Clicktracking 2: Electric Boogaloo

A week or so back I talked about clicktracking links, and how to put them together to avoid abuse and blocking issues.
Since then I’ve come across another issue with click tracking links that’s not terribly obvious, and that you’re not that likely to come across, but if you do get hit by it could be very painful – phishing and malware filters in web browsers.

First, some background about how a lot of malware is distributed, what’s known as “drive-by malware”. This is where the hostile code infects the victims machine without them taking any action to download and run it, rather they just visit a hostile website and that website silently infects their computer.
The malware authors get people to visit the hostile website in quite a few different ways – email spam, blog comment spam, web forum spam, banner ads purchased on legitimate websites and compromised legitimate websites, amongst others.
That last one, compromised legitimate websites, is the type we’re interested in. The sites compromised aren’t usually a single, high-profile website. Rather, they tend to be a whole bunch of websites that are running some vulnerable web application – if there’s a security flaw in, for example, WordPress blog software then a malware author can compromise thousands of little blog sites, and embed malware code in each of them. Anyone visiting any of those sites risks being infected, and becoming part of a botnet.
Because the vulnerable websites are all compromised mechanically in the same way, the URLs of the infected pages tend to look much the same, just with different hostnames – http://example.com/foo/bar/baz.html, http://www.somewhereelse.invalid/foo/bar/baz.html and http://a.net/foo/bar/baz.html – and they serve up just the same malware (or, just as often, redirect the user to a site in russia or china that serves up the malware that infects their machine).
A malware filter operator might receive a report about http://example.com/foo/bar/baz.html and decide that it was infected with malware, adding example.com to a blacklist. A smart filter operator might decide that this might be just one example of a widespread compromise, and go looking for the same malware elsewhere. If it goes to http//a.net/foo/bar/baz.html and finds the exact same content, it’ll know that that’s another instance of the infection, and add a.net to the blacklist.
What does this have to do with clickthrough links?
Well, an obvious way to implement clickthrough links is to use a custom hostname for each customer (“click.customer.com“), and have all those pointing at a single clickthrough webserver. It’s tedious to setup the webserver to respond to each hostname as you add a new customer, though, so you decide to have the webserver ignore the hostname. That’ll work fine – if you have customer1 using a clickthrough link like http://click.customer1.com/123/456/789.html you’d have the webserver ignore “click.customer1.com” and just read the information it needs from “123/456/789.html” and send the redirect.
But that means that if you also have customer2, using the hostname click.customer2.com, then the URL http://click.customer2.com/123/456/789.html it will redirect to customer1’s content.
If a malware filter decides that http://click.customer1.com/123/456/789.html redirects to a phishing site or a malware download – either due to a false report, or due to the customers page actually being infected – then they’ll add click.customer1.com to their blacklist, meaning no http://click.customer1.com/ URLs will work. So far, this isn’t a big problem.
But if they then go and check http://click.customer2.com/123/456/789.html and find the same redirect, they’ll blacklist click.customer2.com, and so on for all the clickthrough hostnames of yours they know about. That’ll cause any click on any URL in any email a lot of your customers send out to go to a “This site may harm your computer!” warning – which will end up a nightmare even if you spot the problem and get the filter operators to remove all those hostnames from the blacklist within a few hours or a day.
Don’t let this happen to you. Make sure your clickthrough webserver pays attention to the hostname as well as the path of the URL.
Use different hostnames for different customers clickthrough links. And if you pick a link from mail sent by Customer A, and change the hostname of that link to the clickthrough hostname of Customer B, then that link should fail with an error rather than displaying Customer A’s content.