SSH scans vs connection ratelimiting

[Joerg Pulz]

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1


On Sat, 19 Aug 2006, Pieter de Boer wrote:

> Gang,
>
> For months now, we're all seeing repeated bruteforce attempts on SSH. I've 
> configured my pf install to ratelimit TCP connections to port 22 and to 
> automatically add IP-addresses that connect too fast to a table that's 
> filtered:
>
> table <lamers> { }
>
> block quick from <lamers> to any
>
> pass in  quick on $ext_if inet proto tcp from any to ($ext_if) port 22 
> modulate state (source-track rule max-src-nodes 8 max-src-conn 8 
> max-src-conn-rate 3/60 overload <lamers> flush global)
>
>
> This works as expected, IP-addresses are added to the 'lamers'-table every 
> once in a while.
>
> However, there apparently are SSH bruteforcers that simply use one connection 
> to perform a brute-force attack:
>
> Aug 18 00:00:01 aberdeen sshd[87989]: Invalid user serwis from 83.19.113.122
> Aug 18 00:00:03 aberdeen sshd[88010]: Invalid user serwis from 83.19.113.122
> Aug 18 00:00:05 aberdeen sshd[88012]: Invalid user serwis from 83.19.113.122
> Aug 18 00:00:10 aberdeen sshd[88014]: Invalid user serwis from 83.19.113.122
> Aug 18 00:00:13 aberdeen sshd[88019]: Invalid user serwis from 83.19.113.122
> Aug 18 00:00:14 aberdeen sshd[88021]: Invalid user serwis from 83.19.113.122
>
>
> My theory was/is that this particular scanner simply multiplexes multiple 
> authentication attempts over a single connection. I 'used the source luke' of 
> OpenSSH to find support for this theory, but found the source a bit too 
> wealthy for my brain to find such support.
>
> So, my question is: Does anyone know how this particular attack works and if 
> there's a way to stop this? If my theory is sound and OpenSSH does not have 
> provisions to limit the authentication requests per TCP session, I'd find 
> that an inadequacy in OpenSSH, but I'm probably missing something here :)

Isn't it the "MaxAuthTries" option for sshd which provides such 
functionality?
Please look for "MaxAuthTries" in the sshd_config(5) manpage for details.

regards
Joerg

- -- 
The beginning is the most important part of the work.
 				-Plato
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.5 (FreeBSD)

iD8DBQFE54MNSPOsGF+KA+MRAh0GAJ45v4C9+xJ5vy+4BPltXwBxpKzzIwCePWa8
o/XSdoB2tFdMXQv1Yo1rwFU=
=dHjL
-----END PGP SIGNATURE-----