Hello to the list and RA guard evasion technique
mh at mh-sec.de
Sun May 29 11:40:19 CEST 2011
as Fernando Gont, Eric Vyncke and quite some more clever IPv6 heads are
on this list, I subscribed and will join the (security) discussions.
I am the author of the thc-ipv6 toolkit and have so far done quite some
ipv6 security/vulnerability research. The newest issue I published is
bypassing the RA Guard security features on Cisco switches.
Note that this technique also bypasses the following configuration Eric
recommended for switches that have layer 3 ACL capabilities but do not
support RA guard:
deny icmp any any router-advertisement
permit any any
And it also bypasses NDPmon/RAfixd/RAmond.
Make the evil Router Advertisement fragmented and put the ICMPv6 into
the second fragment, eg. by putting a very large Destination extension
header before the ICMPv6 part.
So the packets look like:
Destination Header (~1400 bytes)
Destination Header (continued with some bytes)
ICMPv6 with RA
To prevent this attack, put the following IPv6 ACL on all ports:
deny ip any any undetermined-transport
This will drop all packets where the switch is not able to identify the
IPv6 transport type like in this attack. Note that this might drop some
unusual valid traffic too.
Craft the packets in a way so that the first fragment has an ICMPv6 echo
request and the second fragment overwrites the first fragment with the
ICMPv6 router advertisement.
Destination Header (8 bytes)
ICMPv6 with Echo Request
Fragmentation Header with offset == 1 (equals position of 8th byte ==
start of Echo Request in first fragment)
ICMPv6 with RA
Note that the handling of overlapping fragments differs between
platforms, some take the first fragment received, others the latest, so
send the packets accordingly to your target.
Works on all implementations so far I tested, on some e.g. NDPmon it is
way simpler, you have have to add an empty hop-by-hop header and it goes
blind for NDP and RA attacks.
Basically, if just want to prevent accidental RA's on the network, then
all the tools and mechanisms are fine.
But if you want to prevent attacks, the only secure way is packet
reassembling/verification in the switches - and that is not a good idea
for performance and availability reasons (RAM, CPU, ...).
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Marc Heuse - IT-Security Consulting
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