Cisco Applied Mitigation Bulletin-
This Applied Mitigation Bulletin is a companion document to the Cisco Alert, Financial Institution Websites Targeted by Distributed Denial of Service Attacks, and provides identification techniques that administrators can deploy on Cisco network devices.
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The distributed denial of service (DDoS) attacks against financial institutions target excessive resource utilization within the network and do not target a specific vulnerability. This is exploited remotely without authentication and without end-user interaction. Repeated attempts could result in a sustained DoS condition.
The attack vectors for exploitation are through IPv4 packets using the following protocols and ports:
- UDP port 53
- UDP port 80
- TCP port 53
- TCP port 80
An attacker could use spoofed packets.
Traffic on UDP port 53, TCP port 53, and TCP port 80 represent normally valid traffic. Traffic destined to UDP port 80 does not represent a normal port and protocol combination that is used by common applications. This port and protocol combination should be blocked implicitly by the deny at the end of most access list rule sets.
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The DDoS attacks targeting financial institutions do not involve a vulnerability, but rather a resource exhaustion condition caused by overwhelming traffic levels.
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Cisco devices provide several countermeasures for these attacks. Administrators are advised to consider these protection methods to be general security best practices for infrastructure devices and the traffic that transits the network. This section of the document provides an overview of these techniques.
Cisco IOS Software can provide effective means of exploit prevention using the following methods:
- Transit access control lists (tACLs)
- Unicast Reverse Path Forwarding (uRPF)
These protection mechanisms filter and drop, as well as verify the source IP address of, packets that are used in these attacks.
The proper deployment and configuration of uRPF provides an effective means of protection against attacks that use packets with spoofed source IP addresses. uRPF should be deployed as close to all traffic sources as possible.
Effective means of 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 the following:- tACLs
- Application Layer Protocol Inspection
- Threat Detection
- uRPF
These protection mechanisms filter and drop, as well as verify the source IP address of, packets that are attempting to exploit these attacks.
Cisco IOS NetFlow records can provide visibility into network-based exploitation attempts.
Cisco IOS Software, Cisco ASA, Cisco ASASM, Cisco 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.
Effective use of Cisco Intrusion Prevention System (IPS) event actions provides visibility into and protection against these attacks.
The Cisco Security Manager can also provide visibility through incidents, queries, and event reporting.
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Organizations are advised to follow their standard risk evaluation and mitigation processes to determine the potential impact. 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
Device-Specific Mitigation and Identification
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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
- Cisco IOS NetFlow and Cisco IOS Flexible NetFlow
- Cisco ASA, Cisco ASASM, and Cisco FWSM Firewalls
- Cisco Intrusion Prevention System
- Cisco Security Manager
Cisco IOS Routers and Switches
Mitigation: Transit Access Control Lists
To protect the network from traffic that enters the network at ingress access points, which may include Internet connection points, partner and supplier connection points, or VPN connection points, administrators are advised to deploy transit access control lists (tACLs) to perform policy enforcement. Administrators can construct a tACL by explicitly permitting only authorized traffic to enter the network at ingress access points or permitting authorized traffic to transit the network in accordance with existing security policies and configurations. A tACL workaround cannot provide complete protection against these attacks when the attack originates from a trusted source address.
The tACL policy denies unauthorized IPv4 packets on UDP port 80 that are sent to affected devices. In the following example, 192.168.60.0/24 represent the IP address space that is used by the affected devices. Care should be taken to allow required traffic for routing and administrative access prior to denying all unauthorized traffic. This traffic is normally blocked by an implicit deny at the end of an ACL, however by creating a specific ACL it is easier for administrators to determine if they are being attacked in this manner. It should be noted that tACLs will not protect an oversubscribed pipe prior to the device where the tACL is implemented.
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 protocols and ports ! ! !-- The following vulnerability-specific access control entries !-- (ACEs) can aid in identification of attacks ! access-list 150 deny UDP any 192.168.60.0 0.0.0.255 eq 80 ! !-- 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 !
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.
Identification: Transit Access Control Lists
After the administrator applies the tACL to an interface, show ip access-lists and show ipv6 access-list commands will identify the number of IPv4 packets on UDP port 80 that have been filtered. Administrators are advised to investigate filtered packets to determine whether they are attempts to carry out these attacks. Example output for show ip access-lists 150 follows:
router#show ip access-lists 150 Extended IP access list 150 10 deny udp any 192.168.60.0 0.0.0.255 eq 80 (12 matches) 20 deny ip any any router#In the preceding example, access list 150 has dropped 12 packets on UDP port 80 for access control list entry (ACE) line 30.
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 Intelligence Operations white paper.
Administrators can use Embedded Event Manager to provide instrumentation when specific conditions are met, such as ACE counter hits. The Cisco Security Intelligence Operations 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 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.
Mitigation: Spoofing Protection Using Unicast Reverse Path Forwarding
The attacks that are described in this document can be carried out by using spoofed IP packets. Administrators can deploy and configure Unicast Reverse Path Forwarding (uRPF) as a protection mechanism against spoofing.
uRPF is configured at the interface level and can detect and drop packets that lack a verifiable source IP address. Administrators should not rely on uRPF to provide complete spoofing protection because spoofed packets may enter the network through a uRPF-enabled interface if an appropriate return route to the source IP address exists. Administrators are advised to take care to ensure that the appropriate uRPF 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, uRPF may 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 uRPF, reference the Understanding Unicast Reverse Path Forwarding Cisco Security white paper.
Identification: Spoofing Protection Using Unicast Reverse Path Forwarding
With uRPF 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 uRPF has dropped.
Note: Beginning with Cisco IOS Software Release 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 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 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 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 interface type slot/port internal, show cef drop, show ip interface type slot/port, show ip cef switching statistics feature, and show ip traffic examples, uRPF has dropped 18 IP packets received globally on all interfaces with uRPF 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 and Cisco IOS Flexible NetFlow
Identification: IPv4 Traffic Flow Identification Using Cisco IOS NetFlow
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 perform this denial of service (DoS). Administrators are advised to investigate flows to determine 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 0050 1 Gi0/0 192.168.11.54 Gi0/1 192.168.60.158 06 0911 0035 3 Gi0/1 192.168.150.60 Gi0/0 10.89.16.226 06 0016 12CA 1 Gi0/0 192.168.13.97 Gi0/1 192.168.60.28 11 0B3E 0050 5 Gi0/0 192.168.10.17 Gi0/1 192.168.60.97 11 0B89 0050 1 Gi0/0 10.88.226.1 Gi0/1 192.168.202.22 11 007B 007B 1 Gi0/0 192.168.12.185 Gi0/1 192.168.60.239 11 0BD7 0050 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 UDP port 80 (hex value 0050). In addition there are also flows for TCP port 53 (hex value 0035) and TCP port 80 (hex value 0050).
This traffic is sourced from and sent to addresses within the 192.168.60.0/24 address block, which is used for infrastructure devices. The packets in these flows may be spoofed and may indicate an attempt to carry out these attacks. Administrators are advised to compare the flows for TCP port 53 (hex value 0035) and TCP port 80 (hex value 0050) to normal baselines to aid in determining if an attack is in progress.
As shown in the following example, to view only the packets on UDP port 80 (hex value 0050), use the show ip cache flow | include SrcIf|_11_.*0050 command to display the related Cisco NetFlow records.
UDP Flowsrouter#show ip cache flow | include SrcIf|_11_.*0050 SrcIf SrcIPaddress DstIf DstIPaddress Pr SrcP DstP Pkts Gi0/0 192.168.12.110 Gi0/1 192.168.60.163 11 092A 0050 6 Gi0/0 192.168.11.230 Gi0/1 192.168.60.20 11 0C09 0050 1 Gi0/0 192.168.11.131 Gi0/1 192.168.60.245 11 0B66 0050 18 Gi0/0 192.168.13.7 Gi0/1 192.168.60.162 11 0914 0050 1 Gi0/0 192.168.41.86 Gi0/1 192.168.60.27 11 0B7B 0050 2 router#
Identification: IPv4 Traffic Flow Identification Using Cisco Flexible NetFlow
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 80 Gi0/0 Gi0/1 1128 6 192.168.11.54 192.168.60.158 123 53 Gi0/0 Gi0/1 2212 17 192.168.150.60 10.89.16.226 2567 443 Gi0/0 Gi0/1 13 6 192.168.13.97 192.168.60.28 3451 80 Gi0/0 Gi0/1 1 6 192.168.10.17 192.168.60.97 4231 80 Gi0/0 Gi0/1 146 17 10.88.226.1 192.168.202.22 2678 443 Gi0/0 Gi0/1 10567 6 10.89.16.226 192.168.150.60 3562 80 Gi0/0 Gi0/1 30012 6To view only the packets on UDP port 80, use the show flow monitor FLOW-MONITOR-ipv4 cache format table | include IPV4 DST ADDR |_80_.*_17_ command to display the related NetFlow records.
For more information about Cisco IOS Flexible NetFlow, refer to Flexible Netflow Configuration Guide, Cisco IOS Release 15M&T and Cisco IOS Flexible NetFlow Configuration Guide, Cisco IOS Release 12.4T.
Cisco ASA, Cisco ASASM, and Cisco 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 attacks when the attack originates from a trusted source address.
The tACL policy denies unauthorized IPv4 packets on UDP port 80 that are sent to affected devices. In the following example, 192.168.60.0/24 represents the IP address space that is used by the affected devices. Care should be taken to allow required traffic for routing and administrative access prior to denying all unauthorized traffic. It should be noted that tACLs will not protect an oversubscribed pipe prior to the device where the tACL is implemented.
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 protocols and ports ! ! !-- The following vulnerability-specific access control entries !-- (ACEs) can aid in identification of attacks ! access-list tACL-Policy extended deny UDP any 192.168.60.0 255.255.255.0 eq 80 ! !-- 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 tACLs to interfaces in the ingress direction ! access-group tACL-Policy in interface outside
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 IPv4 packets on UDP port 80 that have been filtered. Administrators are advised to investigate filtered packets to determine whether they are attempts to carry out these attacks. Example output for show access-list tACL-Policy follows:
firewall#show access-list tACL-Policy access-list tACL-Policy; 2 elements; name hash: 0x3452703d access-list tACL-Policy line 1 extended deny udp any 192.168.60.0 255.255.255.0 eq 80 (hitcnt=8)
access-list tACL-Policy line 2 extended deny ip any any (hitcnt=8)In the preceding example, access list tACL-Policy has dropped 8 IPv4 packets on UDP port 80 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 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 carry out the attacks 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 Oct 04 2012 00:15:13: %ASA-4-106023: Deny udp src outside:192.0.2.18/2944 dst inside:192.168.60.191/80 by access-group "tACL-Policy" Sep 04 2012 00:15:13: %ASA-4-106023: Deny udp src outside:192.0.2.200/2945 dst inside:192.168.60.33/80 by access-group "tACL-Policy" firewall#In the preceding example, the messages logged for the tACL tACL-Policy show potentially spoofed IPv4 packets for UDP port 80 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 Intelligence Operations white paper.
Mitigation: Application Layer Protocol Inspection
Application layer protocol inspection is available beginning in software release 7.2(1) for the Cisco ASA 5500 Series Adaptive Security Appliance, software release 8.5 for the Cisco Catalyst 6500 Series ASA Services Module, and in software release 4.0(1) for the Cisco Firewall Services Module. This advanced security feature performs deep packet inspection of traffic that transits the firewall. This protocol inspection helps protect against many common denial of service (DoS) attacks, such as
- SYN Flood Protection – Provides SYN flood protection by minimizing embryonic connections and ensuring proper state.
- DNS application inspection – If DNS requests do not conform to standard DNS protocol guidelines, the packet is dropped. This includes poorly formed requests and requests that are over a certain length.
Administrators may construct an inspection policy for applications that require special handling through the configuration of inspection class maps and inspection policy maps, which are applied via a global or interface service policy.
Additional information about application layer protocol inspection is in the Configuring Application Layer Protocol Inspection section of the Cisco ASA 5500 Series Configuration Guide using the CLI, 8.2 and the Configuring Application Inspection section of the Cisco Catalyst 6500 Series ASA Services Module CLI Configuration Guide, 8.5.
Caution: Application layer protocol inspection will decrease firewall performance. Administrators are advised to test performance impact in a lab environment before this feature is deployed in production environments.
Identification: Threat Detection
Cisco Adaptive Security Appliance supports the threat detection feature in software releases 8.0 and later. Using basic threat detection, the security appliance monitors the rate of dropped packets and security events with the following reasons:
- Denial by access lists
- Bad packet format (such as invalid-ip-header or invalid-tcp-hdr-length)
- Connection limits exceeded (both system-wide resource limits and limits set in the configuration)
- DoS attack detected (such as an invalid stateful packet inspection (SPI), Stateful Firewall check failure)
- Basic firewall checks failed (This option is a combined rate that includes all firewall-related packet drops in this bulleted list. It does not include non-firewall-related drops such as interface overload, packets failed at application inspection, and scanning attack detected.)
- Suspicious ICMP packets detected
- Packets failed application inspection
- Interface overload
- Scanning attack detected (This option monitors scanning attacks; for example, the first TCP packet is not a SYN packet, or the TCP connection failed the three-way handshake. Full scanning threat detection [refer to Configuring Scanning Threat Detection for more information] takes this scanning attack rate information and acts on it by classifying hosts as attackers and automatically shunning them, for example.)
- Incomplete session detection such as TCP SYN attack detected or no data UDP session attack detected.
Caution: Configuring advanced threat detection statistics can have a significant impact on the device's CPU.
For infrastructures that are facing the attacks described in this document, the events to focus on are:
- Connection limits exceeded (if connection limiting is configured on the firewall)
- Interface overload
- Scanning attack detection
- Incomplete session detection
More information about configuring threat detection for the Cisco ASA 5500 Series Adaptive Security Appliance is in Configuring Threat Detection. Information about configuring threat detection for the Cisco Catalyst 6500 Series ASA Services Module is in Configuring Threat Detection.
With basic threat detection properly configured, administrators can use the show threat-detection rate command to show the threat events the Cisco ASA or Cisco ASASM has detected. The following example shows 20 SYN attack-related events per second and 223 SYN Trigger events occurring within the burst interval, and 30 Scanning attack-related events and 451 Scanning Trigger events occurring within the burst interval, which could be an indication of an ongoing SYN flood.
firewall# show threat-detection rate Average(eps) Current(eps) Trigger Total events 10-min ACL drop: 1 10 0 983 1-hour ACL drop: 0 0 0 983 10-min SYN attck: 2 20 223 1982 1-hour SYN attck: 0 0 87 1982 10-min Scanning: 3 30 451 2269 1-hour Scanning: 0 0 154 2269 10-min Bad pkts: 0 0 0 4 1-hour Bad pkts: 0 0 0 4 10-min Firewall: 1 10 0 987 1-hour Firewall: 0 0 0 987 10-min Interface: 1 0 0 851 1-hour Interface: 0 0 0 851 firewall#
With advanced threat detection properly configured, administrators can use the show threat-detection statistics command to display statistical threat detection information on the Cisco ASA and Cisco ASASM. The following example shows the access-list drops recorded by threat detection.
firewall# show threat-detection statistics Top Name Id Average(eps) Current(eps) Trigger Total events 1-hour ACL hits: 01 INSIDE/12 0 2 0 44 02 OUTSIDE/11 0 4 0 36 03 INSIDE/6 0 3 0 24 04 OUTSIDE/50 0 1 0 7 05 INSIDE/5 0 0 0 5 06 WEBPORTS-ACL/1.1 0 0 0 5 07 INSIDE/19 0 0 0 4 08 OUTSIDE/9 0 0 0 1 8-hour ACL hits: 01 INSIDE/12 0 2 0 445 02 OUTSIDE/50 0 4 0 368 03 INSIDE/6 0 3 0 225 04 OUTSIDE/11 0 1 0 213 05 WEBPORTS-ACL/1.1 0 0 0 149 06 INSIDE/5 0 0 0 143 07 OUTSIDE/9 0 0 0 2 24-hour ACL hits: 01 OUTSIDE/50 0 2 0 1644 02 INSIDE/12 0 4 0 1520 03 OUTSIDE/11 0 3 0 928 04 INSIDE/6 0 0 0 713 05 WEBPORTS-ACL/1.1 0 0 0 539 06 INSIDE/5 0 0 0 513 07 INSIDE/19 0 0 0 106 08 OUTSIDE/9 0 0 0 59 09 OUTSIDE/13 0 0 0 57 10 OUTSIDE/15 0 0 0 57
Mitigation: Spoofing Protection Using Unicast Reverse Path Forwarding
The attacks that are described in this document can be carried out by spoofed IP packets. Administrators can deploy and configure uRPF as a protection mechanism against spoofing.
uRPF is configured at the interface level and can detect and drop packets that lack a verifiable source IP address. Administrators should not rely on uRPF to provide complete spoofing protection because spoofed packets may enter the network through a uRPF-enabled interface if an appropriate return route to the source IP address exists. In an enterprise environment, uRPF may 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 uRPF, reference the Cisco Security Appliance Command Reference for ip verify reverse-path and the Understanding Unicast Reverse Path Forwarding Cisco Security white paper.
Identification: Spoofing Protection Using Unicast Reverse Path Forwarding
Firewall syslog message 106021 will be generated for packets denied by uRPF. 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 for 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 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 Sep 21 2012 00:15:13: %ASA-1-106021: Deny UDP reverse path check from 192.168.60.1 to 192.168.60.100 on interface outside Sep 21 2012 00:15:13: %ASA-1-106021: Deny UDP reverse path check from 192.168.60.1 to 192.168.60.100 on interface outside Sep 21 2012 00:15:13: %ASA-1-106021: Deny TCP reverse path check from 192.168.60.1 to 192.168.60.100 on interface outsideThe show asp drop command can also identify the number of packets that the uRPF 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, uRPF has dropped 11 IP packets received on interfaces with uRPF configured. Absence of output indicates that the uRPF 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 Table
Administrators can use the Cisco IPS appliances and services modules to provide threat detection. The following table provides an overview of the respective Cisco IPS signatures that will trigger events on potential attempts.
CVE ID Signature Release Signature ID Signature Name Enabled Severity Fidelity* Notes NA S672 1493/0 Distributed Denial of Service on Financial Institutions Yes High 90
NA
S593 2152/0 ICMP Flood No Medium 100
Retired
NA
S572 4002/0 UDP Host Flood No Low 75
Retired
NA
S520 4004/0 DNS Flood Attack No Medium 85
Retired NA
S593 6009/0 SYN Flood DoS No Medium 85
Retired
NA
S573 6901/0 Net Flood ICMP Reply No Informational 100
Retired
NA
S573 6902/0 Net Flood ICMP Request No Informational 100
Retired
NA
S573 6903/0 Net Flood ICMP Any No Informational 100 Retired NA
S573 6910/0 Net Flood UDP No Informational 100
Retired
NA
S573 6920/0 Net Flood TCP No Informational 100 Retired * 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 carry out the attacks listed in the preceding table.
Attacks that use spoofed IP addresses may cause a configured event action to inadvertently deny traffic from trusted sources.
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 attacker that is attempting to carry out the attacks 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.
Cisco Security Manager
Identification: Cisco Security Manager
Cisco Security Manager, Event Viewer
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 attacks that are described in this document.
Using the IPS Alert Events predefined view in the Event Viewer, the user can enter the search strings in the event filter to return all captured events related to the following Cisco IPS signatures:
- 1493/0
- 2152/0
- 4002/0
- 4004/0
- 6009/0
- 6901/0
- 6902/0
- 6903/0
- 6910/0
- 6920/0
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 carry out the attacks that are described in this document.
- Use the Destination event filter to filter network objects that contain the IP address space that is used by the affected devices (for example, IPv4 address range 192.168.60.0/24)
- Use the Destination Service event filter to filter objects that contain UDP port 80.
An Event Type ID filter can be used with the Firewall Denied Events predefined view in the Event Viewer to filter the syslog IDs shown in the following list to provide all captured Cisco firewall deny syslog messages that could indicate potential attempts to carry out the attacks that are described in this document:
- ASA-4-106023 (ACL deny)
For more information about Cisco Security Manager Events, refer to the Filtering and Querying Events section of the Cisco Security Manager User Guide.
Cisco Security Manager Report Manager
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
- 1493/0
- 2152/0
- 4002/0
- 4004/0
- 6009/0
- 6901/0
- 6902/0
- 6903/0
- 6910/0
- 6920/0
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 carry out the attacks that are described in this document:
- Use the Destination IP network filter to filter network objects that contain the IP address space that is used by the affected devices (for example, IPv4 address range 192.168.60.0/24 and IPv6 address range 2001:DB8:1:60::/64)
- Set an action of Deny on the Criteria settings page
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.
Identification: Event Management System Partner Events
Cisco works with industry-leading Security Information and Event Management (SIEM) companies through the Cisco Developer Network. This partnership helps Cisco deliver validated and tested 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 carry out the attacks that are described in this document. The queries can be made by Sig ID and Syslog ID as shown in the following list:
- 1493/0 Distributed Denial of Service on Financial Institutions
- 2152/0 ICMP Flood
- 4002/0 UDP Host Flood
- 4004/0 DNS Flood Attack
- 6009/0 SYN Flood DoS
- 6901/0 Net Flood ICMP Reply
- 6902/0 Net Flood ICMP Request
- 6903/0 Net Flood ICMP Any
- 6910/0 Net Flood UDP
- 6920/0 Net Flood TCP
- ASA-4-106023 (ACL deny)
For more information about SIEM partners, refer to the Security Management System website.
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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.
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Show LessVersion Description Section Date 4 This Cisco Applied Mitigation Bulletin has been updated to use new destinations for links to various supporting technical documents. 2015-December-01 16:31 GMT 3 This Cisco Applied Mitigation Bulletin has been updated to include using the Cisco ASA and Cisco ASASM threat detection feature to help identify potential resource exhaustion attacks. 2012-October-23 14:06 GMT 2 This Cisco Applied Mitigation Bulletin has been updated to include additional details about threat detection using Cisco ASA and Cisco ASASM.
2012-October-16 16:08 GMT 1 Cisco Applied Mitigation Bulletin initial public release. 2012-October-04 21:38 GMT
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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.
Related Information
Related Information
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The security vulnerability applies to the following combinations of products.
Primary Products IntelliShield Applied Mitigation Bulletin Original Release (Base)
Associated Products
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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 ALERTS AT ANY TIME.
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