Cisco IOS EIGRP Network DoS

[FX]

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Hi there,

please find attached an advisory about an issue with the Cisco IOS Enhanced
IGRP implementation that can be used to cause a network segment wide denial of
service condition.

Regards
FX

--
FX <fx@phenoelit.de>
Phenoelit (http://www.phenoelit.de)
672D 64B2 DE42 FCF7 8A5E E43B C0C1 A242 6D63 B564

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Phenoelit Advisory <wir-haben-auch-mal-was-gefunden #0815 +++->

[ Title ]
Cisco Systems IOS EIGRP Network Denial of Service

[ Authors ]
FX <fx@phenoelit.de>

Phenoelit Group (http://www.phenoelit.de)
Advisory http://www.phenoelit.de/stuff/CiscoEIGRP.txt

[ Affected Products ]
Cisco IOS

Tested on: IOS 11.3
IOS 12.0(19)
IOS 12.2

Cisco Bug ID: <not assigned>
CERT Vu ID: <not assinged>

[ Vendor communication ]
10/08/02 Initial Notification,
gaus@cisco.com
10/08/02
-
11/14/02 Communication with gaus@cisco.com about the issue,
fixes and timelines.
12/18/02 Final advisory going public as coordinated release
*Note-Initial notification by phenoelit
includes a cc to cert@cert.org by default

[ Overview ]
Cisco Systems IOS is vulnerable to a denial-of-service attack using
Cisco's proprietary routing protocol Enhanced IGRP (EIGRP). When
flooding a Cisco router with spoofed EIGRP neighbor announcements,
the router will cause an Address Resultion Protocol (ARP) storm on
the network segment while trying to find the MAC addresses for the
newly discovered neighbors, effectively using all available bandwidth.

[ Description ]
EIGRP uses automatic discovery of neighboring routers. An EIGRP router
announces it's existence via multicast on the enabled interfaces. If
two routers discover each other, they try to exchange information
about the current topology in unicast. On Ethernet, both sides need
to obtain the MAC address of the other router.

When generating EIGRP neighbor announcements with random source IP
addresses and flooding a Cisco router (unicast, only possible in 11.x)
or an entire network (multicast), all receiving Cisco routers will try
to contact the sender(s). The source IP addresses have to be in the
subnet(s) enabled via the "network" statement in the config of the
victim router.

A bug in Cisco IOS causes the router to continiously try to obtain the
MAC address of the sender. This process does not time out unless the
EIGRP neighbor holdtimer expires. This value is supplied by the sender
of the neighbor announcement and has a maximum of over 18 hours.

Multiple neighbor announcements with not existing source IP addresses
will cause the router to use all available CPU power and bandwidth on
the segment for ARP request - creating a segment-wide denial of
service condition.

The possible use of IP multicast poses a high risk for larger
corporate networks using EIGRP. Cisco IOS versions below 12.0 also
accept EIGRP neighbor announcements as unicast packets, which makes
the attack possible via the Internet.

[ Example ]
None provided at this time.

[ Solution ]
Implement EIGRP authentication using MD5 hashes - which should have
been done in the first place. Where MD5 can not be implemented, use
extended access lists to match expected neighbors.

The obvious workaround of using fixed neighbor entries in the
configuration does not work due to another bug in IOS that makes it
ignore the command (Cisco Bug ID CSCdv19648).

[ end of file ($Revision: 1.5 $) ]

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