The Cisco IronPort Email Security Appliances and Cisco IronPort Security Management Appliances contain a buffer overflow vulnerability when a long encryption key is provided via the Telnet protocol. This vulnerability can be exploited remotely without authentication and without end-user interaction. Successful exploitation of this vulnerability could allow arbitrary code execution. The attack vector for exploitation is through the Telnet protocol using TCP port 23. The Telnet packets can be sent to a vulnerable device using IP version 4 (IPv4) or IP version 6 (IPv6).
This vulnerability has been assigned Common Vulnerabilities and Exposures (CVE) ID CVE-2011-4862.
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 infrastructure access control lists (iACLs). This protection mechanism filters and drops packets that are attempting to exploit this vulnerability.
Effective exploit prevention can also be provided by the Cisco ASA 5500 Series Adaptive Security Appliance, Cisco Catalyst 6500 Series ASA Services Module (ASASM), and the Firewall Services Module (FWSM) for Cisco Catalyst 6500 Series Switches and Cisco 7600 Series Routers using transit access control lists (tACLs).
Cisco IOS NetFlow records can provide visibility into network-based exploitation attempts.
Cisco IOS Software, Cisco ASA, Cisco ASASM, and Cisco FWSM firewalls can provide visibility through syslog messages and counter values displayed in the output from show commands.
Effective use of Cisco Intrusion Prevention System (IPS) event actions provides visibility into and protection against attacks that attempt to exploit this vulnerability.
The Cisco Security Manager can also provide visibility through incidents, queries, and event reporting.
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.
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:
To protect infrastructure devices and minimize the risk, impact, and effectiveness of direct infrastructure attacks, administrators are advised to deploy infrastructure access control lists (iACLs) to perform policy enforcement of traffic sent to infrastructure equipment. Administrators can construct an iACL by explicitly permitting only authorized traffic sent to infrastructure devices in accordance with existing security policies and configurations. For the maximum protection of infrastructure devices, deployed iACLs should be applied in the ingress direction on all interfaces to which an IP address has been configured. An iACL workaround cannot provide complete protection against this vulnerability when the attack originates from a trusted source address.
The iACL policy denies unauthorized Telnet IPv4 and IPv6 packets on TCP port 23 that are sent to affected devices. In the following example, 192.168.60.0/24 and 2001:DB8:1:60::/64 represent the IP address space that is used by the affected devices, and the hosts at 192.168.100.1 and 2001:DB8::100:1 are considered trusted sources that require access to the affected devices. Care should be taken to allow required traffic for routing and administrative access prior to denying all unauthorized traffic. 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 protocol and port ! permit tcp host 192.168.100.1 192.168.60.0 0.0.0.255 eq 23 ! !-- 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 23 ! !-- Explicit deny ACE for traffic sent to addresses configured within !-- the infrastructure address space ! deny ip any 192.168.60.0 0.0.0.255 ! !-- Permit or deny all other Layer 3 and Layer 4 traffic in accordance !-- with existing security policies and configurations ! ! !-- Create the corresponding IPv6 tACL ! ipv6 access-list IPv6-Infrastructure-ACL-Policy ! !-- Include explicit permit statements for trusted sources !-- that require access on the vulnerable protocol and port ! permit tcp host 2001:DB8::100:1 2001:DB8:1:60::/64 eq 23 ! !-- The following vulnerability-specific access control entries !-- (ACEs) can aid in identification of attacks to global and !-- link-local addresses ! deny tcp any 2001:DB8:1:60::/64 eq 23 ! !-- Permit other required traffic to the infrastructure address !-- range and allow IPv6 neighbor discovery packets, which !-- include neighbor solicitation packets and neighbor !-- advertisement packets ! permit icmp any any nd-ns permit icmp any any nd-na !
!-- Explicit deny for all other IPv6 traffic to the global !-- infrastructure address range !
deny ipv6 any 2001:DB8:1:60::/64 !
!-- Permit or deny all other Layer 3 and Layer 4 traffic !-- in accordance with existing security policies and configurations ! ! !-- Apply tACLs to interfaces in the ingress direction ! interface GigabitEthernet0/0 ip access-group Infrastructure-ACL-Policy in ipv6 traffic-filter IPv6-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 commands no ip unreachables and no ipv6 unreachables. ICMP unreachable rate limiting can be changed from the default using the global configuration commands ip icmp rate-limit unreachable interval-in-ms and ipv6 icmp error-interval interval-in-ms.
After the administrator applies the iACL to an interface, the show ip access-lists and show ipv6 access-list commands will identify the number of Telnet IPv4 and IPv6 packets on TCP port 23 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 and show ipv6 access-list IPv6-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 23 (60 matches)
20 deny tcp any 192.168.60.0 0.0.0.255 eq 23 (9 matches)
30 deny ip any 192.168.60.0 0.0.0.255 (17 matches)
router#
In the preceding example, access list Infrastructure-ACL-Policy has dropped 9 Telnet packets on TCP port 23 for access control list entry (ACE) line 20.
router#show ipv6 access-list IPv6-Infrastructure-ACL-Policy
IPv6 access list IPv6-Infrastructure-ACL-Policy
permit tcp host 2001:DB8::100:1 2001:DB8:1:60::/64 eq 23 (71 matches) sequence 10
deny tcp any 2001:DB8:1:60::/64 eq 23 (58 matches) sequence 20
permit icmp any any nd-ns (80 matches) sequence 30
permit icmp any any nd-na (80 matches) sequence 40
deny ipv6 any 2001:DB8:1:60::/64 (5 matches) sequence 50
In the preceding example, access list IPv6-Infrastructure-ACL-Policy has dropped 58 Telnet packets on TCP port 23 for ACE line 20.
For additional information about investigating incidents using ACE counters and syslog events, reference the Identifying Incidents Using Firewall and IOS Router Syslog Events Cisco Security white paper.
Administrators can use Embedded Event Manager to provide instrumentation when specific conditions are met, such as ACE counter hits. The Cisco Security white paper Embedded Event Manager in a Security Context provides additional details about how to use this feature.
To protect the network from traffic that enters the network at ingress access points, which may include Internet connection points, partner and supplier connection points, or VPN connection points, administrators are advised to deploy transit access control lists (tACLs) to perform policy enforcement. Administrators can construct a tACL by explicitly permitting only authorized traffic to enter the network at ingress access points or permitting authorized traffic to transit the network in accordance with existing security policies and configurations. A tACL workaround cannot provide complete protection against this vulnerability when the attack originates from a trusted source address.
The tACL policy denies unauthorized Telnet IPv4 and IPv6 packets on TCP port 23 that are sent to affected devices. In the following example, 192.168.60.0/24 and 2001:DB8:1:60::/64 represent the IP address space that is used by the affected devices, and the hosts at 192.168.100.1 and 2001:DB8::100:1 are considered trusted sources that require access to the affected devices. Care should be taken to allow required traffic for routing and administrative access prior to denying all unauthorized traffic.
Additional information about tACLs is in Transit Access Control Lists: Filtering at Your Edge.
! !-- Include explicit permit statements for trusted sources !-- that require access on the vulnerable protocol and port ! access-list 150 permit tcp host 192.168.100.1 192.168.60.0 0.0.0.255 eq 23 ! !-- 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 23 ! !-- 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 ! !-- Create the corresponding IPv6 tACL ! ipv6 access-list IPv6-Transit-ACL-Policy ! !-- Include explicit permit statements for trusted sources !-- that require access on the vulnerable protocol and port ! permit tcp host 2001:DB8::100:1 2001:DB8:1:60::/64 eq 23 ! !-- The following vulnerability-specific ACEs can !-- aid in identification of attacks to global and !-- link-local addresses ! deny tcp any 2001:DB8:1:60::/64 eq 23 ! !-- Permit or deny all other Layer 3 and Layer 4 traffic in !-- accordance with existing security policies and configurations !-- and allow IPv6 neighbor discovery packets, which !-- include neighbor solicitation packets and neighbor !-- advertisement packets ! permit icmp any any nd-ns permit icmp any any nd-na !
!-- Explicit deny for all other IPv6 traffic !
deny ipv6 any any ! ! !-- Apply tACLs to interfaces in the ingress direction ! interface GigabitEthernet0/0 ip access-group 150 in ipv6 traffic-filter IPv6-Transit-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 commands no ip unreachables and no ipv6 unreachables. ICMP unreachable rate limiting can be changed from the default using the global configuration commands ip icmp rate-limit unreachable interval-in-ms and ipv6 icmp error-interval interval-in-ms.
After the administrator applies the tACL to an interface, show ip access-lists and show ipv6 access-list commands will identify the number of Telnet IPv4 and IPv6 packets on TCP port 23 that have been filtered. Administrators are advised to investigate filtered packets to determine whether they are attempts to exploit this vulnerability. Example output for show ip access-lists 150 and show ipv6 access-list IPv6-Transit-ACL-Policy 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 23
20 deny tcp any 192.168.60.0 0.0.0.255 eq 139 (9 matches)
30 deny ip any any
router#
In the preceding example, access list 150 has dropped 9 Telnet packets on TCP port 23 for access control list entry (ACE) line 20.
router#show ipv6 access-list IPv6-Transit-ACL-Policy
IPv6 access list IPv6-Transit-ACL-Policy
permit tcp host 2001:DB8::100:1 2001:DB8:1:60::/64 eq 23 (55 matches) sequence 10
deny tcp any 2001:DB8:1:60::/64 eq 23 (38 matches) sequence 20
permit icmp any any nd-ns (41 matches) sequence 30
permit icmp any any nd-na (41 matches) sequence 40
deny ipv6 any any (21 matches) sequence 50
In the preceding example, access list IPv6-Transit-ACL-Policy has dropped 38 Telnet packets on TCP port 23 for access control list entry (ACE) line 20.
For additional information about investigating incidents using ACE counters and syslog events, reference the Identifying Incidents Using Firewall and IOS Router Syslog Events Cisco Security white paper.
Administrators can use Embedded Event Manager to provide instrumentation when specific conditions are met, such as ACE counter hits. The Cisco Security white paper Embedded Event Manager in a Security Context provides additional details about how to use this feature.
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 IPv4 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 Cisco Security white paper.
Administrators can configure Cisco IOS NetFlow on Cisco IOS routers and switches to aid in the identification of IPv4 traffic flows that may be attempts to exploit this vulnerability. Administrators are advised to investigate flows to determine whether they are attempts to exploit this 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 0017 1
Gi0/0 192.168.11.54 Gi0/1 192.168.60.158 06 0911 0017 3
Gi0/1 192.168.150.60 Gi0/0 10.89.16.226 06 0016 12CA 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 Telnet on TCP port 23 (hex value 0017).
As shown in the following example, to view only the Telnet packets on TCP port 23 (hex value 0017), use the show ip cache flow | include SrcIf|_06_.*0017_ command to display the related Cisco NetFlow records:
TCP Flowsrouter#show ip cache flow | include SrcIf|_06_.*0017 SrcIf SrcIPaddress DstIf DstIPaddress Pr SrcP DstP Pkts Gi0/0 192.168.10.201 Gi0/1 192.168.60.102 06 0984 0017 1 Gi0/0 192.168.11.54 Gi0/1 192.168.60.158 06 0911 0017 3 router#
Administrators can configure Cisco IOS NetFlow on Cisco IOS routers and switches to aid in the identification of IPv6 traffic flows that may be attempts to exploit the vulnerability that is described in this document. Administrators are advised to investigate flows to determine whether they are attempts to exploit this vulnerability or whether they are legitimate traffic flows.
The following output is from a Cisco IOS device running Cisco IOS Software 12.4 mainline train. The command syntax will vary for different Cisco IOS Software trains.
router#show ipv6 flow cache
IP packet size distribution (50078919 total packets):
1-32 64 96 128 160 192 224 256 288 320 352 384 416 448 480
.000 .990 .001 .008 .000 .000 .000 .000 .000 .000 .000 .000 .000 .000 .000
512 544 576 1024 1536 2048 2560 3072 3584 4096 4608
.000 .000 .000 .000 .000 .000 .000 .000 .000 .000 .000
IP Flow Switching Cache, 475168 bytes
8 active, 4088 inactive, 6160 added
1092984 ager polls, 0 flow alloc failures
Active flows timeout in 30 minutes
Inactive flows timeout in 15 seconds
IP Sub Flow Cache, 33928 bytes
16 active, 1008 inactive, 12320 added, 6160 added to flow
0 alloc failures, 0 force free
1 chunk, 1 chunk added>
SrcAddress InpIf DstAddress OutIf Prot SrcPrt DstPrt Packets
2001:DB...06::201 Gi0/0 2001:DB...28::20 Go0/1 0x06 0x16C4 0x0017 1464
2001:DB...6A:5BA6 Gi0/0 2001:DB...28::21 Gi0/1 0x3A 0x0000 0x8000 1191
2001:DB...6A:5BA6 Gi0/0 2001:DB...128::2 Gi0/1 0x06 0x160A 0x0017 1597
2001:DB...6A:5BA6 Gi0/0 2001:DB...128::3 Gi0/1 0x3A 0x0000 0x8000 1155
To permit display of the full 128-bit IPv6 address, use the terminal width 132 exec mode command.
In the preceding example, there are multiple IPv6 flows for Telnet on TCP port 23 (hex value 0017).
As shown in the following example, to view only the Telnet packets on TCP port 23 (hex value 0017), use the show ipv6 flow cache | include SrcIf|_06_.*0017_ command to display the related Cisco NetFlow records:
TCP Flows
router#show ipv6 flow cache | include SrcIf|_06_.*0017_ SrcAddress InpIf DstAddress OutIf Prot SrcPrt DstPrt Packets 2001:DB...06::201 Gi0/0 2001:DB...28::20 Local 0x06 0x16C4 0x0017 1464 2001:DB...6A:5BA6 Gi0/0 2001:DB...128::2 Gi0/1 0x06 0x160A 0x0017 1597 router#
Introduced in Cisco IOS Software Releases 12.2(31)SB2 and 12.4(9)T, Cisco IOS Flexible NetFlow improves original Cisco NetFlow by adding the capability to customize the traffic analysis parameters for the administrator's specific requirements. Original Cisco NetFlow uses a fixed seven tuples of IP information to identify a flow, whereas Cisco IOS Flexible NetFlow allows the flow to be user defined. It facilitates the creation of more complex configurations for traffic analysis and data export by using reusable configuration components.
The following example output is from a Cisco IOS device that is running a version of Cisco IOS Software in the 15.1T train. Although the syntax will be almost identical for the 12.4T and 15.0 trains, it may vary slightly depending on the actual Cisco IOS release being used. In the following configuration, Cisco IOS Flexible NetFlow will collect information on interface GigabitEthernet0/0 for incoming IPv4 flows based on source IPv4 address, as defined by the match ipv4 source address key field statement. Cisco IOS Flexible NetFlow will also include nonkey field information about source and destination IPv4 addresses, protocol, ports (if present), ingress and egress interfaces, and packets per flow.
! !-- Configure key and nonkey fields !-- in the user-defined flow record ! flow record FLOW-RECORD-ipv4 match ipv4 source address collect ipv4 protocol collect ipv4 destination address collect transport source-port collect transport destination-port collect interface input collect interface output collect counter packets ! !-- Configure the flow monitor to !-- reference the user-defined flow !-- record ! flow monitor FLOW-MONITOR-ipv4 record FLOW-RECORD-ipv4 ! !-- Apply the flow monitor to the interface !-- in the ingress direction ! interface GigabitEthernet0/0 ip flow monitor FLOW-MONITOR-ipv4 input
The Cisco IOS Flexible NetFlow flow output is as follows:
router#show flow monitor FLOW-MONITOR-ipv4 cache format table
Cache type: Normal
Cache size: 4096
Current entries: 6
High Watermark: 1
Flows added: 9181
Flows aged: 9175
- Active timeout ( 1800 secs) 9000
- Inactive timeout ( 15 secs) 175
- Event aged 0
- Watermark aged 0
- Emergency aged 0
IPV4 SRC ADDR ipv4 dst addr trns src port trns dst port intf input intf output pkts ip prot
=============== =============== ============= ============= ============= ============== ======= =======
192.168.10.201 192.168.60.102 1456 23 Gi0/0 Gi0/1 1128 6
192.168.10.17 192.168.60.97 4231 23 Gi0/0 Gi0/1 146 6
10.88.226.1 192.168.202.22 2678 53 Gi0/0 Gi0/1 10567 17
10.89.16.226 192.168.150.60 3562 80 Gi0/0 Gi0/1 30012 6
To view only the Telnet packets on TCP port 23, use the show flow monitor FLOW-MONITOR-ipv4 cache format table | include IPV4 DST ADDR |_23_.*_6_ command to display the related NetFlow records.
For more information about Cisco IOS Flexible NetFlow, refer to Flexible Netflow Configuration Guide, Cisco IOS Release 15.1M&T and Cisco IOS Flexible NetFlow Configuration Guide, Release 12.4T.
! !-- Configure key and nonkey fields !-- in the user-defined flow record ! flow record FLOW-RECORD-ipv6 match ipv6 source address collect ipv6 protocol collect ipv6 destination address collect transport source-port collect transport destination-port collect interface input collect interface output collect counter packets ! !-- Configure the flow monitor to !-- reference the user-defined flow !-- record ! flow monitor FLOW-MONITOR-ipv6 record FLOW-RECORD-ipv6 ! !-- Apply the flow monitor to the interface !-- in the ingress direction ! interface GigabitEthernet0/0 ipv6 flow monitor FLOW-MONITOR-ipv6 input
The Cisco IOS Flexible NetFlow flow output is as follows:
router#show flow monitor FLOW-MONITOR-ipv6 cache format table
Cache type: Normal
Cache size: 4096
Current entries: 6
High Watermark: 2
Flows added: 539
Flows aged: 532
- Active timeout ( 1800 secs) 350
- Inactive timeout ( 15 secs) 182
- Event aged 0
- Watermark aged 0
- Emergency aged 0
IPV6 SRC ADDR ipv6 dst addr trns src port trns dst port intf input intf output pkts ip prot
================== ================== ============= ============= =========== ============ ========== =======
2001:DB...06::201 2001:DB...28::20 123 123 Gi0/0 Gi0/0 17 17
2001:DB...06::201 2001:DB...28::20 1265 23 Gi0/0 Gi0/0 1237 6
2001:DB...06::201 2001:DB...28::20 1890 23 Gi0/0 Gi0/0 5009 6
2001:DB...06::201 2001:DB...28::20 2856 5060 Gi0/0 Gi0/0 486 17
To permit display of the full 128-bit IPv6 address, use the terminal width 132 exec mode command.
To view only the Telnet packets on TCP port 23, use the show flow monitor FLOW-MONITOR-ipv6 cache format table | include IPV6 DST ADDR|_23_.*_6_ command to display the related Cisco IOS Flexible NetFlow records.
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 Telnet packets on TCP port 23 sent over IPv4 or IPv6 to affected devices. In the following example, 192.168.60.0/24 and 2001:DB8:1:60::/64 represent the IP address space that is used by the affected devices, and the hosts at 192.168.100.1 and 2001:DB8::100:1 are considered trusted sources that require access to the affected devices. Care should be taken to allow required traffic for routing and administrative access prior to denying all unauthorized traffic.
Additional information about tACLs is in Transit Access Control Lists: Filtering at Your Edge.
! !-- Include explicit permit statements for trusted sources !-- that require access on the vulnerable protocol and port ! access-list tACL-Policy extended permit tcp host 192.168.100.1 192.168.60.0 255.255.255.0 eq 23 ! !-- 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 23 ! !-- 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 ! !-- Create the corresponding IPv6 tACL ! !-- Include explicit permit statements for trusted sources !-- that require access on the vulnerable protocol and port ! ipv6 access-list IPv6-tACL-Policy permit tcp host 2001:DB8::100:1 2001:db8:1:60::/64 eq 23 ! !-- The following vulnerability-specific access control entries !-- (ACEs) can aid in identification of attacks ! ipv6 access-list IPv6-tACL-Policy deny tcp any 2001:db8:1:60::/64 eq 23 ! !-- 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 ! ipv6 access-list IPv6-tACL-Policy deny ip any any ! !-- Apply tACLs to interfaces in the ingress direction ! access-group tACL-Policy in interface outside access-group IPv6-tACL-Policy in interface outside
After the tACL has been applied to an interface, administrators can use the show access-list command to identify the number of Telnet packets sent to TCP port 23 over IPv4 and IPv6 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 and show access-list IPv6-tACL-Policy follows:
firewall#show access-list tACL-Policy
access-list tACL-Policy; 3 elements; name hash: 0x3452703d
access-list tACL-Policy line 1 extended permit tcp host 192.168.100.1
192.168.60.0 255.255.255.0 eq telnet (hitcnt=31)
access-list tACL-Policy line 2 extended deny tcp any 192.168.60.0
255.255.255.0 eq telnet (hitcnt=13)
access-list tACL-Policy line 3 extended deny ip any any (hitcnt=8)
In the preceding example, access list tACL-Policy has dropped 13 Telnet packets on TCP port 23 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.
firewall#show access-list IPv6-tACL-Policy
ipv6 access-list IPv6-tACL-Policy; 3 elements; name hash: 0x566a4229
ipv6 access-list IPv6-tACL-Policy line 1 permit tcp host 2001:db8:1:100::1
2001:db8:1:60::/64 eq telnet (hitcnt=59)
ipv6 access-list IPv6-tACL-Policy line 2 deny tcp any
2001:db8:1:60::/64 eq telnet (hitcnt=137)
ipv6 access-list IPv6-tACL-Policy line 3 deny ip any any (hitcnt=27)
In the preceding example, access list IPv6-tACL-Policy has dropped 137 Telnet packets on TCP port 23 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.
Firewall syslog message 106023 will be generated for packets denied by an access control entry (ACE) that does not have the log keyword present. Additional information about this syslog message is in Cisco ASA 5500 Series System Log Message, 8.2 - 106023.
Information about configuring syslog for the Cisco ASA 5500 Series Adaptive Security Appliance is in Monitoring - Configuring Logging. Information about configuring syslog on the Cisco Catalyst 6500 Series ASA Services Module is in Configuring Logging. Information about configuring syslog on the FWSM for Cisco Catalyst 6500 Series Switches and Cisco 7600 Series Routers is in Monitoring the Firewall Services Module.
In the following example, the show logging | grep regex command extracts syslog messages from the logging buffer on the firewall. These messages provide additional information about denied packets that could indicate potential attempts to exploit the 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
Jan 26 2011 00:15:13: %ASA-4-106023: Deny tcp src outside:192.0.2.18/2944
dst inside:192.168.60.191/telnet by access-group "tACL-Policy"
Jan 26 2011 00:15:13: %ASA-4-106023: Deny tcp src outside:192.0.2.200/2945
dst inside:192.168.60.33/telnet by access-group "tACL-Policy"
Jan 26 2011 00:15:13: %ASA-4-106023: Deny tcp src outside:2001:db8:2::2:172/2951
dst inside:2001:db8:1:60::23/telnet by access-group "IPv6-tACL-Policy"
firewall#
In the preceding example, the messages logged for the tACL tACL-Policy show Telnet packets for TCP port 23 sent to the address block assigned to the affected devices.
Additional information about syslog messages for Cisco ASA Series Adaptive Security Appliances is in Cisco ASA 5500 Series System Log Messages, 8.2. Additional information about syslog messages for Cisco Catalyst 6500 Series ASA Services Module is in the Analyzing Syslog Messages section of the Cisco ASASM CLI Configuration Guide. Additional information about syslog messages for the Cisco FWSM is in Catalyst 6500 Series Switch and Cisco 7600 Series Router Firewall Services Module Logging System Log Messages.
For additional information about investigating incidents using syslog events, reference the Identifying Incidents Using Firewall and IOS Router Syslog Events Cisco Security white paper.
Administrators can use the Cisco IPS appliances and services modules to provide threat detection and help prevent attempts to exploit the vulnerability described in this document. The following table provides an overview of CVE identifiers and the respective Cisco IPS signatures that will trigger events on potential attempts to exploit this vulnerability.
| CVE ID | Signature Release | Signature ID | Signature Name | Enabled | Severity | Fidelity* |
|---|---|---|---|---|---|---|
| CVE-2011-4862 | S622 | 41506/0 | krb5-telnet Encryption Key ID Buffer Overflow | Yes | High | 90 |
* Fidelity is also referred to as Signature Fidelity Rating (SFR) and is the relative measure of the accuracy of the signature (predefined). The value ranges from 0 through 100 and is set by Cisco Systems, Inc.
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 vulnerability listed in the preceding table.
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 7.x and 6.x sensors that are deployed in inline protection mode provides threat prevention against an attack that is attempting to exploit the vulnerability that is 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.
Beginning in software version 4.0, Cisco Security Manager can collect syslogs from Cisco firewalls and Cisco IPS devices and provides the Event Viewer, which can query for events that are related to the vulnerability that is described in this document.
Using the IPS Alert Events predefined view in the Event Viewer, the user can enter the search string 41506/0 in the event filter to return all captured events related to Cisco IPS signature 41506/0.
Using the following filters in the Firewall Denied Events predefined view in the Event Viewer provides all captured Cisco firewall access list deny syslog messages that could indicate potential attempts to exploit the vulnerability that is described in this document.
An Event Type ID filter can be used with the Firewall Denied Events predefined view in the Event Viewer to filter the syslog ID syslog ID ASA-4-106023 (ACL deny) to provide all captured Cisco firewall deny ASA-4-106023 (ACL deny) syslog messages that could indicate potential attempts to exploit the vulnerability that is described in this document.
For more information about Cisco Security Manager Events, refer to the Filtering and Querying Events section of the Cisco Security Manager User Guide.
Beginning in software version 4.1, Cisco Security Manager supports the Report Manager, the Cisco IPS event reporting feature. This feature allows an administrator to define reports based on Cisco IPS events of interest. Reports can be scheduled or users can run ad hoc reports as required.
Using the Report Manager, the user can define an IPS Top Signatures report for Cisco IPS devices of interest based on time-range and signature characteristics. When the Signature ID is set to 41506/0, Cisco Security Manager will generate a comprehensive report that ranks the count of the alerts fired for the signature of interest compared to the total sum of all signature alerts shown in the report.
Also in the Report Manager, the Top Services report can be used with the following configuration to generate a report of events that indicate potential attempts to exploit the vulnerability that is described in this document:
For more information about Cisco Security Manager IPS Event Reporting, refer to the Understanding IPS Top Reports section of the Cisco Security Manager User Guide.
Cisco works with industry-leading Security Information and Event Management (SIEM) companies through the Cisco Developer Network. This partnership helps Cisco deliver validated and pretested SIEM systems that address business concerns such as long-term log archiving and forensics, heterogeneous event correlation, and advanced compliance reporting. Security Information and Event Management partner products can be leveraged to collect events from Cisco devices and then query the collected events for the incidents created by a Cisco IPS signature or deny syslog messages from firewalls that could indicate potential attempts to exploit the vulnerability that is described in this document. The queries can be made by Sig ID and Syslog ID as shown in the following list:
For more information about SIEM partners, refer to the Security Management System website.
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 1.0 | 2012-JANUARY-26 | Initial public release |
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 https://sec.cloudapps.cisco.com/security/center/resources/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.