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RE: IGMP denial of service vulnerability
From: "Nick Roffey" <nick () 4thline net>
Date: Sat, 15 Jun 2002 00:49:36 +0100

Might be that I'm missing something here, but....

In the scenario mentioned, since the IGMP report is still addressed 
to a group, any attached routers will receive it, which means they'll
continue to know there's at least one group member on the subnet and
carry on forwarding multicast traffic.

IGMP doesn't have any effect on what a host recieves or doesn't - 
as long as there's at least one member of a multicast group on 
a particular segment sending IGMP reports, an attached router will
continue to forward multicast traffic, which all hosts will receive.

CGMP, which limits multicast forwarding on switches to the ports that 
have multicast group members on them, also from my documentation does
not seem to be affected.  There appears to be no facility for a single 
host to 'leave' via CGMP or to time out - as I understand it, only an 
attached router can 'leave' CGMP - done when all IGMP group members have
left or timed out, which deletes the switch state for all members of the

group.  My documentation might be out of date, but it appears that once 
a group member is known to CGMP, that switch port will continue to 
receive multicast traffic until all group members have gone from the 

The result -  the only situation where a DOS could be possible is when 
the multicast sender and receiver are on different subnets AND the 
multicast receiver is the only group member on that subnet. 

Having said all that, unlikely though it is that this will cause any 
major problems, accepting IGMP packets that are not addressed to a 
group is pretty poor!


-----Original Message-----
From: Arun D. Qamra [mailto:arun () cs ucsb edu]
Sent: 14 June 2002 23:21
To: Marty Schoch
Cc: bugtraq () securityfocus com
Subject: Re: IGMP denial of service vulnerability

Thats an interesting scenario. We did test the same, and DOS doesnt take
place, atleast in our test setup. 

In our setup, the router (a Cisco 2514 running IOS ver12.0 in our case)
does process such a report in the scenario you suggested.

However we agree that the code should be tightened, in all systems.

On 14 Jun 2002, Marty Schoch wrote:

All IGMP packets that are not multicast ethernet addresses should be 

Depending on the implementation of router R in linked document, couldn't
there still be a problem in the following scenario.

Host H1 is a member of two groups and
Host H2 sends a membership report for group to group

Host H1 will obviously see this report as well.
Looking briefly at the code it appears host H1 may still consider this
an acceptable report from another host.  If, and I haven't tested any
router configurations, router R does not consider this a valid report
for the group then the same DOS effect may occur.

The RFC says that membership reports should be sent to the group for
which the report applies.  Why not tighten the code down all the way, to
check not just that the report is multicast, but that all the addresses

Marty Schoch
<mschoch () multicasttech com>

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