ssh bruteforce attacks become sophisticated

SSH scans and bruteforce attacks that have have been common since my first SSH enabled server was attacked in 1996. Back then, attacks were so rare that monitoring logins and manually adding attackers IPs to /etc/hosts.allow (TCP Wrapper) was sufficient to keep systems secure.

In the mid 2000’s, the rise of botnets resulted in distributed bruteforce attacks, in which dozens of IPs (bots) would attempt to bruteforce my SSH daemons. I wrote a shell script that collected the IPs and added them to the TCP Wrappers deny list. A while later, denyhosts, was released and I started using it instead of my shell script.

Since installing denyhosts, I only monitor logins and scan the nightly security reports. In the past few years, the frequency of attacks has slowly risen but occasionally there are significant changes in attack frequency and duration. The last significant escalation I can recall was in the months leading up to McColo being shut down. Immediately after their shutdown, I noticed a dramatic reduction in bruteforce SSH attacks.

It was during that time of increased activity that I wrote Sentry. Like my original shell script and denyhosts, it adds attacking IPs to the TCP Wrappers deny list. Sentry also adds their IPs to my PF firewall. Sentry worked much the same as denyhosts, except that when someone attacks one of my IPs, they got blacklisted on all of them. The number of attacks that made it into my security logs dropped accordingly.

Months later, after McColo was shut down, the distributed attacks all but ceased. Since then, attacks have remained sporadic, perhaps 10 a week. In the last couple weeks, the number of attacks spiked. I’m seeing dozens of new IPs getting blacklisted each day. Unlike previous attacks, the usernames the attackers are using are not being duplicated, which means the command & control network behind this latest round of attacks is more intelligent than most.

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