SSH brute force attempts seem to be on the rise again, at the SANS Internet Storm Center we have received a number of reports that a number of networks are seeing them. The source IP addresses vary with each new attempted username in the wordlist, which would indicate that the attempts are distributed through botnet(s).
It only takes a single user with a weak password for a breach to occur, then with that foothold escalation and further attacks are likely next. This is certainly not a new phenomenon, however I think it is a good time to raise awareness about it once again.

Reader xemaps wrote in with this log snippet:

"Whole day my server has been targeted by a botnet, attacker also changed ip each new dictionary user."

Jun 17 23:02:03 pro sshd[17444]: Invalid user mailer from 217.37.x.x
Jun 17 23:03:24 pro sshd[17460]: Invalid user mailer from 87.66.x.x
Jun 17 23:05:27 pro sshd[17617]: Invalid user mailman from 89.97.x.x
Jun 17 23:09:30 pro sshd[17639]: Invalid user mailtest from 62.2.x.x
Jun 17 23:15:44 pro sshd[17894]: Invalid user maker from 83.236.x.x
Jun 17 23:16:47 pro sshd[17925]: Invalid user mama from 84.73.x.x

The link for this article located at SANS is no longer available.