Security researchers have identified a botnet that borrows an idea from steganography by burying commands in jpg images.

The DlKhora botnet, which is primarily geared towards downloading other strains of malware, encodes instructions so that the command and control server appears to be serving up image files, SecureWorks reports.

The server sets the HTTP Content-Type header to “image/jpeg” and prefaces the bot commands with a fake 32-byte JPEG header. The bot checks if the header matches and decodes the rest of the response to retrieve its commands. The commands are encoded using a single byte XOR with 0×4.

The botnet makes no attempt to pad files such that they resemble genuine image files, a factor that marks the servers used by DlKhora out for detection. Malware installed by the botnet agent, as identified by SecureWorks to date, largely consists of ad hijacking nuisances.

Hackers need a method for passing instructions to the Trojan of compromised machines that form part of zombie (botnet) networks. IRC channels used to be the preferred venue for command and control channel but recently this has changed with miscreants experimented with different control channels such as Google Groups, Twitter and now "image" servers. ®