Security firm Cyren issued a new report, “Botnets: The Clone Army of Cybercrime,” that described a new method of circumventing security blocking efforts. The researchers referred to this evasive technique as a “ghost host.”

Ghost Host in the Machine

SecurityWeek noted that, to carry out the procedure, malware authors place unknown host names in the HTTP host fields of a botnet’s communications. These host names can be registered and unregistered. Because of the indeterminate state of the included host names, which may or may not resolve in the Domain Name System (DNS), this can easily fool usual web security and URL-filtering systems.

The total IP range associated with a bad URL is not usually blocked because legitimate domains may also exist at the IP. Blocking the entire IP would block those legitimate domains.

The impetus for this discovery originated when Cyren detected communications between bot and server even after it blocked the malware server’s URL in the filters protecting a system. It noticed that a botnet member informed the command-and-control (C&C) server that a new infection had occurred. The researchers said they were unsure just how this communication took place at all.

Playing the Name Game

While the destination IP address of the communication packet was still known to be a bad server, the HTTP host fields that were used for requests were assigned to completely different domains. These domains are stuffed into the host fields that Cyren referred to as ghost hosts.

In one case, the researchers noted, the fake domains were “” and “” The resolved domains were blocked, but not the ghost hosts. Changing the ghost host name sent in a communication might prompt different response actions from the C&C server. One host might document a new addition to the botnet, but another ghost name could serve new exploit code.

This is a novel technique that establishes a side-channel communication with a malware’s C&C server, survives a primary blacklist and delivers a resiliency that could impact the overall malware landscape. The increase in resiliency that it brings to malware will have to be evaluated in the near future.

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