SSH Security

I run a linux server that uses SSH as an authentication method. And my server is under attack. There has been over 1000 login attempts from a handful of systems in the past 2 days alone. Since June 1st, 19 different systems have attacked my system over 10,000 times. Chances are, if you're running a system that allows SSH authentication, your system is under attack as well.

For the past few years there has been a remarkable increase in the number of SSH brute force attacks. There are automated scripts out there that scan networks for SSH daemons on port 22, and when found, attempt to log into them using dictionaries of common usernames and passwords.

If you look in your logs, you'll likely see attempts that look something like this:
Jun 30 09:51:58 localhost sshd[32357]: Failed password for invalid user mark from 211.171.202.87 port 41435 ssh2
Jun 30 09:52:02 localhost sshd[32359]: Failed password for invalid user tomas from 211.171.202.87 port 42307 ssh2
Jun 30 09:52:06 localhost sshd[32388]: Failed password for invalid user rpm from 211.171.202.87 port 43192 ssh2
Jun 30 09:52:10 localhost sshd[32390]: Failed password for invalid user jean from 211.171.202.87 port 44095 ssh2

If one of these common username/password pairs actually work against your system, the attacker will gain access to your machine, and likely will use your computer to continue their search for more systems to attack. Your system may also be used as a jumping point for other types of attacks. Your data may be stolen. And who knows what will be traced back to you.

So is there anything you can do to protect yourself? Sure there is. You have a number of options, with different levels of effectiveness.

1) Switch to key based authentication. If you generate an SSH key and use it to provide all access to your system then you're effectively immune against these sorts of attacks. Brute forcing a password using SSH may take days or weeks. Attempting to brute force an RSA based SSH key would take years. Note that you need to REQUIRE keys for authentication, not just use keys. As long as your server still accepts passwords, you're still vulnerable. Regardless of what your standard method for logging in is.

2) If key based authentication isn't an option for you, your next best option is to use a strong password. The longer your password is, and the more random it is, the less likely a password based attack will work. In this case, it's important to ensure that ALL users of the system have strong passwords. Consider using an application like John the Ripper to test the strength of passwords if you have a number of users on the system.

3) An additional option is to move your SSH daemon to a different tcp port. By default SSH listens on port 22. A daemon listening on a different port would be harder to find. Note that unless you mix this will strong passwords you're depending on Security through Obscurity. Obscuring a system like this isn't a bad thing, but it is not a fix all. Assume that someone will eventually find the daemon, and the attack will continue.

4) Restrict who can talk to your SSH daemon. If you're lucky enough to only have a few sources of logins, you can use either application level IP restrictions or firewall rules to ensure you limit who can attempt to login. This works wonders if you trust the people on the allowed hosts list.

The final item I wanted to mention isn't so much a prevention method as a community good deed. Most IPs you see in your logs represent a system that has been compromised by a similiar attack. In most cases, the owner of the system is not aware of what has happened. Or if they are aware, their ISP might be interested to find out about their activities.

If you have some spare time, try tracking down an attacking system. Use tools like dig and whois to attempt to track down the owner of the system and the hosting ISP, and notify them about the attack. Include the log files showing their attack, and explain the issue in plain language. Treat them as a victim, and talk to them in a non-accusitory way.

In a best case situation you'll get a response from the person and they will work to solve the problem. In the worst case scenario you won't get any response. Maybe they solved the problem without saying anything. Maybe they're ignoring you. In either case, at least you tried.