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Cisco Applied Mitigation Bulletin

Identifying and Mitigating Exploitation of the Multiple Vulnerabilities in Cisco ASA Adaptive Security Appliance and Cisco PIX Security Appliances

Advisory ID: cisco-amb-20090408-asa

http://tools.cisco.com/security/center/content/CiscoAppliedMitigationBulletin/cisco-amb-20090408-asa

Revision 1.2

For Public Release 2009 April 8 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 Multiple Vulnerabilities in Cisco ASA and Cisco PIX Security Appliances and provides identification and mitigation techniques that administrators can deploy on Cisco network devices.

Vulnerability Characteristics

There are multiple vulnerabilities in Cisco ASA Adaptive Security Appliances and Cisco PIX Security Appliances. The following subsections summarize these vulnerabilities:

VPN Authentication Bypass Vulnerability: This vulnerability can be exploited remotely with authentication and with end-user interaction. Successful exploitation of this vulnerability allows VPN users to bypass authentication.

Note that this vulnerability will not be addressed in this AMB. No network mitigations are available because of the nature of the vulnerability.

This vulnerability has been assigned CVE identifier CVE-2009-1155.

Crafted HTTP Packet DoS Vulnerability: This vulnerability can be exploited remotely without authentication and without end-user interaction. Successful exploitation of this vulnerability may cause the affected device to reload, resulting in a denial of service (DoS) condition. Repeated attempts to exploit this vulnerability could result in a sustained DoS condition. The attack vector for exploitation is through SSL packets using TCP port 443 and HTTP packets using TCP port 80.

This vulnerability has been assigned CVE identifier CVE-2009-1156.

Crafted TCP Packet DoS Vulnerability: This vulnerability can be exploited remotely without authentication and without end-user interaction. Successful exploitation of this vulnerability may cause the affected device to reload, resulting in a 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 packets using the following protocols and ports:

  • SSL using TCP port 443
  • HTTP using TCP port 80
  • Telnet using TCP port 23
  • SSH using TCP port 22
  • Cisco Tunneling Control Protocol (cTCP) for remote access VPNs using TCP port 10000 (configurable)
  • Virtual Telnet using TCP port 23
  • TLS proxy for encrypted voice inspection
  • Cut-through proxy for network access (virtual HTTP) using TCP port 80
  • TCP Intercept

Note that cTCP, TLS proxy for encrypted voice inspection, and TCP Intercept will not be addressed in this AMB because no network mitigations are available. These protocols are device-specific vulnerabilities or the malicious sources are unable to be identified.

This vulnerability has been assigned CVE identifier CVE-2009-1157.

Crafted H.323 Packet DoS Vulnerability: This vulnerability can be exploited remotely without authentication and without end-user interaction. Successful exploitation of this vulnerability may cause the affected device to reload, resulting in a DoS condition. Repeated attempts to exploit this vulnerability could result in a sustained DoS condition. The attack vector for exploitation is through H.323 packets using TCP port 1720. An attacker could exploit this vulnerability using spoofed packets.

Note that this vulnerability affects transit traffic; therefore the spoofing techniques discussed in this AMB will be the only network mitigations that apply to this vulnerability.

This vulnerability has been assigned CVE identifier CVE-2009-1158.

SQL*Net Packet DoS Vulnerability: This vulnerability can be exploited remotely without authentication and without end-user interaction. Successful exploitation of this vulnerability may cause the affected device to reload, resulting in a DoS condition. Repeated attempts to exploit this vulnerability could result in a sustained DoS condition. The attack vector for exploitation is through SQL*Net packets using TCP port 1521.

Note that this vulnerability will not be addressed in this AMB. No network mitigations are available because of the nature of the protocol's vulnerability.

This vulnerability has been assigned CVE identifier CVE-2009-1159.

Access Control List Bypass Vulnerability: This vulnerability can be exploited remotely without authentication and without end-user interaction. Successful exploitation of this vulnerability may allow traffic to bypass the implicit deny access control entry (ACE).

Note that this vulnerability will not be addressed in this AMB. No network mitigations are available because of the nature of the vulnerability.

This vulnerability has been assigned CVE identifier CVE-2009-1160.

Information about vulnerable, unaffected, and fixed software is available in the PSIRT Security Advisory, which is available at the following link: http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20090408-asa.

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:

  • 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 the vulnerabilities that are described in this document.

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, the Cisco PIX 500 Series Security Appliance, and the Firewall Services Module (FWSM) for Cisco Catalyst 6500 Series switches and Cisco 7600 Series routers using the following:

  • 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 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.

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 flow records can provide visibility into network-based exploitation attempts.

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

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

Cisco IOS Routers and Switches

Mitigation: Infrastructure Access Control Lists

To protect infrastructure devices and minimize the risk, impact, and effectiveness of direct infrastructure attacks, administrators are advised to deploy infrastructure access control lists (iACLs) to perform policy enforcement of traffic sent to infrastructure equipment. Administrators can construct an iACL by explicitly permitting only authorized traffic sent to infrastructure devices in accordance with existing security policies and configurations. For the maximum protection of infrastructure devices, deployed iACLs should be applied in the ingress direction on all interfaces to which an IP address has been configured. An iACL workaround cannot provide complete protection against these vulnerabilities when the attack originates from a trusted source address.

The iACL policy denies unauthorized packets that are sent to affected devices via the following protocols:

  • SSL using TCP port 443
  • HTTP and/or virtual HTTP using TCP port 80
  • Telnet and/or virtual Telnet using TCP port 23
  • SSH using TCP port 22

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 443
  permit tcp host 192.168.100.1 192.168.60.0 0.0.0.255 eq 80
  permit tcp host 192.168.100.1 192.168.60.0 0.0.0.255 eq 23
  permit tcp host 192.168.100.1 192.168.60.0 0.0.0.255 eq 22
 

  !
  !-- 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 443
  deny tcp any 192.168.60.0 0.0.0.255 eq 80
  deny tcp any 192.168.60.0 0.0.0.255 eq 23
  deny tcp any 192.168.60.0 0.0.0.255 eq 22
  

  !
  !-- 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/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

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

Some of the vulnerabilities that are described in this document can be exploited by spoofed IP packets. The proper deployment and configuration of Unicast Reverse Path Forwarding (Unicast RPF) can provide protection mechanisms for spoofing related to the following vulnerabilities:

  • Crafted TCP Packet DoS Vulnerability
  • Crafted H.323 Packet DoS Vulnerability

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. The proper deployment and configuration of IPSG coupled with strict mode Unicast RPF can provide the most effective means of spoofing protection to help mitigate the following vulnerabilities:

  • Crafted TCP Packet DoS Vulnerability
  • Crafted H.323 Packet DoS Vulnerability

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 higher-layer protocol packets, on their respective ports, 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 these vulnerabilities. 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 443
    20 permit tcp host 192.168.100.1 192.168.60.0 0.0.0.255 eq www
    30 permit tcp host 192.168.100.1 192.168.60.0 0.0.0.255 eq telnet
    40 permit tcp host 192.168.100.1 192.168.60.0 0.0.0.255 eq 22
    50 deny tcp any 192.168.60.0 0.0.0.255 eq 443 (72 matches)
    60 deny tcp any 192.168.60.0 0.0.0.255 eq www (165 matches)
    70 deny tcp any 192.168.60.0 0.0.0.255 eq telnet (16 matches)
    80 deny tcp any 192.168.60.0 0.0.0.255 eq 22 (13 matches)
    90 deny ip any 192.168.60.0 0.0.0.255 (351 matches)
router#

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

  • 72 SSL packets on TCP port 443 for ACE line 50
  • 165 HTTP packets on TCP port 80 for ACE line 60
  • 16 Telnet packets on TCP port 23 for ACE line 70
  • 13 SSH packets on TCP port 22 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, and show ip traffic commands to identify the number of packets that Unicast RPF has dropped.

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=38, 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
  11 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          38           0           0
router#


router#show ip traffic

IP statistics:
  Rcvd:  68051015 total, 2397325 local destination
         43999 format errors, 0 checksum errors, 33 bad hop count
         2 unknown protocol, 929 not a gateway
         21 security failures, 190123 bad options, 542768 with options
  Opts:  352227 end, 452 nop, 36 basic security, 1 loose source route
         45 timestamp, 59 extended security, 41 record route
         53 stream ID, 3 strict source route, 40 alert, 45 cipso, 0 ump
         361634 other
  Frags: 0 reassembled, 10008 timeouts, 56866 couldn't reassemble
         0 fragmented, 0 fragments, 0 couldn't fragment
  Bcast: 64666 received, 0 sent
  Mcast: 1589885 received, 2405454 sent
  Sent:  3001564 generated, 65359134 forwarded
  Drop:  4256 encapsulation failed, 0 unresolved, 0 no adjacency
         38 no route, 38 unicast RPF, 0 forced drop
         0 options denied
  Drop:  0 packets with source IP address zero
  Drop:  0 packets with internal loop back IP address 
    
router#

In the preceding show cef drop and show ip traffic examples, Unicast RPF has dropped 38 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 (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 01BB    62
Gi0/0         192.168.31.68   Gi0/1         192.168.60.138  06 0911 0050    43
Gi0/1         192.168.150.60  Gi0/0         10.89.16.226    11 0016 12CA     1
Gi0/0         192.168.13.97   Gi0/1         192.168.60.28   06 0B3E 0017    15
Gi0/0         192.168.50.17   Gi0/1         192.168.60.97   06 0B89 0016    11
Gi0/0         10.88.226.1     Gi0/1         192.168.202.22  11 007B 007B     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 SSL packets on TCP port 443 (hex value 01BB), HTTP packets on TCP port 80 (hex value 0050), Telnet packets on TCP port 23 (hex value 0017), and SSH packets on TCP port 22 (hex value 0016).

This traffic is sent to addresses within the 192.168.60.0/24 address block, which is used for infrastructure devices. Administrators are advised to compare these flows to baseline utilization for the traffic sent on the following protocols and ports and also to investigate the flows to determine whether they are sourced from untrusted hosts or networks:

  • SSL using TCP port 443
  • HTTP and/or virtual HTTP using TCP port 80
  • Telnet and/or virtual Telnet using TCP port 23
  • SSH using TCP port 22

To view only the traffic flows for the packets in the preceding list, the command show ip cache flow | include SrcIf|_06_.*(01BB|0050|0017|0016)_ will display the related TCP NetFlow records as shown here:

router#show ip cache flow | include SrcIf|_06_.*(01BB|0050|0017|0016)_
SrcIf         SrcIPaddress     DstIf         DstIPaddress    Pr SrcP DstP  Pkts
Gi0/0         192.168.10.201   Gi0/1         192.168.60.102  06 0984 01BB    62
Gi0/0         192.168.31.68    Gi0/1         192.168.60.138  06 0911 0050    43
Gi0/0         192.168.13.97    Gi0/1         192.168.60.28   06 0B3E 0017    15
Gi0/0         192.168.50.17    Gi0/1         192.168.60.97   06 0B89 0016    11
router#

Cisco ASA, PIX, 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 packets that are sent to affected devices via the following protocols:

  • SSL using TCP port 443
  • HTTP and/or virtual HTTP using TCP port 80
  • Telnet and/or virtual Telnet using TCP port 23
  • SSH using TCP port 22

Note that the tACL should be configured on a firewall that is upstream of the vulnerable firewall(s) to prevent the unauthorized packets from accessing the affected firewall(s).

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 any 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 443
access-list tACL-Policy extended permit tcp host 192.168.100.1 192.168.60.0 255.255.255.0 eq 80
access-list tACL-Policy extended permit tcp host 192.168.100.1 192.168.60.0 255.255.255.0 eq 23
access-list tACL-Policy extended permit tcp host 192.168.100.1 192.168.60.0 255.255.255.0 eq 22


!
!-- 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 443
access-list tACL-Policy extended deny tcp any 192.168.60.0 255.255.255.0 eq 80
access-list tACL-Policy extended deny tcp any 192.168.60.0 255.255.255.0 eq 23
access-list tACL-Policy extended deny tcp any 192.168.60.0 255.255.255.0 eq 22


!
!-- Permit/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

Some of the vulnerabilities that are described in this document can be exploited by spoofed IP packets. The proper deployment and configuration of Unicast RPF can provide protection mechanisms for spoofing related to the following vulnerabilities:

  • Crafted HTTP Packet DoS Vulnerability
  • Crafted TCP Packet DoS Vulnerability
  • Crafted H.323 Packet DoS Vulnerability

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 higher-layer protocol packets on their respective ports 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 https (hitcnt=455)  
access-list tACL-Policy line 2 extended permit tcp host 192.168.100.1 192.168.60.0 255.255.255.0 eq www (hitcnt=332)  
access-list tACL-Policy line 3 extended permit tcp host 192.168.100.1 192.168.60.0 255.255.255.0 eq telnet (hitcnt=112)  
access-list tACL-Policy line 4 extended permit tcp host 192.168.100.1 192.168.60.0 255.255.255.0 eq ssh (hitcnt=75)  
access-list tACL-Policy line 5 extended deny tcp any 192.168.60.0 255.255.255.0 eq https (hitcnt=44)  
access-list tACL-Policy line 6 extended deny tcp any 192.168.60.0 255.255.255.0 eq www (hitcnt=35)  
access-list tACL-Policy line 7 extended deny tcp any 192.168.60.0 255.255.255.0 eq telnet (hitcnt=86)  
access-list tACL-Policy line 8 extended deny tcp any 192.168.60.0 255.255.255.0 eq ssh (hitcnt=31)    
access-list tACL-Policy line 9 extended deny ip any any (hitcnt=672)  
firewall#

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

  • 44 SSL packets on TCP port 443 (https) for ACE line 5
  • 35 ASDM (HTTP) packets on TCP port 80 (www) for ACE line 6
  • 86 Telnet packets on TCP port 23 (telnet) for ACE line 7
  • 31 SSH packets on TCP port 22 (ssh) 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 Security Appliance System Log Message - 106023.

Information about configuring syslog for the Cisco ASA 5500 Series Adaptive Security Appliance or the Cisco PIX 500 Series Security Appliance is in Monitoring the Security Appliance - Configuring and Managing Logs. 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
  Mar 31 2009 00:15:13: %ASA-4-106023: Deny tcp src outside:192.0.2.18/2944 
         dst inside:192.168.60.191/443 by access-group "tACL-Policy"
  Mar 31 2009 00:15:13: %ASA-4-106023: Deny tcp src outside:192.2.0.210/2945 
         dst inside:192.168.60.33/80 by access-group "tACL-Policy"
  Mar 31 2009 00:15:13: %ASA-4-106023: Deny tcp src outside:192.0.2.100/2946 
         dst inside:192.168.60.240/23 by access-group "tACL-Policy"
  Mar 31 2009 00:15:13: %ASA-4-106023: Deny tcp src outside:192.0.2.90/2947 
         dst inside:192.168.60.115/22 by access-group "tACL-Policy"
firewall#

In the preceding example, the messages logged for the tACL tACL-Policy show higher-layer protocol packets for the following protocols and ports sent to the address block assigned to the affected devices:

  • SSL using TCP port 443
  • HTTP and/or virtual HTTP using TCP port 80
  • Telnet and/or virtual Telnet using TCP port 23
  • SSH using TCP port 22

Additional information about syslog messages for ASA and PIX security appliances is in Cisco Security Appliance System Log Messages. 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 Security Appliance System Log Message - 106021.

Information about configuring syslog for the Cisco ASA 5500 Series Adaptive Security Appliance or the Cisco PIX 500 Series Security Appliance is in Monitoring the Security Appliance - Configuring and Managing Logs. 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
  Mar 31 2009 00:15:13: %ASA-1-106021: Deny TCP reverse path check from
         192.168.60.1 to 192.168.60.100 on interface outside
  Mar 31 2009 00:15:14: %ASA-1-106021: Deny TCP reverse path check from
         192.168.60.1 to 192.168.60.100 on interface outside
  Mar 31 2009 00:15:15: %ASA-1-106021: Deny TCP reverse path check from
         192.168.60.1 to 192.168.60.100 on interface outside

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

firewall#show asp drop frame rpf-violated
  Reverse-path verify failed                          17
firewall#

In the preceding example, Unicast RPF has dropped 17 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:

  • 16393/0 - Cisco ASA Crafted TCP Packet DoS Vulnerability
  • 16294/0 - ASA Crafted H.323 Packet DoS

16393/0 - Cisco ASA Crafted TCP Packet DoS Vulnerability

Beginning with signature update S392 for sensors running Cisco IPS version 6.x or 5.x, these vulnerabilities can be detected by signature 16393/0 (Signature Name: Cisco ASA Crafted TCP Packet DoS Vulnerability). Signature 16393/0 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.

Signature 16393/0 fires when a malicious TCP packet that can cause a denial of service on Cisco ASA/PIX devices is detected. Firing of this signature may indicate a potential exploit of these vulnerabilities.

16294/0 - ASA Crafted H.323 Packet DoS

Beginning with signature update S392 for sensors running Cisco IPS version 6.x or 5.x, these vulnerabilities can be detected by signature 16294/0 (Signature Name: ASA Crafted H.323 Packet DoS). Signature 16294/0 is enabled by default, triggers a Medium severity event, has a signature fidelity rating (SFR) of 95, and is configured with a default event action of produce-alert.

Signature16294/0 fires when attempts to exploit the Crafted H.323 packet denial of service vulnerability in the Cisco ASA 5500 Series Adaptive Security Appliances and Cisco PIX Security Appliances are detected. Firing of this signature may indicate a potential exploit of these vulnerabilities.

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 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.

Cisco IPS 5.x sensors that are deployed in inline protection mode require an event action configured on a per-signature basis. Alternatively, administrators can configure an override that can perform an event action for any signatures that are triggered and are calculated as a high-risk threat. Using an event action on sensors deployed in inline protection mode provides the most effective exploit prevention.

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

Identification: IPS Signature Events

Signature: 16393/0 Cisco ASA Crafted TCP Packet DoS Vulnerability

IPS#show events alert
evIdsAlert: eventId=1226247114084901519 severity=medium vendor=Cisco 
  originator: 
    hostId: IPS
    appName: sensorApp
    appInstanceId: 32765
  time: 2009/04/16 15:47:04 2009/04/16 15:47:04 UTC
  signature: description=Cisco ASA Crafted TCP Packet DoS
Vulnerability id=16393 created=20090409 type=vulnerability
version=S392 
    subsigId: 0
    sigDetails: Crafted TCP Packet
    marsCategory: DoS/MiscServer
  interfaceGroup: vs0
  vlan: 10


!
!-- Packet details removed
!
 
                               
  riskRatingValue: attackRelevanceRating=relevant
targetValueRating=medium 77
  threatRatingValue: 77
  interface: ge0_0
  protocol: tcp

Signature: 16294/0 ASA Crafted H.323 Packet DoS

IPS#show events alert
evIdsAlert: eventId=1228976905083633523 severity=medium vendor=Cisco 
  originator: 
    hostId: IPS
    appName: sensorApp
    appInstanceId: 8784
  time: 2009/04/14 10:04:41 2009/04/14 10:04:41 UTC
  signature: description=ASA Crafted H.323 Packet DoS
Vulnerability id=16294 created=20090409 type=exploit 
version=S392 
    subsigId: 0
    sigDetails: ASA Crafted H.323 Packet DoS
    marsCategory: DoS/NetworkDevice
  interfaceGroup: vs0
  vlan: 30


!
!-- Packet details removed
!
 
                               
  riskRatingValue: attackRelevanceRating=relevant
targetValueRating=medium 76
  threatRatingValue: 76
  interface: ge0_0
  protocol: tcp

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 16393/0 (Signature Name: Cisco ASA Crafted TCP Packet DoS Vulnerability) and 16294/0 (Signature Name: ASA Crafted H.323 Packet DoS). After the S392 dynamic signature update has been downloaded, using keyword NR-16393/0 for IPS signature 16393/0 and keyword NR-16294/0 for IPS signature 16294/0 and a query type of All Matching Events 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 1.2

2009-April-27

Added Intrusion Prevention System section; updated Cisco Security Monitoring, Analysis, and Response System section.

Revision 1.1

2009-April-08

Removed Crafted HTTP > Packet DoS Vulnerability information.

Revision 1.0

2009-April-08

Initial public release.

Cisco Security Procedures

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

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