061 (hitcnt=478)
access-list tACL-Policy line 9 extended deny ip any any (hitcnt=563)
firewall#

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

  • 78 SIP packets on TCP port 5060 (sip) for ACE line 5
  • 39 SIP packets on TCP port 5061 for ACE line 6
  • 437 SIP packets on UDP port 5060 (sip) for ACE line 7
  • 478 SIP packets 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 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
        Aug 04 2010 08:45:44: %ASA-4-106023: Deny tcp src outside:192.168.60.5/22724 
            dst inside:192.168.60.21/5060 by access-group "tACL-Policy"
        Aug 04 2010 08:45:44: %ASA-4-106023: Deny tcp src outside:192.168.0.4/40011 
            dst inside:192.168.60.15/5060 by access-group "tACL-Policy"
        Aug 04 2010 08:45:44: %ASA-4-106023: Deny tcp src outside:192.168.208.144/61650 
            dst inside:192.168.60.11/5060 by access-group "tACL-Policy"
        Aug 04 2010 08:45:48: %ASA-4-106023: Deny tcp src outside:192.168.0.2/59865 
            dst inside:192.168.60.31/5061 by access-group "tACL-Policy"
        Aug 04 2010 08:45:48: %ASA-4-106023: Deny tcp src outside:192.168.48.42/12345 
            dst inside:192.168.60.3/5061 by access-group "tACL-Policy"
        Aug 04 2010 08:45:48: %ASA-4-106023: Deny tcp src outside:192.168.126.168/5053 
            dst inside:192.168.60.9/5061 by access-group "tACL-Policy"
        Aug 04 2010 08:45:52: %ASA-4-106023: Deny udp src outside:192.168.60.134/22670 
            dst inside:192.168.60.11/5061 by access-group "tACL-Policy"
        Aug 04 2010 08:45:52: %ASA-4-106023: Deny udp src outside:192.168.44.68/18777 
            dst inside:192.168.60.13/5061 by access-group "tACL-Policy"
        Aug 04 2010 08:45:52: %ASA-4-106023: Deny udp src outside:192.68.214.152/13391 
            dst inside:192.168.60.41/5061 by access-group "tACL-Policy"
        Aug 04 2010 08:45:54: %ASA-4-106023: Deny udp src outside:192.168.23.3/21826 
            dst inside:192.168.60.10/5060 by access-group "tACL-Policy"
        Aug 04 2010 08:45:54: %ASA-4-106023: Deny udp src outside:192.168.34.173/29006 
            dst inside:192.168.60.8/5060 by access-group "tACL-Policy"
        Aug 04 2010 08:45:54: %ASA-4-106023: Deny udp src outside:192.168.28.109/16289 
            dst inside:192.168.60.99/5060 by access-group "tACL-Policy"
        Aug 04 2010 08:45:54: %ASA-4-106023: Deny udp src outside:192.168.81.251/9919 
            dst inside:192.168.60.1/5060 by access-group "tACL-Policy"
firewall#

In the preceding example, the messages logged for the tACL tACL-Policy show potentially spoofed SIP packets for TCP ports 5060 and 5061 and UDP ports 5060 and 5061 sent to the address block assigned to the affected 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 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
        Aug 04 2010 08:52:46: %ASA-1-106021: Deny UDP reverse path check from 
            192.168.60.202 to 192.168.60.1 on interface outside
        Aug 04 2010 08:52:46: %ASA-1-106021: Deny UDP reverse path check from 
            192.168.60.126 to 192.168.60.1 on interface outside
        Aug 04 2010 08:52:46: %ASA-1-106021: Deny UDP reverse path check from 
            192.168.60.22 to 192.168.60.1 on interface outside
        Aug 04 2010 08:52:46: %ASA-1-106021: Deny UDP reverse path check from 
            192.168.60.75 to 192.168.60.1 on interface outside
        Aug 04 2010 08:52:46: %ASA-1-106021: Deny UDP reverse path check from 
            192.168.60.248 to 192.168.60.1 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 (rpf-violated)                                   10

In the preceding example, Unicast RPF has dropped 10 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.

Cisco Intrusion Prevention System

Mitigation: Cisco IPS Signature Event Actions

Administrators can use Cisco Intrusion Prevention System (IPS) appliances and services modules to provide threat detection and help prevent attempts to exploit the vulnerabilities that are described in this document. These vulnerabilities may be detected by the following signatures:

  • 29219-0 CUCM Malformed REGISTER Message DoS
  • 29239-0 Cisco CUP Memory Corruption Vulnerability

29219-0 CUCM Malformed REGISTER Message DoS

Beginning with signature update S510 for sensors running Cisco IPS version 6.x and greater, this vulnerability can be detected by signature 29219/0 (Signature Name: CUCM Malformed REGISTER Message DoS). Signature 29219/0 is enabled by default, triggers a Medium severity event, has a signature fidelity rating (SFR) of 90, and is configured with a default event action of produce-alert.

This signature fires upon detecting a malformed SIP REGISTER message that can cause a Denial of Service in Cisco Unified Communications Manager. The vulnerability is documented in Cisco bug ID CSCtf66305 and has been assigned the CVE identifier CVE-2010-2838. Firing of this signature may indicate a potential exploit of this vulnerability.

29239-0 Cisco CUP Memory Corruption Vulnerability

Beginning with signature update S510 for sensors running Cisco IPS version 6.x and greater, this vulnerability can be detected by signature 29239/0 (Signature Name: Cisco CUP Memory Corruption Vulnerability). Signature 29239/0 is enabled by default, triggers a High severity event, has a signature fidelity rating (SFR) of 90, and is configured with a default event action of produce-alert.

This signature fires on attempts to exploit a memory corruption bug present in Cisco CUP using TCP port 5070. The vulnerability is documented in Cisco bug ID CSCtd39629 and has been assigned the CVE identifier CVE-2010-2840. Firing of this signature may indicate a potential exploit of this vulnerability.

Administrators can configure Cisco IPS sensors to perform an event action when an attack is detected. The configured event action performs preventive or deterrent controls to help protect against an attack that is attempting to exploit the vulnerabilities that are described in this document.

Cisco IPS sensors are most effective when deployed in inline protection mode combined with the use of an event action. Automatic Threat Prevention for Cisco IPS 6.x and greater sensors that are deployed in inline protection mode provides threat prevention against an attack that is attempting to exploit the vulnerabilities that are described in this document. Threat prevention is achieved through a default override that performs an event action for triggered signatures with a riskRatingValue greater than 90.

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

Cisco Security Monitoring, Analysis, and Response System

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

The Cisco Security Monitoring, Analysis, and Response System (Cisco Security MARS) appliance can create incidents regarding events that are related to the vulnerabilities that are described in this document using IPS signatures 29219-0 (Signature Name: CUCM Malformed REGISTER Message DoS) and 29239-0 (Signature Name: Cisco CUP Memory Corruption Vulnerability). After the S510 dynamic signature update has been downloaded, using keywords NR-29219/0 for IPS signature 29219/0 and NR-29239/0 for IPS signature 29239/0 and a query type of All Matching Event Raw Messages on the Cisco Security MARS appliance will provide a report that lists the incidents created by the IPS signature.

Beginning with the 4.3.1 and 5.3.1 releases of Cisco Security MARS appliances, support for the Cisco IPS dynamic signature updates feature has been added. This feature downloads new signatures from Cisco.com or from a local web server, correctly processes and categorizes received events that match those signatures, and includes them in inspection rules and reports. These updates provide event normalization and event group mapping, and they also enable the MARS appliance to parse new signatures from the IPS devices.

Caution: If dynamic signature updates are not configured, events that match these new signatures appear as unknown event type in queries and reports. Because MARS will not include these events in inspection rules, incidents may not be created for potential threats or attacks that occur within the network.

By default, this feature is enabled but requires configuration. If it is not configured, the following Cisco Security MARS rule will be triggered:

System Rule: CS-MARS IPS Signature Update Failure

When this feature is enabled and configured, administrators can determine the current signature version downloaded by MARS by selecting Help > About and reviewing the IPS Signature Version value.

Additional information about dynamic signature updates and instructions for configuring dynamic signature updates are available for the Cisco Security MARS 4.3.1 and 5.3.1 releases.

Additional Information

THIS DOCUMENT IS PROVIDED ON AN "AS IS" BASIS AND DOES NOT IMPLY ANY KIND OF GUARANTEE OR WARRANTY, INCLUDING THE WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR USE. YOUR USE OF THE INFORMATION ON THE DOCUMENT OR MATERIALS LINKED FROM THE DOCUMENT IS AT YOUR OWN RISK. CISCO RESERVES THE RIGHT TO CHANGE OR UPDATE THIS DOCUMENT AT ANY TIME.

Revision History

Revision 1.0

2010-August-25

Initial public release

Cisco Security Procedures

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

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