Cisco IOS Interface Blocked by IPv4 Packets

[admin]

Cisco routers and switches running Cisco IOS� software and configured to process Internet Protocol version 4 (IPv4) packets are vulnerable to a Denial of Service (DoS) attack. Multiple IPv4 packets with specific protocol fields sent directly to the device may cause the input interface to stop processing traffic once the input queue is full. Traffic passing through the device cannot block the input queue. No authentication is required to process the inbound packet. Processing of IPv4 packets is enabled by default. Devices running only IP version 6 (IPv6) are not affected. Multiple valid workarounds are available in the form of best practices for situations where software upgrades are not currently feasible.
Cisco has made software available, free of charge, to correct the problem.
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Affected Products:
This issue affects all Cisco devices running Cisco IOS software and configured to process Internet Protocol version 4 (IPv4) packets. This includes routers as well as switches and line cards that run Cisco IOS software. Cisco devices that do not run Cisco IOS software are not affected. Devices which run only Internet Protocol version 6 (IPv6) are not affected.

To determine the software running on a Cisco product, log in to the device and issue the show version command to display the system banner. Cisco IOS software will identify itself as "Internetwork Operating System Software" or simply "IOS�". On the next line of output, the image name will be displayed between parentheses, followed by "Version" and the IOS release name. Other Cisco devices will not have the show version command or will give different output.

The following example identifies a Cisco product running IOS release 12.0(3) with an installed image name of C2500-IS-L:
Cisco Internetwork Operating System Software IOS (TM)

2500 Software (C2500-IS-L), Version 12.0(3), RELEASE SOFTWARE

The release train label is "12.0".

The next example shows a product running IOS release 12.0(2a)T1 with an image name of C2600-JS-MZ:

Cisco Internetwork Operating System Software IOS (tm)

C2600 Software (C2600-JS-MZ), Version 12.0(2a)T1, RELEASE SOFTWARE (fc1)

Additional information about Cisco IOS Banners is available at [Only registered and activated users can see links].

Details:
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

Impact:
A device receiving these specifically crafted IPv4 packets will force the inbound interface to stop processing traffic. The device may stop processing packets destined to the router, including routing protocol packets and ARP packets. No alarms will be triggered, nor will the router reload to correct itself. This issue can affect all Cisco devices running Cisco IOS software. This vulnerability may be exercised repeatedly resulting in loss of availability until a workaround has been applied or the device has been upgraded to a fixed version of code.

Software Versions and Fixes
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).

A table listing all affected products and their corresponding fixes is available at: [Only registered and activated users can see links]

Obtaining Fixed Software
Customers with contracts should obtain upgraded software free of charge through their regular update channels. For most customers, this means that upgrades should be obtained through the Software Center on the Cisco worldwide website at [Only registered and activated users can see links]

Customers whose Cisco products are provided or maintained through prior or existing agreement with third-party support organizations such as Cisco Partners, authorized resellers, or service providers should contact that support organization for assistance with obtaining the free software upgrade(s).

Customers who purchase direct from Cisco but who do not hold a Cisco service contract and customers who purchase through third-party vendors but are unsuccessful at obtaining fixed software through their point of sale should get their upgrades by contacting the Cisco Technical Assistance Center (TAC). TAC contacts are as follows.
* +1 800 553 2447 (toll free from within North America)
* +1 408 526 7209 (toll call from anywhere in the world)
* e-mail: [Only registered and activated users can see links]

[admin]

Please have your product serial number available and give the URL of this notice as evidence of your entitlement to a free upgrade. To ensure prompt service by email or by phone, please provide your name, company name, address, product serial number, and current version of Cisco IOS software that you are using. This can be documented by pasting the output of the show version command into the text of an email. Free upgrades for non-contract customers must be requested through the TAC.

Please do not contact either "[Only registered and activated users can see links]" or "[Only registered and activated users can see links]" for software upgrades.

See [Only registered and activated users can see links] for additional TAC contact information, including special localized telephone numbers, instructions, and e-mail addresses for use in various languages.

Workarounds
Cisco recommends that all IOS devices that 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 that 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 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 that 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 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 ****** 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: [Only registered and activated users can see links]

Infrastructure ACLs
Although it is often difficult to block traffic transiting your network, it is possible to identify traffic that 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: [Only registered and activated users can see links]