I've been noticing an increasing number of attacks of the type you are seeing here. It seems to be a DDoS attack but it has to be treated as a possible attempt to access the unit as well. It's best to block them with an access group since this takes pressure off the CPU. Check out our Guide to Using Access-Groups for the details. This has to be done by command line, it can't be set up in the GUI.
The access-list you use to block the traffic should look like this:
ip access-list ex BLOCK
permit tcp host 76.164.174.115 any eq 22
deny tcp any any eq 22
permit ip any any
This blocks ssh traffic from all ip addresses except the Adtran ip address but allows all other traffic to flow normally. If you normally access the unit by SSH from the internet, you can allow other ip addresses in as well but we want to be as specific as possible so we can stop the rogue traffic. Once the access-list is set put 'ip access-group BLOCK in' on your WAN interface. At that point all ssh traffic not explicitly allowed will be blocked.
We also have a Security Best Practices for AOS Products that is worth checking out.
