-----Original Message----- From: Ivan Jager [mailto:aij+@xxxxxxxx] Sent: Thursday, April 29, 2010 1:22 PM To: Jason T. Masker Cc: bugtraq@xxxxxxxxxxxxxxxxx Subject: Re: STP mitm attack idea On Wed, Apr 28, 2010 at 05:26:09PM -0400, Jason T. Masker scribbled thusly: > Best practice is to implement layer 2 security mechanisms which would > identify these ports as "access" ports and shut them down if any STP > traffic was received through these interfaces. On Cisco equipment, > this is known as BPDU guard. > http://www.cisco.com/en/US/customer/tech/tk389/tk621/technologies_tech_n ote09186a008009482f.shtml >I don't have an account with Cisco any more, but why would >shutting down the port be the right thing to do? CMU does that, >and it means you have to be very careful when plugging in a >higher-end switch, which was recently a problem for Computer >Club. It seems like simply ignoring STP packets from that port >would be just as effective and much less disruptive. >Ivan Shutting down the port is useful for security in the way that it helps prevent the type of attack that Xperience has described. When BPDU Guard is implemented the port will be shut down if any Spanning Tree packets are seen. It is risky turning off Spanning Tree as any loops in the network will create a denial of service by causing broadcast traffic to be sent out every port on the switch in a continuous loop. An interesting thing to note is what happens if a cable is plugged into two ports on a switch, essentially creating a loop. For this reason when BPDU is implemented and a port comes up it will send out two Spanning Tree packets. The opposing port sees these packets and shuts down. One other feature of BPDU guard is that it can be configured to stay in an error state for a specified period of time by using the "errdisable recovery cause bpduguard" command. When configured using the "errdisable recovery interval xxx" This allows the port to return to normal usage after the error condition has been resolved. Another reason to implement these features is that it prevents Access ports from "sharing" Spanning Tree information and "leaking" the network topology. From a security stand point it might be useful disabling CDP on Access ports as well. Dan
