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Cisco IOS Stack Group Bidding Protocol Crafted Packet DoS
------------------------------------------------------------------------
SUMMARY
"Multilink PPP (MLP) allows users to combine multiple PPP links into a
single logical network connection, thus enabling on demand bandwidth
allocation. When implemented across multiple device chassis, this is known
as Multichassis Multilink PPP (MMP). The Stack Group Bidding Protocol is
the mechanism by which devices participating in MMP locate each other and
negotiate for a connection termination point".
The Cisco IOS Stack Group Bidding Protocol (SGBP) feature in certain
versions of Cisco IOS software is vulnerable to a remotely-exploitable
denial of service condition. Successful exploitation of this vulnerability
may cause the affected device to become unresponsive and trigger a
hardware reset, resulting in a denial of service condition.
DETAILS
Vulnerable Systems:
* Check original avisory for full system list (
<http://www.cisco.com/warp/public/707/cisco-sa-20060118-sgbp.shtml> here)
The SGBP implementation provided by the Cisco Internetwork Operating
System (IOS) is susceptible to a denial of service attack when presented
with a crafted UDP packet. Sending such a packet to port 9900 of an
affected device will cause it to freeze and stop responding to or passing
traffic. After a delay, the system watchdog timer will detect this
condition and force a reset of the device. The system recovery behavior
will be controlled by the device configuration register; for example, the
router may reload or drop to the ROM monitor.
This vulnerability affects any device that runs Cisco IOS and has enabled
the SGBP protocol. SGBP is enabled by defining a stack group, which is
done using the global IOS command "sgbp group <name>". The presence of
this command will cause the device to begin listening on port 9900, even
if the remaining SGBP parameters are not fully configured.
The following examples demonstrate device configurations for which SGBP is
enabled:
Router#show sgbp
Group Name: test Ref: 0xA3728C00
Seed bid: default, 50, default seed bid setting
Or:
Router#show running-config | include sgbp
sgbp group test_group
If your device displays output similar to either of the above examples,
please consult the IOS software table below to determine whether your
version of IOS is affected.
Cisco products that do not run IOS, do not contain support for SGBP, or do
not have SGBP enabled are not affected by this vulnerability.
Systems on which SGBP is not supported or enabled will return either blank
output or an error message. The following examples demonstrate device
configurations that are not affected by this vulnerability:
* A system that supports but is not enabled for SGBP returns this
output:
Router#show sgbp
Router#
* A system that does not support SGBP returns this error message:
Router#show sgbp
Router#show sgbp
^
% Invalid input detected at '^' marker.
Workaround:
* Configure Access Control Lists (ACLs)
For sites that require the SGBP protocol to be enabled, it may be possible
to apply Access Control Lists (ACLs) to prevent untrusted hosts from
exploiting this vulnerability. The following extended access-list can be
adapted to your network. This example assumes that the SGBP members
communicate using the 192.168.10.0 network.
access-list 101 permit udp 192.168.10.0 0.0.0.255 192.168.10.0
0.0.0.255 port eq 9900
access-list 101 deny udp any 192.168.10.0 0.0.0.255 port eq
9900
access-list 101 permit ip any any
The access-list must then be applied to all interfaces using configuration
commands such as:
interface ethernet 0/0
ip access-group 101 in
* Enable Control Plane Policing
The Control Plane Policy (CoPP) feature may be used to mitigate this
vulnerability, as in this example:
! Do not police SGBP traffic from the trusted network
access-list 140 deny udp 192.168.10.0 0.0.0.255 any eq 9900
! Police SGBP traffic from untrusted hosts and networks
access-list 140 permit udp any any eq 9900
! Do not police any other type of traffic going to the router
access-list 140 deny ip any any
!
class-map match-all sgbp-class
match access-group 140
!
policy-map control-plane-policy
! Drop all traffic that matches the class "sgbp-class"
class sgbp-class
drop
!
control-plane
service-policy input control-plane-policy
Note: CoPP is only available on certain platforms and IOS release trains.
Additional information on the configuration and use of the CoPP feature
can be found at the following URL:
<http://www.cisco.com/en/US/products/sw/iosswrel/ps1838/products_white_paper09186a0080211f39.shtml>
Deploying Control Plane Policing White Paper
* Infrastructure ACLs (iACL)
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 iACLs:
<http://www.cisco.com/warp/public/707/iacl.html>
http://www.cisco.com/warp/public/707/iacl.html
* Configuring Receive Access Lists (rACLs)
For distributed platforms, rACLs may be an option starting in Cisco IOS
Software Versions 12.0(21)S2 for the 12000 series GSR and 12.0(24)S for
the 7500 series. 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 identify and allow legitimate
traffic to your device and deny all unwanted packets:
<http://www.cisco.com/warp/public/707/racl.html>
http://www.cisco.com/warp/public/707/racl.html
ADDITIONAL INFORMATION
The original article can be found at:
<http://www.cisco.com/warp/public/707/cisco-sa-20060118-sgbp.shtml>
http://www.cisco.com/warp/public/707/cisco-sa-20060118-sgbp.shtml
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