Phenoelit Advisory <wir-haben-auch-mal-was-gefunden #0815 +++->
[ Title ] Cisco Systems IOS EIGRP Network Denial of Service
[ Authors ] FX <firstname.lastname@example.org>
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, email@example.com 10/08/02 - 11/14/02 Communication with firstname.lastname@example.org 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 email@example.com 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).
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