Protect your network against HSRP attacks!

As in my earlier post, I’m writing a series of postings to help protect your network against attacks. A good hacker will always find a way to hurt your network. But with the recommendations described in this series of postings it will be more difficult for the hacker to find a way in.

In this post I will describe the hacking of HSRP operations and off course I will describe a solution to this problem.

As always I use GNS3. Below the topology and the configuration of multilayer switch R2.

interface FastEthernet0/0
switchport mode trunk
interface FastEthernet0/5
switchport access vlan 13
no cdp enable
interface Vlan10
ip address
standby 10 ip
interface Vlan11
ip address
standby 11 ip
interface Vlan12
ip address
standby 12 ip
interface Vlan13
ip address
standby 13 ip

The website explains HSRP as follows:

“HSRP is Cisco’s standard method of providing high network availability by providing first-hop redundancy for IP hosts on an IEEE 802 LAN configured with a default gateway IP address. HSRP routes IP traffic without relying on the availability of any single router. It enables a set of router interfaces to work together to present the appearance of a single virtual router or default gateway to the hosts on a LAN. When HSRP is configured on a network or segment, it provides a virtual Media Access Control (MAC) address and an IP address that is shared among a group of configured routers. HSRP allows two or more HSRP-configured routers to use the MAC address and IP network address of a virtual router. The virtual router does not exist; it represents the common target for routers that are configured to provide backup to each other. One of the routers is selected to be the active router and another to be the standby router, which assumes control of the group MAC address and IP address should the designated active router fail.”

So in short, HSRP provides a virtual default gateway. And that’s the problem, when someone or something finds a way to hack into the HSRP operations it is possible to change the default gateway and reroute networktraffic or blackhole the networktraffic. This turns out to be very simple with Kali. Below a simple explanation how to change the behaviour of HSRP operations.

Start Kali, start yersinia, click the HSRP tab, choose to become active router, give in an ip address and that’s al there is:

hsrp ip


When you have enable terminal monitor on your switch, you’ll see a state change
state change
Now check the HSRP settings with “show standby vlan 13” and you’ll see that the gateway has changed to
sh stand vlan 13

When you look closer to the config you’ll notice something else:
config change
For Yersinia to be able to take over the HSRP operations, it alters the config by entering a static entry to the mac address table!

Again this al there is to! To prevent this, make sure u use authetication on your HSRP interfaces as described in the following example:

First create a key-chain
key chain


Then configure the authentication on the vlan interface. If your are in a production network. First configure the standby unit, then the active unit. Otherwise the actions are disruptive!


stand auth

Now try the HSRP attack again and you’ll see it will not work this time!
stand vlan 13 auth


And to make it even more clear, when terminal monitor is enabled you’ll get the message depicted in the picture below
bad auth