Cisco Security Advisory-
The performance of Cisco 12000 series routers can be degraded when they have to send a large number of ICMP unreachable packets. This situation usually can occur during heavy network scanning. This vulnerability is tracked by three different bug IDs: CSCdr46528 ( registered customers only) , CSCdt66560 ( registered customers only) , and CSCds36541 ( registered customers only) . Each bug ID is assigned to a different Engine the line card is based upon.
The rest of the Cisco routers and switches are not affected by this vulnerability. It is specific for Cisco 12000 Series.
No other Cisco product is vulnerable.
The workaround is to either prevent the router from sending unreachable Internet Control Message Protocol (ICMPs) at all or to rate limit them.
This advisory is available at https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20011114-gsr-unreachable.
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This section provides details on affected products.
Vulnerable Products
Only Cisco 12000 Series Internet Routers are affected with this vulnerability. No other routers or switches are affected. Not all line cards of the Cisco 12000 Series are affected by this vulnerability. Vulnerability is present in the underlying technology an individual line card is based upon. That technology is called "Engine". Currently, Cisco is shipping line cards based on the following Engines: 0, 1, 2, 3, and 4.
To determine what Engine your card is based on, you need to log on the Cisco 12000 router and issue the show diag command while in enable mode. The engine type will be displayed as L3 Engine: x , where x will be the corresponding number.
The following example shows the output for an Engine 2 based line card.
c12000#show diag SLOT 1 (RP/LC 1 ): 1 Port Packet Over SONET OC-48c/STM-16 Single Mode/SR SC-SC connector MAIN: type 41, 800-5271-01 rev A0 dev 0 HW config: 0x04 SW key: 00-00-00 PCA: 73-3295-05 rev A0 ver 5 HW version 1.1 S/N SDK034004AY MBUS: Embedded Agent Test hist: 0x00 RMA#: 00-00-00 RMA hist: 0x00 DIAG: Test count: 0x00000000 Test results: 0x00000000 L3 Engine: 2 - Backbone OC48 (2.5 Gbps) ^^^^^^^^^^^^ <- Note the engine type [further output truncated]
All line cards that are based on the Engines 0, 1 and 2 are vulnerable. Line cards based on the Engine 3 and 4 are not affected.
The following table depicts which Cisco IOS® Software Release is vulnerable to a particular issue:
DDTS
12.0S
12.0ST
CSCdr46528 ( registered customers only)
Vulnerable
Vulnerable
CSCds36541 ( registered customers only)
Vulnerable
Vulnerable
CSCdt66560 ( registered customers only)
Vulnerable
Vulnerable
Products Confirmed Not Vulnerable
No other Cisco products are currently known to be affected by these vulnerabilities.
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The received packet will be dropped when either there is no valid path to the destination or when the packet should be routed to the Null0 interface. The packets are either fast dropped (Engine 0 Line Cards) or hardware dropped (all other application-specific integrated circuit (ASIC) based forwarding Line Cards). Given the fast and hardware drop capabilities of the Cisco 12000, a large volume of traffic can be dropped without impacting the capabilities of the router. Whenever a packet is dropped the router must send an ICMP unreachable packet back to the source. That is mandated by the Internet Standards.
When a high volume of traffic is sent to the router that requires ICMP unreachable replies, the processing of the replies can saturate the CPU. This condition can happen when the router is "Black Hole" filtering, dropping packets sent to it as the network's default path, or from a direct Denial of Service (DOS) against the router. For further information on "Black Hole" filtering, refer to Essential IOS Features Every ISP Should Consider, section "Black Hole Routing as a Packet Filter".
The following table shows the relationship between the vulnerabilities and Engine the line card is based on.
DDTS
Engine 0
Engine 0
Engine 2
Engine 4
CSCdr46528
Vulnerable
CSCds36541
Vulnerable
CSCdt66560
Vulnerable
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There are two workarounds for this issue. The first one is to prevent the router from sending ICMP unreachables at all. That behavior is governed with the no ip unreachables command. This command should be applied on an interface, such as in this example:
router(config)#interface ethernet 0 router(config-if)#no ip unreachables
It is possible to mitigate the problem by rate limiting number of ICMP unreachables packets that are sent. Here is the example:
router(config)#ip icmp rate-limit unreachable n
Where n is the number of milliseconds between two consecutive ICMP unreachable packets. The default value is 500. That means that one ICMP unreachable packet is send every 500 ms.
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When considering software upgrades, also consult http://www.cisco.com/go/psirt and any subsequent advisories to determine exposure and a complete upgrade solution.
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 Technical Assistance Center ("TAC") or your contracted maintenance provider for assistance.
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 contains the fix and the anticipated date of availability for each are listed in the "Rebuild", "Interim", and "Maintenance" columns. 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 defect. Although it receives less testing, it contains only the minimal changes necessary to effect the repair.
Interim
Built at regular intervals between maintenance releases and receives less testing. Interim releases 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 via manufacturing, and usually they are not available for customer download from CCO without prior arrangement with the Cisco TAC.
More information on Cisco IOS software release names and abbreviations is available at http://www.cisco.com/warp/public/620/1.html.
Train
Description of Image or Platform
Availability of Fixed Releases*
Vulnerability CSCdr46528
Rebuild
Interim**
Maintenance
12.0S
Core/ISP support: GSR, RSP, c7200
12.0(16)S1
12.0(16.5)S
12.0(17)S
12.0ST
Cisco IOS software Release 12.OST is an early deployment (ED) release for the Cisco 7200, 7500/7000RSP and 12000 (GSR) series routers for Service Providers (ISPs).
12.0(15.6)ST3
12.0(16.5)ST
12.0(16)ST
Vulnerability CSCds36541
Rebuild
Interim**
Maintenance
12.0S
Core/ISP support: GSR, RSP, c7200
12.0(13.6)S2
12.0(14.1)S
12.0(14)S
12.0ST
Cisco IOS software Release 12.OST is an early deployment (ED) release for the Cisco 7200, 7500/7000RSP and 12000 (GSR) series routers for Service Providers (ISPs).
12.0(14.3)ST
Vulnerability CSCdt66560
Rebuild
Interim**
Maintenance
12.0S
Core/ISP support: GSR, RSP, c7200
12.0(16)S1
12.0(16.6)S
12.0(17)S
Notes
* All dates are estimates and subject to change.
** Interim releases are subjected to less rigorous testing than regular maintenance releases, and may have serious bugs.
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The Cisco PSIRT is not aware of any public announcements or malicious use of the vulnerability described in this advisory.
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To learn about Cisco security vulnerability disclosure policies and publications, see the Security Vulnerability Policy. This document also contains instructions for obtaining fixed software and receiving security vulnerability information from Cisco.
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Revision 1.2
2006-Nov-06
Fixed URL to Black Hole filtering.
Revision 1.1
2001-Nov-15
Changed tables entries for the Affected Products and Software Versions and Fixes sections.
Revision 1.0
2001-Nov-14
Initial public release.
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