The mad lads at watchTowr are back with their unique blend of zany humor and impressive security research. And this time, it’s the curious case of backdoors within popular backdoors, and the list of unclaimed domains that malicious software would just love to contact.
OK, that needs some explanation. We’re mainly talking about web shells here. Those are the bits of code that get uploaded to a web server, that provide remote access to the computer. The typical example is a web application that allows unrestricted uploads. If an attacker can upload a PHP file to a folder where .php files are used to serve web pages, accessing that endpoint runs the arbitrary PHP code. Upload a web shell, and accessing that endpoint gives a command line interface into the machine.
The quirk here is that most attackers don’t write their own tools. And often times those tools have special, undocumented features, like loading a zero-size image from a .ru domain. The webshell developer couldn’t be bothered to actually do the legwork of breaking into servers, so instead added this little dial-home feature, to report on where to find all those newly backdoored machines. Yes, many of the popular backdoors are themselves backdoored.
This brings us to what watchTowr researchers discovered — many of those backdoor domains were either never registered, or the registration has been allowed to expire. So they did what any team of researchers would do: Buy up all the available backdoor domains, set up a logging server, and just see what happens. And what happened was thousands of compromised machines checking in at these old domains. Among the 4000+ unique systems, there were a total of 4 .gov. domains from governments in Bangladesh, Nigeria, and China. It’s an interesting romp through old backdoors, and a good look at the state of still-compromised machines.
