Thursday 26 September 2013

Problems with the CBL

We've recently started having a lot of problems with the Composite Blocking List (CBL).  This is supposed to be a list of IP addresses that are known to be sending spam emails, so its employed by people running email servers to automatically reject connections from these senders to reduce the volume of spam email they are getting.  The contents of the CBL is also aggregated into other third party block lists, such as Spamhaus's Zen list.

Block lists are a pretty fundamental part of most anti-spam systems, and in general they are a good idea.  Unfortunately, the way IP addresses are added to the CBL seems to be very questionable to me - they run "honeypot" servers and if you're caught connecting to one of these servers then you get added to the block list.  This makes a lot of sense when the honeypot is just detecting people sending spam email.  Unfortunately the CBL's honeypot also looks for people making web requests to it, and this is the problem.

I'll give an example of a couple of typical small office networks:
1. An email smarthost and web proxy are operated on the same server.  All the workstations are firewalled off from direct internet access, so they have to use the (authenticated) smarthost to send email and the proxy to access the web.
2. Everything on the network sits behind a single router that does NAT.  The router is set to firewall off SMTP so the only machine that can send email is the mail server, but the workstations either have unrestricted web access, or go via a proxy server that also sits behind the same router.

In both of these example networks, the outgoing email and the web traffic comes from the same IP address...  And I'm sure the problem is immediately obvious: someone plugs a virus-infected machine into the network, which starts making web requests, the IP address ends up on the CBL and suddenly no one in the office can send email.  So anyone using the CBL to reject email, is rejecting email from any network that has had a virus infection, irrespective of whether that infection could have actually sent spam.  Anyone who has run a network for a while (especially one full of Windows laptops) knows that virus infections happen all the time.

Recently we seem to be having a lot of customers being hit by the ZBot virus, and ending up with all their email being blocked by people using the CBL because of this.

One solution is to move the email traffic onto a different IP address to the web traffic.  In some cases this isn't too hard, but in others the customer may be using an ISP who will only provide them with a single IP address, so implementing this would mean changing ISP.

We could reconfigure everyone to use their ISP's email smarthosts for outgoing email, but we don't routinely do this because they add another possible point of failure, seem to be forever getting black listed themselves since you're sharing them with potentially badly-behaved people, and in my experience ISPs frequently seem to configure them in crazy ways that causes unexpected breakage.

I've been in contact with CBL, suggesting that they could make a list of the honeypot domains available to us.  This would allow us to set our customers' proxy servers to block the connections (avoiding being added to the CBL), and also automatically alert the administrator to the virus infection.  Unfortunately they say they can't do this due to the rapidly changing nature of the honeypot servers - this seems like a solved problem to me though, they could easily distribute a rapidly changing list of domains using DNS.

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