Attacking HSRP and how to protect it

I covered HSRP (Hot Standby Router Protocol) in a previous post that went into great detail on how HSRP functioned and a few enhancements to it. This time around I figured it would be fun to see what we could do to a typical HSRP deployment, and then research ways to further protect it.

For this scenario the following topology will be used:


. The topology includes a basic HSRP configuration between two routers (R01 and R02), a basic layer 2 switch (sw1) and a webserver sitting on the “Internet”. For verification purposes, the active HSRP router is checked:  12

With the verification completed its now time to start the reconnaissance. For this Kali Linux was used along with a hand full of tools running on it. This post is not intended to explain how to use the tools and will not go over them, a simple Google search will give you all the information you need for that…

A quick wireshark trace shows there is in fact HSRP traffic flowing on the subnet:Screenshot from 2015-07-19 09_37_13

The attack is launched and a new HSRP takes over as active:

Screenshot from 2015-07-19 09_41_253

Executing the attack alone is enough to cause a DDoS event for that subnet. All traffic is routed to the rouge Active HSRP “router” and if it is not setup properly will cause all traffic destined for an outside subnet to be dropped:4

For some a DDoS is the end goal of the attack, but that’s not as fun as being able to be in the middle of the traffic without users even knowing. For the MITM to be successful a few things needed to be configured on the kali Linux device in order to get traffic flowing properly. Once setup traffic was able to leave the subnet:


Now that the attacker is in the middle of all traffic leaving the subnet, it is possible to sniff any traffic going through with wireshark. If anything is sent in clear text (unencrypted) the attacker is able to see it in wireshark including login information:Screenshot from 2015-07-19 10_05_10

The easiest way to add a layer of protection onto HSRP is to utilize MD5 authentication. This can be accomplished through key chains, key strings, or a mixture of the two. The configuration to use a key string is very straight forward. On all routers participating in HSRP for that group add the single command line: standby [HSRP group #] authentication text  [Authentication String]. Once configured, all devices will use the string to hash message data. If a router attempts to join the HSRP group with a incorrect key the following warning is seen:


SNMP traps can be setup so that an alert is sent immediately if the bad authentication is seen allowing for network staff to investigate the issue.


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s