full-disclosure - Counteroffensive help on bruteforce attacks on SSHD

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From: Valdis.Kletnieks at vt.edu (Valdis.Kletnieks@...edu)
Subject: Counteroffensive help on bruteforce attacks on SSHD 

On Fri, 29 Oct 2004 14:34:21 BST, Andrew Poodle said:
> I'm seeing lots of ssh login attempts with user=root from two or three
> IP addresses, after I blocked access at the firewall based on host.

> Can anyone point me at some good resources where I can bone up and learn
> more about counter-measures....  I'm not looking to take this guy out
> (although would'nt be a bad thing).. But would be interesting to find
> out more.

1) set your firewall up *beforehand* to deny all SSH connects except from
hosts/networks that you need inbound SSH from.  If you're never going to SSH
in except from 3 specific machines and one dial-up net, just allow those 3
machines and the /24 or whatever that the dial-up uses.

2) In your sshd_config file, "PermitRootLogin no" and "PermitEmptyPasswords no"
will help security a lot.  If you're ambitious, you might consider forcing
the use of RSA keys and "PasswordAuthentication no".  Note that this *DOES*
require that the hosts you're ssh'ing in from *also* be secure (because if an
attacker gets the private key on that machine, they just got a login on
your box too...)

3) If you're ambitious, drop the network admin a "Please whack your user who
has a compromised box" (almost *all* of the recent plague of SSH scans have been
from ancient, unsecured, unpatched boxes). Offer void in Korea or anyplace else
that doesn't have a net admin who gives a damn, YMMV, etc.. ;)

4) That should stop the anklebiters.  Deterrence measures for more determined
attackers are a separate issue. ;)

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