Cisco Applied Mitigation Bulletin

Identifying and Mitigating Exploitation of the Cisco IOS Software TCP Denial of Service Vulnerability

Advisory ID: cisco-amb-20100812-tcp

http://tools.cisco.com/security/center/content/CiscoAppliedMitigationBulletin/cisco-amb-20100812-tcp

Revision 1.0

For Public Release 2010 August 12 21:30  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 Cisco IOS Software TCP Denial of Service Vulnerability and provides identification and mitigation techniques that administrators can deploy on Cisco network devices.

Vulnerability Characteristics

Cisco IOS® Software version 15.1(2)T contains a denial of service (DoS) vulnerability when it processes TCP packets. This vulnerability can be exploited remotely without authentication and without end-user interaction. Successful exploitation of this vulnerability may result in a DoS condition. Repeated attempts to exploit this vulnerability could result in a sustained DoS condition. The attack vector for exploitation is through TCP packets. An attacker could exploit this vulnerability using spoofed packets.

This vulnerability has been assigned CVE identifier CVE-2010-2827.

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-20100812-tcp.

Mitigation Technique Overview

The mitigating factors for this vulnerability are the effective filtering and restricting of traffic from trusted sources only, to the affected devices. 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

  • tACLs
  • Unicast RPF
  • TCP Intercept

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

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

Cisco IOS Routers and Switches

Mitigation: Infrastructure Access Control Lists

To protect infrastructure devices and minimize the risk, impact, and effectiveness of direct infrastructure attacks, administrators are advised to deploy 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 TCP packets 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. 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 TCP services on the device  
  !

  permit tcp host 192.168.100.1 192.168.60.0 0.0.0.255

  !
  !-- The following vulnerability-specific access control entry
  !-- (ACE) can aid in identification of attacks
  !

  deny tcp any 192.168.60.0 0.0.0.255 
  
  !
  !-- 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

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 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. 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 TCP packets 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
    20 deny tcp any 192.168.60.0 0.0.0.255 (22 matches)
    30 deny ip any 192.168.60.0 0.0.0.255 
router#

In the preceding example, access list Infrastructure-ACL-Policy has dropped 22 TCP packets for access control list entry (ACE) line 20.

For additional information about investigating incidents using ACE counters and syslog events, reference the Identifying Incidents Using Firewall and IOS Router Syslog Events 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 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 00A1     1
Gi0/0         192.168.11.54   Gi0/1         192.168.60.158  06 0911 00A1     3
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 00A1     5
Gi0/0         192.168.10.17   Gi0/1         192.168.60.97   06 0B89 00A1     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  06 0BD7 00A1     1
Gi0/0         10.89.16.226    Gi0/1         192.168.150.60  11 12CA 0016     1
router#

In the preceding example, there are multiple flows for TCP packets (Protocol Pr hex value 06).

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

To view only the traffic flows for TCP packets, the command show ip cache flow | include SrcIf|192.168.60.*_06_ will display the related TCP NetFlow records as shown here:

router#show ip cache flow | include SrcIf|192.168.60.*_06_
SrcIf         SrcIPaddress     DstIf         DstIPaddress    Pr SrcP DstP  Pkts
Gi0/0         192.168.10.201   Gi0/1         192.168.60.102  06 0984 00A1     1
Gi0/0         192.168.11.54    Gi0/1         192.168.60.158  06 0911 00A1     3
Gi0/0         192.168.13.97    Gi0/1         192.168.60.28   06 0B3E 00A1     5
Gi0/0         192.168.10.17    Gi0/1         192.168.60.97   06 0B89 00A1     1
Gi0/0         192.168.12.185   Gi0/1         192.168.60.239  06 0BD7 00A1     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 TCP packets 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 to the TCP services on the vulnerable devices
!

access-list tACL-Policy extended permit tcp host 192.168.100.1 
     192.168.60.0 255.255.255.0 

!
!-- The following vulnerability-specific access control entry
!-- (ACEs) can aid in identification of attacks
!

access-list tACL-Policy extended deny tcp any 192.168.60.0 255.255.255.0 

!
!-- 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.

Mitigation: Embryonic Connection Limiting with TCP Intercept

TCP Intercept can be used to mitigate the vulnerability described in this document. TCP Intercept will force connecting source endpoints to validate themselves through the use of SYN cookies. Administrator can configure this form of protection by using static NAT or static identity NAT for ASA, PIX, and FWSM firewalls. In addition, the ASA firewall and the Cisco PIX 500 Series Security Appliance may be configured to use TCP Intercept through the use of the Modular Policy Framework (MPF).

Additional information is available regarding on Static NAT, Static Identity NAT, and the Modular Policy Framework.

In the following example, an embryonic connection limit of one will be set. This limit will, in effect, force simultaneous TCP connections to be validated using SYN cookies. The following commands will set an embryonic connection limit of one for connections to TCP ports for hosts in the 192.168.60.0/24 subnet. It is possible to use the MPF configuration method to set an embryonic connection timeout. The default is 30 seconds. In the following example, the policy is applied globally to all interfaces.


!
!-- Configure an access list that will be used to match on TCP traffic 
!-- to affected devices
!


access-list TCP-Intercept-ACL extended permit tcp any 192.168.60.0 255.255.255.0 


!
!-- Configure a class that uses the above-configured access list
!-- to match TCP packets that are destined to TCP services on affected devices 
!


class-map TCP-Intercept-Class
 match access-list TCP-Intercept-ACL


!
!
!-- Add the above-configured "TCP-Intercept-Class" that matches
!-- TCP packets, to the default
!-- policy "global_policy" and use it to inspect 
!-- TCP traffic that transits the firewall
!


policy-map global_policy


!
!-- For the TCP-Intercept-Class, set the embryonic connection maximum to 1, 
!-- which will trigger SYN cookies when there is more than one simultaneous connection.
!


 class TCP-Intercept-Class
  set connection embryonic-conn-max 1 


!  
!-- By default, the policy "global_policy" is applied 
!-- globally, which results in the inspection of 
!-- traffic that enters the firewall from all interfaces 
!


service-policy global_policy global

Additional information about connection limits is in the Preventing Network Attacks section of the Cisco Security Appliance Command Line Configuration Guide.

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 TCP packets that have been filtered. Administrators are advised to investigate filtered packets to determine whether they are attempts to exploit this vulnerability. Example output for show access-list tACL-Policy follows:

firewall#show access-list tACL-Policy
access-list tACL-Policy; 3 elements
access-list tACL-Policy line 1 extended permit tcp host 192.168.100.1 
     192.168.60.0 255.255.255.0 (hitcnt=34)
access-list tACL-Policy line 2 extended deny tcp any 
     192.168.60.0 255.255.255.0 (hitcnt=119)
access-list tACL-Policy line 3 extended deny ip any any (hitcnt=8)
firewall#

In the preceding example, access list tACL-Policy has dropped 119 TCP packets received from an untrusted host or network. In addition, syslog message 106023 can provide valuable information, which includes the source and destination IP address, the source and destination port numbers, and the IP protocol for the denied packet.

Identification: Firewall Access List Syslog Messages

Firewall syslog message 106023 will be generated for packets denied by an 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 11 2010 00:15:13: %ASA-4-106023: Deny tcp src outside:192.0.2.18/2944 
         dst inside:192.168.60.191/23 by access-group "tACL-Policy"
  Aug 11 2010 00:15:13: %ASA-4-106023: Deny tcp src outside:192.0.2.200/2945 
         dst inside:192.168.60.33/80 by access-group "tACL-Policy"
  Aug 11 2010 00:15:13: %ASA-4-106023: Deny tcp src outside:192.0.2.99/2946 
         dst inside:192.168.60.240/22 by access-group "tACL-Policy"
  Aug 11 2010 00:15:13: %ASA-4-106023: Deny tcp src outside:192.0.2.100/2947 
         dst inside:192.168.60.115/53 by access-group "tACL-Policy"
firewall#

In the preceding example, the messages logged for the tACL tACL-Policy show potentially spoofed TCP packets sent to the address block assigned to 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 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 11 2010 00:15:13: %ASA-1-106021: Deny TCP reverse path check from
         192.168.60.1 to 192.168.60.100 on interface outside
  Aug 11 2010 00:15:13: %ASA-1-106021: Deny TCP reverse path check from
         192.168.60.1 to 192.168.60.100 on interface outside
  Aug 11 2010 00:15:13: %ASA-1-106021: Deny TCP reverse path check from
         192.168.60.1 to 192.168.60.100 on interface outside

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

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

In the preceding example, Unicast RPF has dropped 11 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.

Identification: Threat Detection Statistics

TCP Intercept Statistics

Beginning with ASA and PIX version 8.0(4), TCP Intercept threat detection statistics are enabled by the threat-detection statistics tcp-intercept command documented in the Cisco Security Appliance Command Reference. These statistics reflect mitigation performed as a result of embryonic connection limits that are configured by static NAT, static identity NAT or by using the Modular Policy Framework (MPF). In the following example, the show threat-detection statistics top tcp-intercept command provides information on the top ten protected servers under attack.

ASA#show threat-detection statistics top tcp-intercept 
Top 10 protected servers under attack (sorted by average rate)
Monitoring window size: 30 mins    Sampling interval: 30 secs
<Rank> <Server IP:Port> <Interface> <Ave Rate> <Cur Rate> <Total> <Source IP (Last Attack Time)>
--------------------------------------------------------------------------------
1    192.168.128.26:22 outside 12 109 22744 <various> Last: 192.168.60.101 (0 secs ago)
2    192.168.1.7:8200 outside 0 0 4 198.18.80.77 (50 secs ago)
3    192.168.206.5:80 outside 0 0 2 192.168.180.202 (3 mins ago)
4    192.168.210.13:80 outside 0 0 2 192.168.180.202 (3 mins ago)
5    192.168.206.5:443 outside 0 0 2 192.168.180.202 (3 mins ago)
6    192.168.206.5:21 outside 0 0 2 192.168.180.202 (3 mins ago)
7    192.168.206.5:22 outside 0 0 2 192.168.180.202 (3 mins ago)
8    192.168.206.5:5900 outside 0 0 2 192.168.180.202 (3 mins ago)
9    192.168.206.40:80 outside 0 0 2 192.168.180.202 (3 mins ago)
10   192.168.154.43:80 outside 0 0 2 192.168.180.202 (3 mins ago)

In the preceding example, 22744 TCP packets that were received inbound on the interface outside have been dropped. The average rate of attack during the monitoring window was 12 packets per second, and 109 packets per second is the attack rate in the current sampling interval.

Information about configuring threat protection for the Cisco ASA 5500 Series Adaptive Security Appliance is in Configuring and Viewing Threat Statistics.

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-12

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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