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Crafted Packet Causes Reload on Cisco Routers
------------------------------------------------------------------------
SUMMARY
Cisco Routers running Internetwork Operating System (IOS) that supports
Multi Protocol Label Switching (MPLS) are vulnerable to a Denial of
Service (DoS) attack on MPLS disabled interfaces. A system that supports
MPLS is vulnerable even if that system is not configured for MPLS.
The vulnerability is only present in Cisco IOS release trains based on
12.1T, 12.2, 12.2T, 12.3 and 12.3T. Releases based on 12.1 mainline, 12.1E
and all releases prior to 12.1 are not vulnerable.
Cisco has made free software available to address this vulnerability.
There are workarounds available to mitigate the effects.
DETAILS
Affected Products:
Vulnerable Products
Only the following products running a vulnerable version of IOS that
support MPLS are affected.
* 2600 and 2800 series routers
* 3600, 3700 and 3800 series routers
* 4500 and 4700 series routers
* 5300, 5350 and 5400 series Access Servers
Products that are not listed above are not affected.
MPLS is not supported in IP and IP Plus feature sets. Therefore, products
running an IOS version with an IP or IP Plus feature set are not
vulnerable.
An attack can only be launched at systems that are not configured for MPLS
Traffic Engineering and on the interfaces where MPLS is not enabled. MPLS
enabled interfaces can be determined by the show mpls interfaces command.
An unaffected system where MPLS is not supported will give an output
similar to the following.
Router#show mpls interfaces
^
% Invalid input detected at '^' marker.
Router#
MPLS can be enabled in different ways on a router. In the below output, a
router is shown that has MPLS enabled for IP on interface Ethernet0/0.
&nsbp;&nsbp;Router#show&nsbp;mpls&nsbp;interfaces
&nsbp;&nsbp;Interface&nsbp;&nsbp;&nsbp;&nsbp;&nsbp;&nsbp;&nsbp;&nsbp;&nsbp;&nsbp;&nsbp;&nsbp;&nsbp;&nsbp;IP&nsbp;&nsbp;&nsbp;&nsbp;&nsbp;&nsbp;&nsbp;&nsbp;&nsbp;&nsbp;&nsbp;&nsbp;Tunnel&nsbp;&nsbp;&nsbp;Operational
&nsbp;&nsbp;Ethernet0/0&nsbp;&nsbp;&nsbp;&nsbp;&nsbp;&nsbp;&nsbp;&nsbp;&nsbp;&nsbp;&nsbp;&nsbp;Yes&nsbp;(tdp)&nsbp;&nsbp;&nsbp;&nsbp;&nsbp;No&nsbp;&nsbp;&nsbp;&nsbp;&nsbp;&nsbp;&nsbp;Yes
&nsbp;&nsbp;Router#
When MPLS for IP is enabled on an interface, the router is immune to the
attacks coming from that interface but vulnerable to the attacks coming
from other interfaces. Enabling MPLS for IP on all interfaces of the
router will make the router immune to attacks coming from any interface.
An interface that has MPLS for IP enabled will have mpls ip or
tag-switching ip command in the interface configuration.
MPLS Traffic Engineering (TE) provides a better protection against this
vulnerability. If MPLS TE is enabled globally, the router will be immune
to the attacks coming from any interface. A router that has MPLS TE
enabled will have mpls traffic-eng tunnels command in the show
running-config output.
Products Confirmed Not Vulnerable
* Products that are not running Cisco IOS are not vulnerable.
* Products running Cisco IOS versions 12.0 and earlier and 12.1 mainline
are not vulnerable.
* Products that are not mentioned in the Affected Products section are
not vulnerable (including but not limited to Cisco 7200, 7500, 12000
series and Catalyst systems).
No other Cisco products are currently known to be affected by these
vulnerabilities.
Details:
Multi Protocol Label Switching (MPLS) is a vendor-independent protocol
that integrates layer-2 (as defined in the Open System Interconnection
Reference Model) information into layer-3. More information on MPLS can be
found at <http://www.cisco.com/warp/public/732/Tech/mpls>
http://www.cisco.com/warp/public/732/Tech/mpls.
A vulnerability exists in the processing of an MPLS packet that is
received on an interface where MPLS is disabled. A router that is
configured for MPLS Traffic Engineering is immune to attacks coming from
any interface.
A Cisco device receiving a crafted packet on an MPLS disabled interface
will reset and may take several minutes to become fully functional. This
vulnerability may be exploited repeatedly resulting in an extended DoS
attack. This issue is documented in bugs ID CSCeb56909 ( registered
customers only) and CSCec86420 ( registered customers only) .
Such crafted packets can only be sent from the local network segment.
Software Versions and Fixes:
A table listing all the vulnerable versions and their corresponding fixes
can be found at:
<http://www.cisco.com/warp/public/707/cisco-sa-20050126-les.shtml#software>
http://www.cisco.com/warp/public/707/cisco-sa-20050126-les.shtml#software
Workarounds:
The effectiveness of any workaround is dependent on specific customer
situations such as product mix, network topology, traffic behavior, and
organizational mission. Due to the variety of affected products and
releases, customers should consult with their service provider or support
organization to ensure any applied workaround is the most appropriate for
use in the intended network before it is deployed.
Warning: Using this workaround may affect the operation of your network
and might cause problems. Therefore it is strongly recommended that you do
a code upgrade if you are affected. It is not recommended that you use the
workaround as a long term solution.
Enabling MPLS Traffic Engineering (MPLS TE) globally can be used as a
workaround to mitigate this vulnerability. Since MPLS requires Cisco
Express Forwarding (CEF) in order to work, CEF needs to be enabled first
in order to enable MPLS TE.
CEF and MPLS TE can be enabled by the following commands.
Router(config)# ip cef
Router(config)# mpls traffic-eng tunnels
Having MPLS TE enabled will make the router immune to the attacks coming
from any interface.
ADDITIONAL INFORMATION
The information has been provided by <mailto:psirt@cisco.com> Cisco
Systems Product Security Incident Response Team.
The original article can be found at:
<http://www.cisco.com/warp/public/707/cisco-sa-20050126-les.shtml>
http://www.cisco.com/warp/public/707/cisco-sa-20050126-les.shtml
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