Products & Services

Contents

Summary
Affected Products
Details
Vulnerability Scoring Details
Impact
Software Versions and Fixes
Workarounds
Obtaining Fixed Software
Exploitation and Public Announcements
Status of this Notice: Final
Distribution
Revision History
Cisco Security Procedures

Vulnerable Products

Products Confirmed Not Vulnerable

Cisco routers are configured to process and accept Internet Protocol version 4 (IPv4) packets by default. IPv4 packets handled by the processor on a Cisco IOS device with protocol types of 53 (SWIPE), 55 (IP Mobility, or 77 (Sun ND), all with Time-to-Live (TTL) values of 1 or 0, and 103 (Protocol Independent Multicast - PIM) with any TTL value, may force the device to incorrectly flag the input queue on an interface as full. A full input queue will stop the device from processing inbound traffic on that interface and may result in routing protocols dropping due to dead timers.

Routers that have the PIM process running are not affected by traffic with protocol type 103. This process will be created when PIM is configured on any interface of the router. An interface with PIM enabled will have one of the following three commands in the interface configuration: ip pim dense-mode, ip pim sparse-mode, or ip pim sparse-dense-mode. Devices with input queues blocked with only PIM packets may have additional workaround options, which are listed in the Workarounds section.

On a blocked Ethernet interface, Address Resolution Protocol (ARP) times out after a default time of four hours, and no traffic can be processed. The device must be rebooted to clear the input queue on the interface, and will not reload without user intervention. The attack may be repeated on all interfaces causing the router to be remotely inaccessible. A workaround is available, and is documented in the Workarounds section. Other types of interfaces, including but not limited to ATM, Serial and POS interfaces, may still be affected, but ARP is no longer a factor. 

The Cisco vulnerabilities are documented in the following two bug IDs: CSCea02355 (registered customers only) affects all Cisco routers running Cisco IOS software, documents the flaw with protocols 53, 55, and 77, and was introduced with bug ID CSCdi22941 (registered customers only) . CSCdz71127 (registered customers only) was introduced by an earlier code revision, and documents an input queue vulnerability to protocol 103 with a device which is not configured for PIM. Any version of software which has the fix for CSCdx02283 (registered customers only) is vulnerable.

To identify a blocked input interface, use the show interfaces command and look for the Input Queue line. If the current size (in this case, 76) is larger than the maximum size (75), the input queue is blocked.

Use the show buffers command and look for the prot field. Below are two examples:

Router#show interface ethernet 0/0
Ethernet0/0 is up, line protocol is up  
  Hardware is AmdP2, address is 0050.500e.f1e0 (bia 0050.500e.f1e0)   
  Internet address is 172.16.1.9/24
  MTU 1500 bytes, BW 10000 Kbit, DLY 1000 usec, rely 255/255, load 1/255
  Encapsulation ARPA, loopback not set, keepalive set (10 sec)
  ARP type: ARPA, ARP Timeout 04:00:00  
  Last input 00:00:41, output 00:00:07, output hang never
  Last clearing of "show interface" counters 00:07:18
  Input queue: 76/75/1091/0 (size/max/drops/flushes); Total output drops: 0  
!--- The 76/75 shows that this is blocked

Router#show buffers input-interface serial 0/0 packet
 Buffer information for Small buffer at 0x612EAF3C
 data_area 0x7896E84, refcount 1, next 0x0, flags 0x0
 linktype 7 (IP), enctype 0 (None), encsize 46, rxtype 0
 if_input 0x6159D340 (FastEthernet3/2), if_output 0x0 (None)
 inputtime 0x0, outputtime 0x0, oqnumber 65535
 datagramstart 0x7896ED8, datagramsize 728, maximum size 65436
 mac_start 0x7896ED8, addr_start 0x7896ED8, info_start 0x0
 network_start 0x7896ED8, transport_start 0x0
 source: 10.0.0.1, destination: 192.168.10.10, id: 0xAAB8, ttl: 41, prot: 103

!--- prot: 103 is proof that this is one of the attack packets

In addition to the command-line instructions to definitively show impacted interfaces, the white paper entitled "Uses of Network Management for Monitoring of the "IP Packet Blocks Input Queue" PSIRT Advisory" details methods to identify impacted devices via SNMP, RMON, and Network Management products.

http://www.cisco.com/warp/public/707/inputqueue.html

Cisco IOS Software

Each row of the table describes a release train and the platforms or products for which it is intended. If a given release train is vulnerable, then the earliest possible releases that contain the fix and the anticipated date of availability for each are listed in the Rebuild, Interim, and Maintenance columns. In some cases, no rebuild of a particular release is planned; this is marked with the label "Not scheduled." A device running any release in the given train that is earlier than the release in a specific column (less than the earliest fixed release) is known to be vulnerable, and it should be upgraded at least to the indicated release or a later version (greater than the earliest fixed release label).

When selecting a release, keep in mind the following definitions:

• Maintenance
  Most heavily tested and highly recommended release of any label in a given row of the table.
• Rebuild
  Constructed from the previous maintenance or major release in the same train, it contains the fix for a specific vulnerability. Although it receives less testing, it contains only the minimal changes necessary to effect the repair. Cisco has made available several rebuilds of mainline trains to address this vulnerability, but strongly recommends running only the latest maintenance release on mainline trains.
• Interim
  Built at regular intervals between maintenance releases and receives less testing. Interims should be selected only if there is no other suitable release that addresses the vulnerability, and interim images should be upgraded to the next available maintenance release as soon as possible. Interim releases are not available through manufacturing, and usually they are not available for customer download from CCO without prior arrangement with the Cisco Technical Assistance Center (TAC).

In all cases, customers should exercise caution to be certain the devices to be upgraded contain sufficient memory and that current hardware and software configurations will continue to be supported properly by the new release. If the information is not clear, contact the Cisco TAC for assistance, as shown in the section following this table.

Notes:

** Marked versions of code are not available on CCO. Please contact the Cisco TAC and request the specific images you need posted.

Cisco ONS 15454 Software

This vulnerability is repaired in Cisco ONS 15454 Software Release 4.1.0 and later and is expected to be available in early August, 2003.

Cisco recommends that all IOS devices which process IPv4 packets be configured to block unwanted traffic, or any traffic directed to the router from an unauthorized source with the use of Access Control Lists (ACLs). This can be done at multiple locations, and it is recommended that you review all methods and use the combination which fits your network best. Although the following ACLs are listed as a workaround for this vulnerability, in cases where performance is not impacted,  these techniques can be considered best practices and may be left in place as a long-term solution rather than a temporary fix.

Best practices dictate that legitimate traffic is defined as management protocols such as telnet, snmp, or ssh, and configured routing protocols from explicitly allowed peers. All other traffic destined to the device should be blocked at the input interface. Traffic entering the network should also be carefully evaluated and filtered at the network edge if destined to an infrastructure device. Although network service providers must often allow unknown traffic to transit their network, it is not necessary to allow that same traffic destined to their network infrastructure. Several white papers have been written to assist in deploying these recommended security best practices.

For devices with interfaces that are currently blocked due to exploitation of this vulnerability, ACL workarounds may be applied. AFTER APPLYING THE WORKAROUND, the input queue depth may be raised with the hold-queue <new value> in interface command to something larger than the default size of 75. This will allow traffic flow on the interface. The device may then be reloaded at a convenient time to release the blocked packets.

For interfaces blocked with PIM packets only, the PIM process may be enabled on the router after applying a workaround which will clear protocol type 103 packets from the blocked input queue. This does not clear packets with protocol type 53, 55, or 77 from the input queue. Although a device with PIM enabled is not vulnerable to attacks with protocol 103 packets, enabling PIM is not recommended as a workaround to this vulnerability, as it does not block protocols 53, 55, or 77, and may have performance implications.

ACLs can have performance impact on certain platforms, so care should be taken when applying the recommended workarounds.

Transit ACLs

The following access list is specifically designed to block attack traffic. Note that the attack traffic can include spoofed source addresses. This access list should be applied to all interfaces of the device, both entering and leaving your network, and should include topology-specific filters. This could include filtering routing protocol traffic, management protocols, and traffic destined for the internal network. Protocol 103 is Protocol Independent Multicast (PIM), which is a commonly deployed application in multicast networks. Interfaces with PIM enabled have not been found to be vulnerable to exploit traffic with protocol 103; PIM traffic may be permitted to those select devices.

access-list 101 permit tcp any any
access-list 101 permit udp any any
access-list 101 deny 53 any any
access-list 101 deny 55 any any
access-list 101 deny 77 any any
access-list 101 deny 103 any any
!--- insert any other previously applied ACL entries here
!--- you must permit other protocols through to allow normal
!--- traffic -- previously defined permit lists will work
!--- or you may use the permit ip any any shown here
access-list 101 permit ip any any 

Prior to deploying ACLs that filter transit traffic, a classification ACL can be used to help identify required permit statements. A classification ACL is an ACL that permits a series of protocols. Displaying access-list entry hit counters helps determine required protocols: entries with zero packets counted are likely not required. Classification access-lists are detailed in the link below for infrastructure access-lists.

Receive ACLs

For distributed platforms, receive path access lists may be an option starting in Cisco IOS Software Versions 12.0(21)S2 for the c12000 and 12.0(24)S for the c7500. The receive access lists protect the device from harmful traffic before the traffic can impact the route processor. Receive path ACLs are considered a network security best practice, and should be considered as a long-term addition to good network security, as well as a workaround for this specific vulnerability. The CPU load is distributed to the line card processors and helps mitigate load on the main route processor. The white paper entitled "GSR: Receive Access Control Lists" will help you identify and allow legitimate traffic to your device and deny all unwanted packets:

http://www.cisco.com/warp/public/707/racl.html

Infrastructure ACLs

Although it is often difficult to block traffic transiting your network, it is possible to identify traffic which should never be allowed to target your infrastructure devices and block that traffic at the border of your network. Infrastructure ACLs are considered a network security best practice and should be considered as a long-term addition to good network security as well as a workaround for this specific vulnerability. The white paper entitled "Protecting Your Core: Infrastructure Protection Access Control Lists" presents guidelines and recommended deployment techniques for infrastructure protection ACLs:

http://www.cisco.com/warp/public/707/iacl.html

Customers with Service Contracts

Customers using Third Party Support Organizations

Customers without Service Contracts

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.

A stand-alone copy or Paraphrase of the text of this document that omits the distribution URL in the following section is an uncontrolled copy, and may lack important information or contain factual errors.

This advisory is posted on Cisco's worldwide website at:

http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20030717-blocked

In addition to worldwide web posting, a text version of this notice is clear-signed with the Cisco PSIRT PGP key and is posted to the following e-mail and Usenet news recipients.

• cust-security-announce@cisco.com
• first-teams@first.org
• bugtraq@securityfocus.com
• vulnwatch@vulnwatch.org
• cisco@spot.colorado.edu
• cisco-nsp@puck.nether.net
• full-disclosure@lists.grok.org.uk
• comp.dcom.sys.cisco@newsgate.cisco.com

Future updates of this advisory, if any, will be placed on Cisco's worldwide website, but may or may not be actively announced on mailing lists or newsgroups. Users concerned about this problem are encouraged to check the above URL for any updates.

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/en/US/products/products_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.