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Re: Question re: load balancers as a security device
From: David Howe <David.Howe () ansgroup co uk>
Date: Thu, 24 Jan 2008 09:44:06 +0000

 dan.tesch () comcast net wrote:
I'm new to a company that has a large number of sites parked on
managed servers at a hosting facility - the servers, firewalls and
load balancers are exclusive to our use but managed by the ISP.

In reviewing our site design I have seen that the VPN between our LAN
and the hosting facility permits all IP traffic in both directions -
effectively making these public facing servers part of our LAN in my

For obvious reasons I'm looking to change this.  Nobody is lobbying
against the change but a senior developer that was involved in the
original design points out that because of the load balancers in
front of the servers, the world at large is not able to touch the
machines and thus the potential for compromise is limited.

Could I get some comments from this community about how vulnerable or
not this type of setup might be? I'm looking for specific info
related to the load balancers not commentary about the corporate LAN
in this situation - even if the combination of the firewalls and load
balancers provide 99.9% protection I think it is a bad idea and would
most likely not pass PCI scrutiny.

There isn't an easy answer; depending on your *outbound* firewalling from the webservers, threat model and the exploit used, an attacker may still be able to get a working shell on the machine - at which point he is effectively on your network.

There are exploits "in the wild" where a shell is created and the control connection for it is set up *outbound* to the attacker (or more probably, to a relay controlled by the attacker). There are also exploits "in the wild" where individual command shell commands can be run, and the results returned in the http reply. Assuming your webserver (s) are vulnerable to such exploits, the load balancer would only be a minor impediment to the attacker (in that he would have to structure any sequence of commands so as to take into account they may not run sequentially on the same server; that could be as simple as retrying the commands until you *are* on the right server)

Another question could be if your hosts at the colo are vlanned away from the *other* hosts at the colo - if not, then an attacker could theoretically compromise or even buy another host at the same site, and attack your servers at his leisure, play games with ARP to intercept traffic/gain access to the VPN link, or generally ignore the existence of the load balancing. Even with vlanning, there may be a route to your boxes that doesn't go though the balancing solution - so reliance on it to block ports other than web is probably unwise.

David Howe
Senior SysCare  Engineer

david.howe () ansgroup co uk
Office number: 0161 227 1010
Fax: 0161 227 1020

ANS group plc
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