Cisco IOS EIGRP Network DoS

James-lists hackerwacker at
Thu Dec 19 20:50:17 UTC 2002

 ----- Original Message -----
 From: "FX" <fx at>
 To: <bugtraq at>; <darklab at>
 Sent: Thursday, December 19, 2002 10:06 AM
 Subject: Cisco IOS EIGRP Network DoS

 Hi there,

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


          FX           <fx at>
       Phenoelit   (
 672D 64B2 DE42 FCF7 8A5E E43B C0C1 A242 6D63 B564

Attached text moved here:

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

[ Title ]
 Cisco Systems IOS EIGRP Network Denial of Service

[ Authors ]
 FX  <fx at>

 Phenoelit Group (

[ 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 at
 11/14/02 Communication with gaus at 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 at 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 $) ]

 Cisco comments on Bug traq:


 We can confirm the statement made by FX from Phenoelit in his message
 "Cisco IOS EIGRP Network DoS" posted on 2002-Dec-19. The EIGRP
 implementation in all versions of IOS is vulnerable to a denial of
 service if it receives a flood of neighbor announcements. EIGRP is a
 Ciscos' extension of IGP routing protocol used to propagate routing
 information in internal network environments.

 The workaround for this issue is to apply MD5 authentication that will
 permit the receipt of EIGRP packets only from authorized hosts.
 You can find an example of how to configure MD5 authentication for
 EIGRP at the following URL (possibly wrapped):

 If you are using EIGRP in the unicast mode then you can mitigate
 this issue by placing appropriate ACL which will block all EIGRP
 packets from illegitimate hosts. In the following example the
 EIGRP neighbor has IP address of and the local router
 has address

 Router#config t
 Router(config)#access-list 111 permit eigrp host host
 Router(config)#access-list 111 deny eigrp any host

 The previous example will permit all EIGRP packet throughout the router
 and into the rest of the network. If you want to block these packets
 as well then use the following commands instead of the previous

 Router#config t
Router(config)#access-list 111 permit eigrp host host
Router(config)#access-list 111 deny eigrp any any

 An ACL will not be effective if you are using the default multicast
of EIGRP neighbor discovery. However, multicast packets should not be
propagated through the Internet so an attacker must be on the same local
network segment as the target router in order to exploit this issue with
multicast advertisements.

The issue with EIGRP neighbor command FX is referring to is assigned
Cisco Bug ID CSCdv19648 and is visible to all registered users through
Cisco's Bug Toolkit at
At the time of writing this notice Cisco PSIRT does not have a current
estimate on when the fix will be available.


 Version: PGP 6.5.3

 Damir Rajnovic <psirt at>, PSIRT Incident Manager, Cisco Systems
 <>      Telephone: +44 7715 546 033
 200 Longwater Avenue, Green Park, Reading, Berkshire RG2 6GB, GB
 There is no insolvable problems.
 The question is can you accept the solution?

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